Chemistry A-Level

Chemistry A-Level glossary

2646 key terms for Chemistry A-Level, A to Z, each linked to the topic pages that use it.

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A

absence as evidence: Deducing the functional group from the combination of peaks present and absent, since the absence of a second peak is what settles it.AQA
absolute entropy: Every substance above absolute zero has a positive value of this, quoted in J K⁻¹ mol⁻¹.AQA
absorbance and concentration: Absorbance is proportional to concentration, and that is what makes the technique quantitative.AQA
Absorption and re-emission of infrared radiation: Gases that absorb outgoing infrared radiation emitted by the warmed Earth gain vibrational energy and re-emit radiation in all directions, so some returns…OCR
Absorption of infrared radiation: Infrared radiation is absorbed when its energy matches the energy gap between vibrational energy levels of a covalent bond.OCR
Absorption of infrared radiation by bond vibrations: An organic compound absorbs at wavenumbers that match the vibrational energies of its bonds, so particular absorption peaks act as fingerprints for particular…OCR
absorption of visible light: Some of the wavelengths of visible light are absorbed when light passes through a solution of a transition metal complex.AQA
Absorption of visible light and electron promotion: An electron absorbs visible light of a specific energy and is promoted to a higher d level.OCR
absorption spectrum: A plot of absorbance against wavelength, produced by passing light of each wavelength in turn through a sample and recording how much is absorbed.AQA
abundance ratios: The ratio of the peak heights is fixed for that element, and combining the isotopic masses with their abundances gives a relative atomic mass.AQA
acceleration to constant kinetic energy: An electric field accelerates every ion through the same potential difference, so each ion leaves with the same kinetic energy.AQA
Accuracy: Describes how close a measurement is to the true value.OCR
Accurate measurement of mass: Use a balance of appropriate precision, tare or zero it, and record the mass to the balance's resolution with units.OCR
Accurate reading of the meniscus: The burette is read at eye level against the bottom of the meniscus to avoid parallax error.OCR
Accurate substitution and evaluation: Substituting values correctly into the chosen equation and evaluating the result accurately.OCR
Accurate weighing and dissolving: Weigh the solid accurately and dissolve it in a small volume of distilled water in a beaker.OCR
achiral exceptions: If the original groups are identical, no chiral centre forms, so methanal and symmetrical ketones like propanone yield achiral products that do not exhibit…AQA
acid anhydride: Two acyl groups sharing a single oxygen, (RCO)₂O, left when two acid molecules lose one molecule of water between them, hence ethanoic anhydride.AQA
Acid as proton donor: A species that donates a proton, H⁺, to another species.OCR
acid catalysis: Concentrated sulfuric acid is used as a catalyst that is regenerated, not a reactant in the equation.AQA, OCR
acid dissociation constant: The equilibrium constant for the dissociation of a weak acid, given by Ka = [H⁺][A⁻]/[HA], with units mol dm⁻³.AQA
Acid dissociation constant and its rearrangement: Ka = [H⁺][A⁻]/[HA] rearranges to [H⁺] = Ka × [HA]/[A⁻], and has units of mol dm⁻³.OCR
Acid dissociation constant Ka: For a weak acid HA dissociating in water, HA ⇌ H⁺ + A⁻, it is given by Ka = [H⁺][A⁻]/[HA], where concentrations are equilibrium values.OCR
Acid hydrolysis of esters: Each ester link is broken by water with acid catalysis, giving the parent dicarboxylic acid and the diol, with heating with dilute acid and water driving the…OCR
Acid hydrolysis of nitriles: A nitrile group, –C≡N, is converted into a carboxylic acid group, –COOH by heating with dilute aqueous acid, such as dilute hydrochloric or sulfuric acid.OCR
Acid indigestion treatment: Indigestion is caused by excess hydrochloric acid in the stomach, and antacids relieve discomfort by neutralising this acid.OCR
Acid member: The species in a pair that has one more proton than its partner.OCR
acid rain: Sulfur dioxide dissolves in atmospheric water to form sulfurous acid, and further oxidation in the air gives sulfuric acid.AQA
Acid strength and hydrogen ion concentration: A smaller [H⁺(aq)] gives a larger pH, and acidic solutions have pH below 7 at 298 K.OCR
Acid versus base conditions: The products depend on whether the medium is acidic or alkaline, with alkali neutralising the acid to its salt.OCR
Acid-base neutralisation: The reaction of an acid with a base to form a salt and water.OCR
Acid-catalysed hydrolysis: Reaction in hot aqueous acid in which an ester reacts with water to give a carboxylic acid and an alcohol, with the acid regenerated at the end.OCR
Acid–base behaviour of products: Under acid conditions the carboxylic acid stays protonated, but under alkali conditions the acid is deprotonated as it forms, giving a carboxylate salt.OCR
Acid–base neutralisation: The reaction of the amine group with an acid is a simple one of these, forming a salt.OCR
Acid–base reaction of the COOH group: The carboxylic acid group of an amino acid reacts with an alkali such as sodium hydroxide to be neutralised, giving a carboxylate salt and water.OCR
Acid–base reaction with hydroxide: The hydroxide ions react with the ammonium ion in a reversible acid–base step: NH₄⁺ + OH⁻ ⇌ NH₃ + H₂O.OCR
Acid–base titration: Uses a burette to add a solution of known concentration to a measured volume of the other solution until the reaction is just complete, marked by an indicator…OCR
Acid–carbonate reaction: Adding an aqueous acid supplies H⁺(aq) ions that react with CO₃²⁻ to release carbon dioxide gas, CO₂(g).OCR
Acid–carbonate reactions: A carboxylic acid is a weak acid but is strong enough to react with the carbonate ion CO₃²⁻, donating a proton to it.OCR
acidic and basic oxides: Sulfur dioxide is an acidic oxide, calcium oxide is a basic oxide and calcium carbonate is the salt of a weak acid, so both neutralise it.AQA
acidic buffer: Made from a weak acid and its conjugate base, for example ethanoic acid and sodium ethanoate.AQA, OCR
Acidic Oxide: An oxide that reacts with bases to form a salt and water.AQA
Acidification before precipitation: Dilute nitric acid is added before silver nitrate to remove interfering ions such as carbonate.OCR
Acidified dichromate(VI) and alcohols: A primary or secondary alcohol turns the orange solution green when warmed with acidified potassium dichromate(VI), while a tertiary alcohol does not.OCR
Acidified dichromate(VI) as oxidising agent: The ion Cr₂O₇²⁻, supplied as potassium dichromate(VI), K₂Cr₂O₇, dissolved in dilute sulfuric acid, H₂SO₄, which supplies the H⁺ ions.OCR
acidified potassium dichromate(VI): A suitable oxidising agent, being potassium dichromate(VI) solution acidified with dilute sulfuric acid.AQA, OCR
acidifying with nitric acid: Dilute nitric acid is added first because carbonate, hydroxide and sulfite ions would otherwise precipitate with silver ions and be mistaken for a halide.AQA
Activating and 2,4-directing effect of the –OH group: The –OH group is electron-donating into the ring through lone-pair overlap with the delocalised π system, and it directs incoming electrophiles to the 2- and…OCR
Activation energy: The minimum energy a collision must have for particles to react; a catalyst provides an alternative route with lower activation energy.AQA, OCR
Activation energy and alternative reaction pathways: A catalyst lowers the activation energy by providing an alternative reaction pathway.OCR
Activation energy and reaction rate: A reaction with a large negative ∆G can still be immeasurably slow at room temperature if its activation energy is high, because few particles have enough…OCR
Activation of the aromatic ring by the –OH group: The –OH group makes the ring sufficiently electron-rich, so no halogen carrier catalyst is needed.OCR
active site: A small cleft created by the tertiary structure, whose shape and charge distribution are complementary to one particular substrate.AQA
active sites: Reactant molecules are adsorbed onto these sites on the surface, which weakens their bonds, holds them in the right orientation and raises their concentration…AQA
acyl chloride: A compound RCOCl in which the chlorine is bonded to the carbonyl carbon, named -oyl chloride, as in ethanoyl chloride.AQA
Acyl chlorides: Reactive derivatives of carboxylic acids with the general formula RCOCl, where the –OH of the acid is replaced by –Cl.OCR
Acyl chlorides as reactive acid derivatives: Compounds RCOCl whose carbonyl carbon carries a partial positive charge and whose chloride is a good leaving group, so nucleophiles attack readily.OCR
Acylation: Reaction of benzene with an acyl chloride to attach an acyl group, forming an aromatic ketone such as phenylethanone.OCR
acylation products: With water an acid, with an alcohol an ester, with ammonia an amide, and with a primary amine an N-substituted amide.AQA
acylium ion: The electrophile RCO⁺ that attacks the ring, produced when AlCl₃ pulls the chloride off the acyl chloride.AQA
adding propagation steps: Writing the two steps one above the other and then adding them, so that species appearing on both sides cancel.AQA
Addition of hydrogen across the C=C double bond: Hydrogen is added across the C=C double bond, which becomes a C–C single bond as each carbon gains a hydrogen atom.OCR
Addition polymerisation: An alkene with a C=C double bond opens that bond and joins with others, so the repeat unit has the same atoms as the monomer and no small molecule is lost.AQA, OCR
Addition polymerisation of alkenes: Alkene monomers are joined through the C=C double bond, so the polymer backbone is a continuous chain of two-carbon units.OCR
Addition polymerisation to plastics: Ethene molecules undergo addition polymerisation, joining through their double bonds to form poly(ethene), a plastic.OCR
Addition reactions forming other organic chemicals: Other organic chemicals are made by adding reagents across the C=C bond, for example steam in hydration to make ethanol, or hydrogen in hydrogenation.OCR
addition-elimination: A mechanism in which the nitrogen lone pair attacks the carbonyl carbon while the C=O electrons move onto the oxygen, giving a tetrahedral intermediate, then…AQA
adjusting for charge: Subtract the positive charge or add the negative charge when counting the outer-shell electrons of the central atom.AQA
adsorption: The specific retention mechanism for solid stationary phases, such as silica in TLC, where molecules bind to the surface of the solid.AQA
adsorption and desorption: Reactant molecules are adsorbed onto active sites, and the products are desorbed, freeing the site for more reactant.AQA
Adsorption and differential movement: Separation depends on how strongly each component is adsorbed on the stationary phase compared with how readily it moves with the mobile phase, so components…OCR
Adsorption and spot position: A component that is strongly adsorbed stays near the origin and has a low Rf, while one that moves readily with the solvent has a high Rf.OCR
adsorption on active sites: In heterogeneous catalysis the reactants adsorb onto active sites, which weakens their bonds and holds them in a favourable orientation before the products…AQA
Agricultural applications of Group 2 compounds: Farmers spread calcium hydroxide on acidic soil to neutralise it and make conditions better for crops.OCR
Alcohol products: One of the two products formed when hot aqueous alkali hydrolyses the ester, alongside the carboxylate salt.OCR
Alcohol versus phenol discrimination: Telling these apart by whether the –OH sits on the ring, as an alcohol has an –OH attached to an sp³ carbon and a phenol has an –OH attached directly to a…OCR
Alcohols to alkenes: Heating an alcohol with an acid catalyst gives an alkene as the organic product, for example ethene from ethanol.OCR
aldehyde then carboxylic acid: The first step gives an aldehyde and the second gives a carboxylic acid.AQA
Aldehyde versus ketone discrimination: Telling these apart by what is bonded to the carbonyl carbon, since an aldehyde has a C=O with H on the carbon and a ketone has a C=O between two carbons.OCR
Aldehyde versus ketone distinction: A follow-up test is needed, since the 2,4-DNPH test confirms a carbonyl group but does not distinguish an aldehyde from a ketone.OCR
Alicyclic: An aliphatic compound arranged in non-aromatic rings with or without side chains.OCR
Aliphatic: A compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings.OCR
alkaline fuel cell: A hydrogen–oxygen cell in which the electrolyte supplies hydroxide ions, so both half-equations must contain OH⁻.AQA
Alkaline nature of hydroxide solutions: The resulting solutions are alkaline because the hydroxide ions are present.OCR
alkalinity of the product: Because magnesium hydroxide is only sparingly soluble the solution reaches about pH 10 rather than pH 14, while barium hydroxide solution approaches pH 13.AQA
Alkalis as water-soluble bases: An alkali is a base that dissolves in water, so all alkalis are bases but not all bases are alkalis.OCR
alkane fuels and uses: Methane is natural gas, propane and butane are bottled as LPG, roughly C₅ to C₁₀ makes petrol, and longer chains give kerosene and diesel.AQA
Alkane general formula: CₙH₂ₙ₊₂, which holds for every member of the alkane series rather than a single molecular formula.OCR
alkene products: Because the fragments have to share out the hydrogen that was there, at least one product is normally unsaturated.AQA
alkene rich products: A high percentage of alkenes, especially ethene and propene, which feed the polymer industry.AQA
alkoxide intermediate: A negatively charged alkoxide intermediate is formed, which then takes a hydrogen ion from the water, completing the hydroxyl group.AQA
alkyl alkanoate naming: The name runs the opposite way round from the drawing, so the alkyl is taken from the alcohol and the alkanoate from the acid.AQA
Alkyl group: A saturated hydrocarbon group of general formula CₙH₂ₙ₊₁, obtained by removing one hydrogen atom from an alkane.OCR
Alkylation: Reaction of benzene with a haloalkane to attach an alkyl group, giving an alkylbenzene.OCR
Alphabetical ordering of substituents: Naming and locating side chains as prefixes such as methyl-, ethyl- and chloro- in alphabetical order, not numerical order.OCR
alphabetical prefixes: Substituents listed as prefixes in alphabetical order, using di, tri and tetra for repeats but ignoring those multipliers when alphabetising.AQA
alternating backbone: A sugar-phosphate-sugar-phosphate polymer chain in which every second unit is a sugar and every other one a phosphate.AQA
Alternative pathway and lower activation energy: A catalyst increases the rate of a reaction by providing an alternative reaction pathway of lower activation energy.OCR
Alternative pathway of lower activation energy: The increase in rate is explained by the catalyst providing an alternative pathway of lower activation energy.OCR
Alternative reaction pathway: A route provided by a catalyst that has a lower activation energy than the uncatalysed route.OCR
alternative reaction route: A different path through different intermediates that a catalyst provides for a reaction.AQA
always endothermic: Bond breaking always takes energy in, so mean bond enthalpies are always positive and any definition saying energy released is refused.AQA
amide: A compound with nitrogen on the carbonyl carbon, RCONH₂, named -amide, gaining an N- prefix if an alkyl group replaces one of the hydrogens on the nitrogen.AQA
Amide and ester formation: The carboxylic acid group reacts with alcohols in esterification and with amines to form amides, and the amine group reacts with acyl chlorides to form amides.OCR
amide formation: Ammonia or a primary amine attacks the carbonyl carbon of an acyl chloride or an anhydride, and the chloride or carboxylate is eliminated, so the product…AQA
Amide functional group: A carbonyl group (C=O) directly bonded to a nitrogen atom, giving the functional group –CONH₂ in a primary amide.OCR
Amide functional group structure: A carbonyl group, C=O, bonded directly to a nitrogen atom, giving the functional group –CONH–.OCR
amide group: The -CO-NH- that is left after the -COOH of one amino acid reacts with the -NH₂ of the next and water is lost.AQA
Amide group formation: Forms when a carboxylic acid group on one monomer reacts with an amine group on another, releasing a small molecule such as water.OCR
Amide linkage: The –CO–NH– unit within the polymer backbone, with the carbonyl carbon bonded directly to the nitrogen and no oxygen in the link.OCR
amino acid self-condensation: An amino acid carries an -NH₂ at one end and a -COOH at the other, so it condenses with itself over and over to give a polyamide.AQA
ammonia and ammonium chloride: The classic pair, where the salt supplies ammonium ions and the base supplies ammonia molecules.AQA
Ammonia solubility as a confirmatory test: Dilute ammonia dissolves silver chloride, concentrated ammonia dissolves silver bromide and silver iodide remains insoluble even in concentrated ammonia.OCR
ammonia tests: Adding ammonia solution dropwise and then in excess, where the copper precipitate redissolves to a deep blue solution while the aluminium one does not.AQA
Ammonium identification: Warming with sodium hydroxide solution releases ammonia gas that turns damp red litmus paper blue.OCR
ammonium ion: The ammonium ion is NH₄⁺, a positive ion made of nitrogen and hydrogen.OCR
Ammonium ion formation: NH₄⁺ forms from NH₃ and H⁺ by donation of the nitrogen lone pair.OCR
Ammonium ion identification: The ammonium ion, NH₄⁺, is identified by adding warm sodium hydroxide solution, NaOH(aq), to the sample, which releases ammonia gas, NH₃.OCR
ammonium salt formation: With ammonia or an amine two moles are needed per mole of acyl chloride, because the second mole takes up the HCl as a salt.AQA
amount of substance: A quantity that counts how many specified particles a sample contains, not how heavy or how large it is.OCR
Amphiprotic: Describes a species such as water that can act as an acid or a base, donating a proton in one reaction and accepting one in another.OCR
Amphoteric: A substance that can act as both an acid and a base.AQA
amphoteric water: Water can act as a base towards HCl and as an acid towards NH₃, so it turns up on both sides of this topic.AQA
anaerobic conditions: Air is excluded because oxygen would allow the ethanol to be oxidised to ethanoic acid, turning the mixture to vinegar.AQA
analysis of the mixture: The mixture is then usually separated and identified by thin-layer chromatography.AQA
angles in ions: The same reasoning gives about 104° for a four-pair ion carrying two lone pairs.AQA
anhydride versus acyl chloride: Ethanoic anhydride gives the same amide but releases ethanoic acid instead of hydrogen chloride, and reacts less violently.AQA
anhydrous conditions: Conditions that must be dry, since water destroys the AlCl₃.AQA
Anion identification: The detection of carbonate, sulfate and halide anions, each confirmed by a distinct observation when the appropriate reagent is added.OCR
anion in alkali: In excess alkali the OH⁻ removes a proton from NH₃⁺, leaving NH₂ and COO⁻.AQA
Anion tests with silver nitrate and barium salts: Acidifying with dilute nitric acid and adding silver nitrate to detect halides, or adding barium chloride or barium nitrate with dilute acid to detect sulfate…OCR
anomalous boiling points: Hydrogen bonding explains why water, ammonia and hydrogen fluoride boil far above the trend set by the other hydrides of its group.AQA
Anomalous melting and boiling points: Water has a small relative molecular mass of 18, yet its melting point is 273 K and its boiling point is 373 K at standard pressure, far higher than expected…OCR
Anomaly: A measurement that does not fit the pattern set by the other readings, identified by comparing each result with its neighbours and with the expected trend…OCR
Apparatus assembly and clamping: The round-bottomed flask is held by a clamp on a retort stand, never by hand, and joints are greased where appropriate.OCR
Apparatus choice in titration: A pipette delivers a fixed accurate volume while a burette delivers variable measured volumes.OCR
Apparatus selection: Matching the piece of equipment to the required measurement so that its precision matches the measurement required.OCR
Apparatus uncertainty: The range within which a reading may lie, often quoted as half the smallest scale division for a single reading, and twice that for a difference between two…OCR
Applying familiar redox method to unfamiliar systems: Any unfamiliar ion, such as a complex or an oxoanion, is balanced for atoms, charge and electrons before an answer is chosen.OCR
Applying general patterns to unfamiliar aromatic reactions: The same logic is applied to unfamiliar substituents and reagents, checking charges and lone pairs at each step.OCR
Applying patterns to unfamiliar halogen pairs: Using the same displacement pattern for any halogen pair, predicting whether a reaction occurs.OCR
Applying stoichiometric coefficients in entropy sums: Multiplying each standard entropy by the stoichiometric coefficient from the balanced equation before summing.OCR
Applying the sequence to specific elements: An element with 11 protons comes after one with 10 and before one with 12, placing a given element correctly relative to its neighbours.OCR
Approximate estimate: The result is approximate because the actual bonds in the reacting molecules differ from the averaged values, and because the calculation assumes all species…OCR
Approximate pH above 7: The approximate pH of the resulting solutions is above 7, typically around 10 to 12 for the more soluble hydroxides.OCR
approximations: The assumptions that the H⁺ from water is negligible, so [H⁺] = [A⁻], and that so little acid dissociates that [HA] at equilibrium equals the concentration…AQA
Aqueous alkali as reagent: The reaction is carried out with aqueous alkali, for example aqueous sodium hydroxide, often under heating.OCR
aqueous complexes: The aqueous ions include [Cu(H₂O)₆]²⁺, which is blue, and [Fe(H₂O)₆]²⁺, which is pale green.AQA
area beyond Eₐ: The shaded area beyond the barrier represents the molecules with energy greater than or equal to Eₐ, and the rate roughly follows it.AQA
area beyond the activation energy: Beyond a marked Eₐ the shaded area under the hotter curve is many times larger, so many more molecules have energy greater than or equal to the activation…AQA
area under the curve: The whole area equals the total number of molecules, so it stays constant at a fixed amount of gas.AQA
Aromatic compound: A compound containing a benzene ring, C₆H₆, drawn as a planar hexagon of six carbon atoms with one hydrogen on each carbon.OCR
aromatic hydrocarbons: One of the main products of catalytic cracking, alongside motor fuels.AQA
aromatic polyamides: Polyamides such as Kevlar in which both monomers are aromatic, which is what makes its chains rigid and flat.AQA
Aromatic ring: A benzene ring whose delocalised π system also counts as unsaturation, even though no isolated double bond is drawn.OCR
Arrhenius equation: An equation in which the rate constant k equals A multiplied by an exponential factor containing −Eₐ/RT.AQA
arrow direction: The arrowhead points from the donor atom towards the acceptor atom, so in the ammonium ion the arrow runs from nitrogen to the added hydrogen.AQA
arrows for dative bonds: An arrow is drawn for a co-ordinate bond, whose head points from the atom supplying the lone pair towards the atom receiving it.AQA
Assessing purity by boiling point: Comparing the boiling point with a data-book value and checking the range is narrow.OCR
assignment from the Data Booklet: A quoted shift is used to name a specific environment, such as a proton on a carbon bonded to the single-bonded oxygen of an ester at about 4.1 ppm.AQA
Assignment of NMR peaks: The shift value is used to name the type of proton environment rather than only counting peaks.OCR
asymptotic tail: The long tail to the right that never touches the axis, because there is no upper limit to the energy a molecule can have.AQA
atmospheric measurements: Ground-based measurements over Antarctica showed a seasonal collapse in ozone, and satellite data confirmed the same hole.AQA
Atom accounting in polymer sections: Checking that the two monomers together account for all the atoms shown in the given polymer section.OCR
atom economy: The relative formula mass of the desired product divided by the total relative formula mass of all the reactants, multiplied by one hundred.AQA, OCR
Atom economy and by-products: Measures how much of the reactant mass ends up in the desired product, using the molar mass of the desired product divided by the total molar mass of all…OCR
atomic number: The number of protons in the nucleus, and it alone fixes which element you have.AQA, OCR
Atomic Number (Z): The number of protons in the nucleus of an atom.AQA
Atomic number and first point of difference: If the directly attached atoms are identical, comparison moves outward along the chain, comparing the sets of atoms attached at the first point of difference.OCR
atomic number comparison: Comparing the atom directly attached to each doubly bonded carbon by atomic number, where the higher atomic number wins.AQA
Atomic number equals electron count in neutral atoms: For a neutral atom, the atomic number Z equals the number of protons and therefore the number of electrons.OCR
Atomic orbital: A region around the nucleus where there is a high probability of finding an electron.OCR
Atomic orbitals: A sub-shell contains one or more of these, each holding up to two electrons of opposite spin, with s having one, p three, d five and f seven.OCR
atomic radius: Falls steadily across Na to Ar because the outer electrons are pulled in harder as nuclear charge rises with similar shielding.AQA, OCR
Atomic radius and shielding down Group 2: Going down the group, atomic radius increases and the outer electrons sit in a higher principal shell, so they are further from the nucleus and more shielded.OCR
atomic radius down a group: Each element adds a complete electron shell, so the atomic radius increases down the group.AQA
attack at a δ⁺ carbon: Behaving as a nucleophile when offered a carbon carrying a partial positive charge, such as in a halogenoalkane or an acyl chloride.AQA
attack by electrophiles: The double bond attracts electrophiles, species that accept a pair of electrons.AQA
attack from either side: A reagent can approach a planar species from above or from below with equal probability, so the two enantiomers are produced in equal amounts.AQA
attraction to negative surfaces: Wet hair and wet fabric fibres carry a slight negative charge on their surface, so the cations are attracted head-first onto the surface.AQA
Aufbau filling order of sub-shells: Sub-shells are filled in order of increasing energy: 1s, then 2s, 2p, 3s, 3p, 4s, then 3d and 4p.OCR
autocatalysis: An autocatalyst is a product of the reaction that then catalyses it, so the rate rises as the reaction proceeds.AQA
availability of the lone pair: How available the lone pair on the nitrogen atom is to a proton, which is what base strength comes down to.AQA
Average bond enthalpies: Mean values for a particular bond type across a range of compounds, usually quoted in kJ mol⁻¹ for gaseous species.OCR
Average bond enthalpy: The enthalpy change when one mole of a specified covalent bond is broken in gaseous molecules, averaged over a range of gaseous compounds containing that bond.OCR
averaged data: Tabulated figures are averages, not exact values for specific molecules, so the answer often disagrees with a Hess's law value.AQA
averaged over compounds: The tabulated value is a compromise over a range of different compounds containing that bond, so it fits neither exactly.AQA
Averaging over gaseous compounds: The same bond has slightly different strengths in different molecules, so a single averaged value is used for calculations.OCR
Averaging over similar compounds and its limitation: Because the value is an average, it measures covalent bond strength only approximately, as the same bond has slightly different enthalpies in different…OCR
Avogadro constant: The number of particles in a mole, 6.022 × 10²³ mol⁻¹.AQA
Avogadro number: The number of particles contained in one mole, equal to 6.02 × 10²³.OCR
Axis labelling with quantities: Each axis is labelled with the quantity name or symbol, for example 'volume of gas' or 'time'.OCR
Axis labelling with units: Each axis label pairs the quantity with its unit, for example 'volume of gas / cm³' or 'time / s'.OCR
azo dyes: Strongly coloured compounds for which aromatic amines are the feedstock, the amine being converted to a diazonium salt below 10 °C and then coupled.AQA

B

Balanced chemical equations: Equations in which the correct formulae are written for every reactant and product and the coefficients are adjusted so that each element balances.OCR
balanced cracking equations: Equations must balance exactly, with carbon and hydrogen atoms tallying on both sides.AQA
Balanced equations: Equations are balanced by counting atoms on each side, especially hydrogen and oxygen, after predicting the products.OCR
Balanced equations and molar mass: The balanced equation identifies the desired product and is used to sum the molar masses of every reactant.OCR
Balanced equations and mole ratios: A balanced equation's coefficients give the mole ratio that links the substances being calculated.OCR
Balanced equations and state symbols: Equations written with correct formulae and state symbols, checking that atoms and charges balance on both sides.OCR
Balanced equations with state symbols: The balanced equation is written with state symbols, such as 2Mg(s) + O₂(g) → 2MgO(s).OCR
Balanced full equations: Equations written with state symbols, checked so that atoms and charge balance.OCR
Balanced ionic equations for precipitation: Equations such as Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) must balance both atoms and charge on each side, with all state symbols present.OCR
Balanced ionic equations with state symbols: Each reaction is written as an ionic equation with state symbols, for example Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s).OCR
Balanced symbol equations: Equations such as Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O and CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, balanced atom by atom with correct formulae.OCR
Balancing atoms and charge: Checking that the final equation balances both atoms and total charge on each side.OCR
balancing charge: The charge of a compound ion is balanced against the counter-ion so the finished formula sums to zero charge.AQA
Balancing complex half-equations: Balance atoms, then oxygen with water, then hydrogen with H⁺, then charge with electrons, giving 2Cr³⁺ + 7H₂O → Cr₂O₇²⁻ + 14H⁺ + 6e⁻.OCR
Balancing equations and state symbols: Equations must be balanced so the atoms match on both sides, with correct state symbols checked before moving on.OCR
Balancing organic combustion equations: Balance carbon as CO₂ first, then hydrogen as H₂O, then oxygen with O₂, adjusting the O₂ coefficient last.OCR
Balancing redox half-equations: Write the half-equation Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O and balance the organic half-equation before combining them.OCR
balancing with H₂O and H⁺: Balance oxygen by adding H₂O and balance hydrogen by adding H⁺.AQA
barium sulfate precipitate: Formed as a dense white precipitate by Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) when barium chloride is added to a solution containing sulfate ions.AQA
Barium sulfate precipitation: Adding dilute hydrochloric acid then barium chloride solution identifies sulfate, giving a white precipitate of barium sulfate.OCR
Base as proton acceptor: A species that accepts a proton, H⁺, from another species.OCR
base bond angles: The angle that stands when there are no lone pairs, set by the total electron pairs.AQA
Base hydrolysis of esters: Hydroxide ions attack the ester link irreversibly, giving the dicarboxylate salt and the diol, because the carboxylic acid formed is deprotonated to the…OCR
base peak: The tallest peak, which is usually a fragment and must not be used to obtain relative molecular mass.AQA, OCR
base sequence stores information: Everything that distinguishes one gene from another is the order of the bases hanging off the sugars, not the backbone.AQA
base to sugar bond: The base joins at carbon one through a nitrogen atom, and phosphate is never bonded straight to base.AQA
Base-10 logarithms: The logarithm in the pH expression is base 10, not logₑ, and for concentrations below 1 mol dm⁻³ it is negative.OCR
basic buffer: Uses a weak base and its conjugate acid, such as ammonia and ammonium chloride.AQA, OCR
Basic Oxide: An oxide that reacts with acids to form a salt and water.AQA
Basicity: The number of moles of H⁺ that one mole of an acid can donate in an acid–base reaction, a property of the acid's formula and structure rather than of its…OCR
Basicity of amines: Distinguishing an aromatic amine from an aliphatic amine when predicting relative base strength, since a phenylamine-type aromatic amine is far less basic…OCR
below 1500 cm⁻¹: Individual peaks there cannot reliably be assigned to single bonds, but the pattern as a whole is unique to one compound.AQA
Benefits versus risks: A strong answer states both sides and reaches a judgement, for example that the disease-prevention benefit outweighs the risks when chlorine is handled and…OCR
Bidentate and multidentate ligands: These create chelate rings that can wind in two mirror-image senses, giving optical isomers.OCR
bidentate ligands: Ligands with two donor atoms, each with a lone pair, so one bidentate ligand forms two coordinate bonds to the same metal ion and occupies two coordination…AQA, OCR
Bidentate ligands giving octahedral complexes: A bidentate ligand such as ethane-1,2-diamine can give six-fold coordination with three bidentate ligands, as in [Cr(en)₃]³⁺, where the octahedral arrangement…OCR
Bifunctional nature of amino acids: An amino acid carries both an amine group and a carboxylic acid group on the same molecule, so it shows the reactions of each functional group and some…OCR
biodegradability: Micro-organisms use enzymes to do the same chemistry slowly in soil and water, which is what makes these polymers biodegradable.AQA
Biodegradable and renewable alternatives: Alternatives include biodegradable polymers such as poly(lactic acid) made from renewable plant material, and polymers designed to degrade.OCR
Biodegradable polymers: Polymers that are broken down by microorganisms, so they decompose in soil and reduce the volume of persistent plastic waste.OCR
Biofuel: A fuel derived from renewable biological sources.AQA
biofuel from biomass: A fuel made from biomass, that is plant material grown recently rather than from fossil carbon.AQA
blank: A blank of pure solvent used to zero the colorimeter so that the cell and the water contribute nothing.AQA
bleach and disinfectant: The uses of the solution formed, because the chlorate(I) ion is a strong oxidising agent that destroys coloured compounds and kills bacteria.AQA
blocked active sites: An impurity binds to the active sites so strongly that it never desorbs, so those sites are permanently out of use.AQA
blood plasma buffering: Blood plasma is held between pH 7.35 and 7.45 by the carbonic acid and hydrogencarbonate pair, fed by dissolved carbon dioxide.AQA
Boiling points compared with alkanes: Hydrogen bonding between alcohol molecules raises boiling points above those of comparable alkanes.OCR
Boltzmann distribution: A qualitative explanation of how molecular kinetic energies are spread at a given temperature, peaking at a moderate energy and tailing off at high energy.OCR
bond angle of 109.5°: The angle found in a tetrahedral complex, against 90° in an octahedral one.AQA
Bond angle of about 109.5°: Electron pair repulsion pushes the four bonds towards the corners of a tetrahedron, giving a bond angle of about 109.5°.OCR
Bond angle of about 120°: The trigonal planar arrangement around each carbon of the C=C gives a bond angle of about 120°.OCR
Bond angles: The characteristic angle for each shape, such as 180° for linear, 120° for trigonal planar, 109.5° for tetrahedral, 107° for pyramidal, 104.5° for non-linear…OCR
Bond angles and shape prediction: Count the donor atoms bonded to the metal, decide whether they are coplanar or three-dimensional, and state the shape and angle together.OCR
Bond angles in octahedral complexes: The angle between adjacent ligands is 90°.OCR
bond breaking and bond formation: Breaking bonds always requires energy input, while forming new σ-bonds releases energy, and the overall change is exothermic when the energy released exceeds…OCR
Bond breaking and bond making: Bonds broken in the reactants and bonds made in the products are summed separately, and the relationship ΔH = Σ(bonds broken) − Σ(bonds made) is applied.OCR
bond breaking arrows: The breaking of a covalent bond is shown by a curly arrow starting from the bond.AQA
bond breaking is endothermic: Breaking bonds takes energy in, and if bond breaking dominates, energy is drawn from the surroundings and the mixture cools.AQA
bond dipole: A polar bond has a separation of charge with a direction.AQA
Bond dipoles and partial charges: A polar bond carries partial charges marked with δ⁺ and δ⁻, with the more electronegative atom chosen for δ⁻.OCR
Bond dipoles and vector cancellation: Each bond dipole is a vector, and where bond dipoles do not cancel the molecule has a permanent dipole.OCR
Bond enthalpy: The enthalpy change when one mole of a specified covalent bond is broken in gaseous molecules, so it is always endothermic and positive.AQA, OCR
bond enthalpy balance: The C=O bonds in carbon dioxide and O–H bonds in water are stronger overall than the C–C and C–H bonds broken in the fuel together with the O=O bonds broken…AQA
bond length and strength: More shared electrons mean a stronger attraction to both nuclei, so multiple bonds are shorter and stronger than single bonds between the same two atoms.AQA, OCR
Bond lengths in benzene: All six carbon–carbon bonds are equal in length and intermediate between typical single and double bond lengths, supporting delocalisation.OCR
bond making is exothermic: Making bonds gives energy out, and if more energy is released forming the new bonds than was needed to break the old ones, the system loses energy to the…AQA
bond polarity: The shared pair in a carbon–halogen bond is pulled towards the halogen, so the bond is polar.AQA
Bond polarity and lone pairs: The polar O–H bonds leave hydrogen partially positive and oxygen partially negative, so lone pairs attract neighbouring hydrogen atoms.OCR
Bond polarity versus molecular polarity: Bond polarity is a property of a bond, whereas whether a whole molecule is polar also depends on its shape and on whether the individual bond dipoles cancel.OCR
Bond strength and reactivity down Group 7: The rate of hydrolysis increases down Group 7 because the carbon–halogen bond becomes weaker and easier to break, so iodoalkanes hydrolyse fastest and…OCR
Bond stretching and bending: In a covalent bond the atoms behave as if joined by a spring, so the bond can stretch (symmetric and asymmetric) or bend.OCR
bond vibration: A covalent bond behaves like two masses joined by a spring, vibrating at a frequency fixed by the strength of the bond and the masses of the atoms it joins.AQA
Bond vibration and absorption of IR radiation: Infrared spectroscopy identifies bonds by the frequencies at which they absorb IR radiation, because each bond vibrates at characteristic wavenumbers.OCR
bonding pairs: Pairs of electrons shared with another atom.AQA
bonds broken minus bonds made: The expression used to calculate an approximate ∆H, taking the total bonds broken minus the total bonds made.AQA
Born–Haber cycle: A cycle used to calculate lattice enthalpies indirectly from values that can be measured, running from the elements in their standard states to the compound…AQA, OCR
Boundary cases scandium and zinc: Scandium does not qualify because Sc³⁺ is [Ar] 3d⁰, an empty d-sub-shell, and zinc does not qualify because Zn²⁺ is [Ar] 3d¹⁰, a complete d-sub-shell.OCR
brackets in formulae: Used when more than one compound ion is needed, with the whole ion bracketed before the subscript.AQA
branched against straight chains: The first poly(ethene) had chains that branched and packed badly, while later chains were straight and packed closely.AQA
Branched chains: One way in which the carbon and hydrogen of an aliphatic compound are joined together.OCR
branching and contact area: Where the same force operates in both substances, compare the surface area in contact, so a longer unbranched chain boils higher than a shorter or more…AQA
breaking C–C bonds: Cracking involves breaking these bonds in alkanes, rather than C–H bonds.AQA
Broad O–H absorption peak: A broad absorption near 2500–3300 cm⁻¹ from the O–H bond of the carboxyl group, broadened because of hydrogen bonding.OCR
broad O–H shapes: A broad absorption between about 2500 and 3000 cm⁻¹ alongside the C=O means a carboxylic acid, while between about 3230 and 3550 cm⁻¹ with no C=O means an…AQA
Broad peak shape caused by hydrogen bonding: The absorption is broad because hydrogen bonding varies the O–H bond strength across many molecules, spreading the absorption over a range of wavenumbers.OCR
Bromine water and alkenes: Alkenes decolourise bromine water from orange to colourless, which is linked to the presence of a carbon–carbon double bond.OCR
bromine water and dichromate: An alkene decolourises orange bromine water at room temperature, and a primary or secondary alcohol warmed with acidified potassium dichromate(VI) turns it…AQA
bromine water test: Benzene does not decolourise bromine water while an alkene does, because the ring cannot polarise a bromine molecule on its own.AQA
Bromine water test for alkenes: Adding bromine water to a small sample and observing whether the orange-brown colour is decolourised, which indicates a carbon-carbon double bond.OCR
Bromine water test for unsaturation: Bromine water is added to the sample and an alkene decolourises the orange-brown bromine water because the bromine reacts with the C=C bond.OCR
Brønsted–Lowry acid: A proton donor, where a proton means H⁺, a hydrogen atom that has lost its only electron.AQA
Brønsted–Lowry base: The amine group behaves as this because the lone pair on the nitrogen accepts a proton from an acid.AQA, OCR
Brønsted–Lowry basicity of amines: An amine behaves as a Brønsted–Lowry base because the nitrogen atom has a lone pair that accepts a proton.OCR
Brønsted–Lowry proton transfer: Classifies a species by what it does with a proton, H⁺, which is a hydrogen atom that has lost its electron.OCR
Buffer: A solution that resists pH change when small amounts of acid or base are added.OCR
Buffer action against added acid and alkali: Added H⁺ ions are removed by the CH₃COO⁻ ions, and added OH⁻ ions are removed by the CH₃COOH molecules, so the pH changes only slightly.OCR
Buffer action and pH resistance: A buffer resists change in pH when small amounts of acid or alkali are added, because it contains a reservoir of both a weak acid and its conjugate base.OCR
Buffer calculation: Uses [H⁺] = Ka × [HA]/[A⁻], then pH = −log₁₀[H⁺], to calculate the pH of a buffer.OCR
buffer definition: A solution that maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.AQA
buffer pH calculation: Substituting into [H⁺] = Ka × [HA]/[A⁻], in which the volumes cancel, then pH = −log₁₀[H⁺] quoted to two decimal places.AQA
buffer region: A flat region shown by a weak acid with a strong base, drawn as a shallow rise rather than a flat horizontal line.AQA
Buffer regions: A gradual early change shown in the curve for a weak acid with a strong base.OCR
Buffer solutions and the conjugate acid–base pair: A buffer resists pH change because a weak acid HA and its conjugate base A⁻ are both present in significant amount.OCR
building peptides: Join the -COOH of one residue to the -NH₂ of the next, remove a water molecule at each junction and keep the residues in the stated order.AQA
Burette technique and concordant titres: Both titrations rely on accurate burette readings and concordant titres.OCR
by-product handling: The anhydride releases ethanoic acid, a weak acid that can be recovered and reused, while the acyl chloride releases hydrogen chloride, a corrosive and toxic…AQA
By-products: Unwanted products; fewer of them reduce waste and the need for separation or disposal.OCR
By-products sulfur dioxide and hydrogen chloride: SO₂ and HCl are gaseous by-products that leave the reaction mixture, which helps drive the reaction and gives a purer product.OCR

C

C–Cl weaker than C–F: The carbon–chlorine bond has the lower bond enthalpy, so it breaks in preference to the carbon–fluorine bond.AQA
C=C bond reactivity: The C=C bond is the reactive site that lets alkenes be converted into plastics and other organic chemicals.OCR
C=O absorption peak: A strong sharp absorption near 1680–1750 cm⁻¹ that is one of the two diagnostic infrared features of a carboxylic acid.OCR
C=O near 1700 cm⁻¹: A strong sharp absorption near 1700 cm⁻¹, whose presence means the compound could be an aldehyde, ketone, carboxylic acid, ester, amide, or acyl chloride.AQA
Cahn–Ingold–Prelog priority by atomic number: The group with the higher atomic number directly attached to the double-bond carbon takes priority.OCR
Cahn–Ingold–Prelog priority rules: Rules that rank the two groups on each carbon of the C=C group by comparing the atoms directly attached and giving higher priority to the atom of greater…OCR
Calcium hydroxide as a base: Calcium hydroxide is a base, so it reacts with the acid rather than adding to it.OCR
calculating ∆S from absolute entropies: Subtract the sum of the reactant entropies from the sum of the product entropies, each multiplied by its balancing number.AQA
Calculating ∆S of the system from products and reactants: Applying ∆S = ΣS(products) − ΣS(reactants) using the standard entropies supplied, keeping products minus reactants.OCR
Calculating an unknown value from known contributions: To find an unknown, multiply each known value by its subscript, subtract from the overall charge and divide by the number of unknown atoms.OCR
Calculating and interpreting the titre: The titre is the difference between the final and initial burette readings, and concordant titres from repeat runs improve the reliability of the result.OCR
Calculating enthalpy changes: Summing the average bond enthalpies of all bonds broken in the reactants and subtracting the sum of the average bond enthalpies of all bonds made in the…OCR
Calculating Kc from equilibrium concentrations: Substitute the supplied equilibrium concentrations into the expression, keeping units consistent in mol dm⁻³, then evaluate and derive the units of Kc from…OCR
calculating molar mass: Adding the relative atomic masses of all atoms in the formula gives the molar mass in g mol⁻¹.OCR
Calculating oxidation state from a formula: Working out the oxidation state from the formula using the usual rules, where oxygen is usually 2⁻, hydrogen 1⁺, and the sum of oxidation states equals the…OCR
Calculating rate and concentration from the rate equation: Related quantities such as rate or a concentration can be found by substituting known values into the rearranged equation.OCR
Calculation accuracy: Keeping the answer between the lightest and heaviest isotope masses as a check on reasonableness, and stating it to a sensible number of significant figures.OCR
Calibration against standards: A calibration or comparison with standards can convert a detector response into an amount.OCR
calibration curve: A plot of absorbance against concentration, made from standard solutions of known concentration, which should be a straight line through the origin.AQA
Calibration with buffer solutions: Calibrating the meter with at least two buffer solutions of known pH that bracket the expected value, rinsing the electrode with distilled water between…OCR
Calorimetry and q = mcΔT: A known mass of water is heated or cooled by a reaction, and q = mcΔT gives the heat, with c the specific heat capacity of water (4.18 J g⁻¹ K⁻¹).OCR
cancellation: If the polar bonds point symmetrically outwards, their dipoles cancel and the molecule is non-polar overall despite its polar bonds.AQA
cancelling catalyst and intermediate: Removing the chlorine radical and ClO• because each appears on both sides of the added equations, leaving 2O₃ → 3O₂.AQA
cancelling dipoles: In a symmetrical arrangement the bond dipoles are equal in size and point outwards in directions that exactly oppose one another, so the vector sum is zero.AQA
candidate formulae: Formulae with the right whole-number mass that are listed and then added up using the precise masses.AQA
carbocation intermediate: The C–O bond breaks heterolytically, the arrow running from that bond to the oxygen, so a water molecule leaves and a carbocation remains.AQA
Carbocation intermediates and stability: A positively charged carbon formed after the π electrons of the C=C bond attack the δ⁺ hydrogen of the polar H–X molecule, and the more stable carbocation…OCR
carbocation stability: A tertiary carbocation is more stable than a secondary one, which is more stable than a primary one, because the charge is spread.AQA
Carbocation stability and major products: Applied to predict the major alcohol for an unsymmetrical alkene such as propene giving mainly propan-2-ol.OCR
Carbon chain extension and carbon counting: The nitrile route adds exactly one carbon, whereas Friedel–Crafts adds the whole acyl group, so carbons must be counted carefully.OCR
carbon chain length: Nitrile reduction maintains the carbon count, as the chain lengthening by one carbon occurs in the prior step when forming the nitrile from a halogenoalkane.AQA
Carbon dioxide test: The observable sign is effervescence as carbon dioxide is released; the gas turns limewater milky, confirming CO₂.OCR
carbon dioxide trade-off: The carbonate route has an obvious drawback, releasing a mole of carbon dioxide for every mole of sulfur dioxide removed.AQA
Carbon environment: The type of carbon present, identified from chemical shift values by matching each observed shift to the typical ranges.OCR
carbon environments: Counted by ¹³C NMR, using the number of peaks in the spectrum.AQA
carbon monoxide and soot: Products of incomplete combustion that are hazardous, since carbon monoxide is a toxic ligand for haemoglobin and soot aggravates respiratory disease.AQA
Carbon neutral fuel: A fuel where the carbon dioxide absorbed equals the carbon dioxide released, as shown by the photosynthesis, fermentation and combustion equations.AQA
Carbon skeleton: The chain of carbon atoms in a skeletal formula, where each vertex and each end of a line is a carbon atom.OCR
Carbon skeleton retention: The carbon skeleton is unchanged, so the number of carbon atoms in the acid equals the number in the nitrile.OCR
carbon-12 scale: The scale on which the mass of an atom of carbon-12 is defined as exactly 12.AQA
Carbon-13 NMR and molecular symmetry: Used to confirm the number of distinct carbon environments in the chosen structure, checking for symmetry so equivalent carbons are not counted as separate…OCR
Carbon-13 NMR prediction: Identifying each distinct carbon environment, remembering that symmetry makes some carbons equivalent, and predicting the number of signals.OCR
Carbon-13 NMR spectroscopy: A technique that tells you how many distinct carbon environments a molecule contains, where each set of equivalent carbons gives one signal.OCR
carbon-13 satellite: A small peak one mass unit higher than the molecular ion, coming from the roughly one per cent of carbon present as carbon-13.AQA
carbon-carbon bond formation: What Friedel-Crafts acylation does by attaching an acyl group to the ring, which is what a synthesis usually needs.AQA
Carbon-Neutral: A process where the net carbon dioxide emissions to the atmosphere are zero.AQA
Carbon-skeleton conservation: The carbon skeleton and carbon number are checked at each stage to confirm they are preserved or deliberately changed.OCR
Carbon–carbon bond formation: The key step that lengthens a carbon chain during synthesis, joining two fragments so the product has more carbon atoms than the starting material.OCR
Carbon–carbon bond formation and chain lengthening: Forming the nitrile adds one carbon to the chain, so the sequence builds a longer carbon skeleton.OCR
Carbon–carbon bond formation in synthesis: A new C–C bond joins the substituent to the aromatic ring, so the reaction extends a carbon skeleton, which is why it matters in synthesis.OCR
carbon–carbon bond too strong: With no hydrogen available, the only route to a carbonyl would be to break a strong carbon–carbon bond, which this reagent cannot do.AQA
carbon–carbon double bond: The feature that makes a hydrocarbon unsaturated, allowing it to add extra atoms without losing anything.AQA
Carbon–halogen bond enthalpy: Bond enthalpies fall down the group from C–F, the strongest at roughly 467 kJ mol⁻¹, through C–Cl and C–Br to C–I, the weakest at roughly 238 kJ mol⁻¹.OCR
Carbon–halogen bond strength and reactivity: The rate of precipitate formation reflects the carbon–halogen bond strength, so iodoalkanes react fastest and chloroalkanes slowest.OCR
carbonate ion: The carbonate ion is CO₃²⁻, a negative ion whose charge must be included when writing its formula.OCR
Carbonate reactions releasing carbon dioxide: With a carbonate such as calcium carbonate, the acid also releases carbon dioxide.OCR
Carbonate test: Carbonate ions are identified by reaction with H⁺(aq), forming CO₂(g).OCR
carbonate test for acids: A carboxylic acid added to sodium carbonate or hydrogencarbonate solution fizzes, and the gas turns limewater milky.AQA
carbonic acid and hydrogencarbonate: The pair H₂CO₃ ⇌ H⁺ + HCO₃⁻ holds blood plasma in its narrow pH range, and HCO₃⁻ absorbs lactic acid produced during exercise.AQA
Carbonyl compounds: aldehydes and ketones: Both give the coloured precipitate with 2,4-DNP, so the test confirms a carbonyl compound but does not distinguish an aldehyde from a ketone.OCR
Carbonyl electrophilicity: The acid protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic so water can attack.OCR
Carbonyl group confirmation: The formation of a yellow-orange precipitate with 2,4-dinitrophenylhydrazine confirms the presence of a carbonyl compound.OCR
Carbonyl group detection: A carbonyl group is detected chemically by 2,4-dinitrophenylhydrazine, which confirms a carbonyl is present but does not by itself distinguish aldehyde from…OCR
Carbonyl group distinction: Separating an aldehyde from a ketone by checking what is bonded to the carbonyl carbon, since a hydrogen attached means an aldehyde and two carbon groups mean…OCR
Carbonyl group structure: Both aldehydes and ketones contain the carbonyl group, C=O, but only the aldehyde has a hydrogen atom attached to the carbonyl carbon.OCR
Carboxyl functional group: A carbonyl group joined to a hydroxyl group on the same carbon, written COOH, present in carboxylic acids.OCR
carboxyl group: A group in which one carbon holds both a C=O and an O-H, so the two oxygen atoms sit on the same carbon and are never bonded to each other.AQA, OCR
Carboxylate ion stability: The carboxylate ion is stabilised because the negative charge is delocalised over the two oxygen atoms.OCR
Carboxylate salt formation: Neutralisation of the COOH group by an alkali produces a carboxylate salt and water, as in the balanced equation with sodium hydroxide.OCR
Carboxylate salts: The product obtained rather than the free acid, because the alkaline conditions deprotonate the acid formed.OCR
Carboxylic acid formation: The carboxylic acid is the organic product of the hydrolysis, for example heating ethanenitrile with dilute acid gives ethanoic acid and ammonium ions.OCR
Carboxylic acid identification: Carboxylic acids can be identified by adding a carbonate, such as sodium carbonate solution or solid calcium carbonate.OCR
carboxylic acid product: The product of oxidising an aldehyde, shown as RCOOH in the equation RCHO + [O] → RCOOH.AQA
Carboxylic acid to acyl chloride conversion: Carboxylic acids react with thionyl chloride to give an acyl chloride, with the acid's –OH replaced by –Cl.OCR
Care and storage of the pH electrode: Rinsing the electrode with distilled water after use and storing it in the recommended solution, keeping the bulb wet and handling it carefully.OCR
carrier gas under pressure: An unreactive gas pushed through the column under pressure while the column sits in a hot oven.AQA
Catalysis and activation energy: A catalyst raises the rate of a reaction by providing an alternative route of lower activation energy, and it is not consumed overall.OCR
Catalysis by concentrated sulfuric acid: A small amount of concentrated sulfuric acid is used, protonating the carbonyl group to make it more susceptible to attack by the alcohol.OCR
catalyst: Provides an alternative route with a lower activation energy and is chemically unchanged at the end of the reaction.AQA
Catalyst examples: Acid catalysis in aqueous solution, where hydrogen ions in solution catalyse a reaction between species also in solution, is an example of homogeneous…OCR
catalyst poisoning: Impurities block the active sites so the catalyst has reduced efficiency, which has a cost implication.AQA
Catalyst poisoning and industrial application: A catalyst may be poisoned by impurities that block active sites, which reduces its effectiveness.OCR
Catalyst regenerated and not consumed: The catalyst takes part in one step and is regenerated in a later step, so its mass and chemical identity are restored at the end.OCR
Catalyst regenerated in the mechanism: The catalyst takes part in one step of the mechanism and is regenerated in a later step, so its mass and identity are restored.OCR
Catalyst regenerated, not consumed: A catalyst is not used up overall and is regenerated, so a small amount can catalyse a large amount of reaction.OCR
catalyst regeneration: The chlorine radical is consumed and then regenerated, so it does not appear in the overall equation and a tiny amount keeps working.AQA
catalysts in the rate equation: A catalyst can appear in the rate equation while being absent from the overall equation.AQA
Catalytic activity: Effective catalysis that comes from the ability to adopt variable oxidation states plus adsorption onto surfaces.AQA
Catalytic behaviour of transition metals and their compounds: Transition metals and their compounds act as catalysts because they can change oxidation state and provide a surface or intermediate species that lowers the…OCR
Catalytic breakdown of ozone: The breakdown of ozone by Cl• and other radicals, in which the radical is regenerated so it acts as a catalyst.OCR
catalytic converters: Devices using a ceramic honeycomb coated with platinum, palladium and rhodium, giving an enormous area over which the gases can meet the metal.AQA
catalytic cycle: A single chlorine radical destroys ozone over and over, being regenerated in the second step so that it acts as a catalyst rather than a reactant.AQA, OCR
cation in acid: In excess acid the carboxylate ion is protonated back to COOH while the nitrogen keeps its extra proton, giving a single positive charge overall.AQA
Cation tests with sodium hydroxide: Adding dilute sodium hydroxide solution and recording the precipitate colour and its behaviour in excess identifies cations such as Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺.OCR
cationic surfactant: An ionic head that is attracted to water and a non-polar tail that is not, made by giving one of the four groups a long hydrocarbon chain.AQA
Cations and delocalised electrons: Each metal atom loses its outer electrons to become a positive ion, and those released electrons are shared across the whole lattice rather than belonging to…OCR
Cations lose electrons, anions gain electrons: A positive ion has lost electrons, so subtract the charge from the electron count; a negative ion has gained electrons, so add the magnitude of the charge.OCR
CDCl₃: An example of a deuterated solvent that dissolves a wide range of organic compounds and is commercially available.OCR
cell diagrams: A shorthand for a whole cell, in which a single vertical line marks a boundary between two phases and a double vertical line marks the salt bridge.AQA
cell polarity: The electrode with the more negative potential is the negative terminal, where oxidation releases electrons into the external circuit.AQA
CFC chemistry: For a CFC such as CF₂Cl₂, the C–Cl bond is the one broken, giving two radicals shown with dot notation.OCR
chain isomers: Isomers that differ in the carbon skeleton, such as one straight and one branched.AQA
chain lengthened by one carbon: The product has one more carbon than the starting material, which makes the reaction useful for lengthening a chain.AQA
Chain lengthening in synthesis: Increasing the length of a carbon chain by forming a new carbon–carbon bond, confirmed by counting carbons before and after.OCR
chain of reasoning: A sequence of linked steps that ends at a consequence, for the cell or for the patient, rather than a single fact.AQA
chains slide: What the chains can then do past one another, making the same polymer flexible.AQA
Change in dipole moment: A bond must change its dipole during the vibration to absorb infrared strongly, so symmetric stretches of symmetrical molecules absorb weakly.OCR
Changes to energy usage: Because the effect is linked to energy use, reducing reliance on fossil fuels and improving efficiency are presented as responses that limit further emissions.OCR
characteristic absorption bands: Bands in an infrared spectrum whose position and shape are read and then linked to bonds present in the structure, such as O–H or C=O.OCR
Characteristic absorptions of common organic bonds: A broad absorption centred near 3300 cm⁻¹ points to an O–H bond, while a sharp strong peak near 1700 cm⁻¹ points to a C=O bond.OCR
Characteristic bond absorptions in IR: Bonds absorb IR radiation at characteristic wavenumbers because each bond vibrates at particular frequencies, measured in cm⁻¹.OCR
Characteristic infrared absorption of gases: Each gas absorbs infrared radiation at a characteristic wavenumber, allowing it to be identified in a mixture.OCR
Characteristic reactions: The reactions by which negatively charged ions in aqueous solution are identified, each anion being confirmed by a distinct observation.OCR
Characteristic retention time under fixed conditions: Different components have different retention times under fixed conditions, so retention time is used for qualitative identification.OCR
Characteristic shifts of proton environments: Each type of hydrogen absorbs at a characteristic shift, such as alkyl CH₃ hydrogens around δ 0.9–1.7 and an aldehyde hydrogen near δ 9–10.OCR
characteristic wavenumbers: Because each type of bond has its own natural frequency, absorptions appear at these values, which you look up on the data sheet rather than recalling.AQA
charge and electron number: Electrons equal protons minus the charge, so subtracting the signed charge handles both a 2⁺ ion that has lost two electrons and a 1⁻ ion that has gained one.AQA
charge and radius: Lattice strength rises as the charge on the ions rises and as the ionic radii fall, because the attraction depends on charge density.AQA
Charge and radius effects on ∆hydH: A larger charge and a smaller ionic radius give a more exothermic ∆hydH, so Mg²⁺ releases more energy than Na⁺.OCR
charge balance: Electrons are added until the charge on each side is equal, and a half-equation must balance for atoms and for total charge.AQA, OCR
Charge balance for neutral compounds: Choosing the ratio of atoms whose total oxidation numbers sum to zero for a neutral species.OCR
Charge balance for polyatomic ions: Choosing the ratio of atoms whose total oxidation numbers sum to the ion charge for a polyatomic ion.OCR
charge clouds: Regions of negative charge that each pair of outer-shell electrons around the central atom is treated as.AQA
Charge density / charge-to-size ratio: A measure of charge concentrated in an ion, increasing with ionic charge and decreasing with ionic radius.AQA
charged intermediate protonated: The negatively charged intermediate has an arrow from its lone pair to H⁺, and omitting this final protonation step leaves a charged product.AQA
charged phosphate groups: After forming two ester links a phosphate keeps one ionisable OH, which at the pH inside a cell loses its proton, so the backbone is a chain of negative…AQA
Charges on common ions and polyatomic ions: The correct charge is applied to each ion, including group numbers and common polyatomic ions, before combining them.OCR
charges on ions: An ion needs its charge written outside a square bracket enclosing the whole species.AQA
charges to learn: The overall charge carried by each compound ion, memorised with the ion as a single list.AQA
chelate effect: The name for the strong tendency of bidentate and multidentate ligands to replace monodentate ligands from complexes, so the substitution runs almost to…AQA
Chemical equivalence and symmetry: Carbons related by a mirror plane or a rotation axis are equivalent and share a signal, so symmetry must be checked before deciding.OCR
Chemical shift: Measured in parts per million relative to tetramethylsilane at 0 ppm, it indicates the type of carbon environment.OCR
Chemical shift and environment identification: The chemical shift, δ in ppm, indicates the environment, and values from a chemical-shift data table are used to assign environments.OCR
Chemical shift ranges: Ranges used to place signals in the correct region for the functional groups present.OCR
Chemical shift scale: Chemical shift is measured relative to a reference so that values from different machines and samples can be compared, with the TMS signal set at δ = 0 ppm.OCR
Chemical shift scale in ppm: Measured in parts per million on the δ scale, it tells you what kind of proton environment is present because each type of hydrogen absorbs at a…OCR
chiral active site: A site with a definite handedness because it is built from amino acids, which are themselves chiral.AQA
Chiral centre: An sp³ carbon bonded to four different groups, so the molecule and its mirror image cannot be superimposed.OCR
Chiral centre / asymmetric carbon atom: A carbon atom bonded to four different groups or atoms.AQA
chiral molecules in an inactive mixture: The molecules themselves are still chiral and the mixture still contains optical isomers; it is only the bulk sample that shows no rotation.AQA
chirality: The feature that produces optical isomers, a chiral centre being typically a carbon atom bonded to four different groups, so that reflection cannot be rotated…AQA, OCR
Chirality and optical isomerism: When the α-carbon has four different groups, the molecule is chiral and can exist as two optical isomers.OCR
Chirality at the α-carbon: A carbon bearing four different groups gives chirality, which matters for the α-carbon of most amino acids.OCR
chlorate(I) ion: The species ClO⁻, in which chlorine is at oxidation state +1, formed along with chloride ions when chlorine reacts with water.AQA
Chloric(I) acid as a disinfectant: HClO is the active disinfectant that oxidises and destroys bacteria and other microorganisms, rather than Cl⁻.OCR
chloric(I) acid as the active species: Chlorine is not itself the direct agent; it reacts with water and the chloric(I) acid or chlorate(I) ion oxidises and destroys the organisms.AQA
chlorination: Chlorine prevents cholera and typhoid, but chlorine is toxic and can react with organic matter to form trace chlorinated compounds suspected of being…AQA
Chlorine in water treatment: Chlorine kills bacteria in drinking water and so reduces the spread of waterborne disease.OCR
chlorine radical: A chlorine atom with an unpaired electron, written Cl•, not a chloride ion.AQA
Chlorine radicals from CFCs: Cl• formed when UV light breaks carbon–chlorine bonds in chloroalkanes such as CFCs.OCR
chlorine-free replacements: Alternative compounds developed by chemists that contain no carbon–chlorine bond, so ultraviolet light cannot release a chlorine radical and no catalytic…AQA
chlorofluorocarbons: Compounds whose C–Cl bonds are broken by ultraviolet radiation in the upper atmosphere.AQA
choice of acid: Nitric acid removes interfering ions without adding an interfering ion, whereas hydrochloric acid would add chloride and sulfuric acid would precipitate…AQA
Choice of scale: A scale is chosen in convenient steps so the data spread across most of the grid, using a sensible step such as 1, 2, 5 or 10 units per major square.OCR
Choice of solvent by solubility: A solvent is chosen in which the solid is appreciably more soluble when hot than when cold.OCR
Chromatogram is a detector response against time: A gas chromatogram plots detector response against time for the components of a vaporised mixture carried through a column by a mobile gas phase.OCR
chromium +6 to +3: The change in oxidation state identified when the dichromate ion is reduced to the Cr³⁺ ion.AQA
chromium and copper anomalies: Their configurations break the filling pattern, chromium as [Ar]3d⁵4s¹ and copper as [Ar]3d¹⁰4s¹, because a half-full or full 3d sub-level is lower in energy.AQA
CIP priority rules: Rules applied to assign priorities to the groups on each doubly bonded carbon so that an alkene can be labelled E or Z.AQA, OCR
cis-platin: A square-planar platinum(II) complex with the formula [Pt(NH₃)₂Cl₂], in which the two chloride ligands sit adjacent to each other rather than opposite.OCR
cis-trans isomerism: A type of isomerism shown by square planar complexes in which two of the four ligands differ, giving a cis form with them 90° apart and a trans form with them…AQA
cis–trans isomerism: A special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same.OCR
cisplatin structure: A square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent.AQA
Classifying a structure: Inspecting every carbon–carbon link and asking whether any bond order exceeds one.OCR
ClO• intermediate: It is made and used up inside the cycle, so it appears in neither the overall equation nor the catalyst role.AQA
Clock reaction: A common example of an initial-rate method in which reactants are mixed and the time taken for a visible change is measured.OCR
closed system: A closed system is required so that no substance enters or leaves, since anything that escapes cannot come back.AQA, OCR
closed system at equilibrium: The principle applies only to a system already at equilibrium in a closed container.AQA
Closer packing in liquid water: When ice melts, some hydrogen bonds break and the lattice collapses, allowing molecules to pack more closely.OCR
Co-ordinate bond: A covalent bond in which both electrons in the shared pair are donated by the same atom or ion.AQA
co-ordinate bonding: A bond in which both electrons come from the ligand, drawn as an arrow pointing from the donor atom to the metal.AQA
co-ordination number: The number of co-ordinate bonds to the central metal atom or ion.AQA
co-ordination number 4: The number of ligands attached to the metal ion when larger ligands such as chloride are present, falling from 6.AQA
cold water versus steam: Magnesium reacts only very slowly with cold water to give the hydroxide, but heated in steam it burns with a bright white flame to give the oxide instead.AQA
collecting a boiling range: Collect only the fraction distilling over the expected boiling range.AQA
Collecting data over time to determine rate: Producing a series of readings from which a rate can be found, for example plotting gas volume against time and taking the gradient.OCR
collision frequency: Reacting particles brought closer together collide more often, and a greater collision frequency scales up successful collisions, increasing the rate.AQA, OCR
Collision frequency and rate: A larger proportion of molecules with energy at or above the activation energy gives more frequent successful collisions, so the rate increases.OCR
collision theory: Reactions can only occur when collisions take place between particles having sufficient energy.AQA
Colorimetry: A technique that exploits the absorption of light by a coloured species, where absorbance is proportional to concentration, so tracking absorbance over time…OCR
Colour as an identification test: The colour of the precipitate, and whether it dissolves in excess reagent, helps identify the metal ion.OCR
Colour change from orange to green: The dichromate ion is reduced, changing the solution from orange to green as chromium goes from +6 to +3.OCR
Colour change of acidified dichromate(VI): Acidified dichromate(VI) is an orange oxidising agent that is reduced to green chromium(III) ions when it oxidises a primary or secondary alcohol or an…OCR
Colour change range: Roughly pKIn ± 1, and it must fall entirely within the steep section of the pH curve for the indicator to be usable.AQA
Colour changes in ligand substitution: The pale blue [Cu(H₂O)₆]²⁺ becomes deep blue with ammonia and green-yellow with chloride.OCR
Colour changes in transition element complexes: The colour change arises because the ligands alter the energy gap between the d orbitals, so a different wavelength of visible light is absorbed and the…OCR
Colour changes of chromium species: The green colour of Cr³⁺ turns orange as oxidation forms Cr₂O₇²⁻.OCR
Colour changes of transition-metal ions: Each ion gives a hydroxide precipitate of a stated colour, such as blue for Cu²⁺, green for Fe²⁺, brown for Fe³⁺, pale brown for Mn²⁺ and grey-green for Cr³⁺.OCR
colour changes on adding reagents: Excess ammonia turns copper(II) deep blue, concentrated hydrochloric acid turns it yellow, and cobalt(II) goes from pink to blue.AQA
colour sequence: Yellow then blue then green then violet, the colours seen as zinc reduces vanadium species in acidic solution.AQA
colours and states: F₂ and Cl₂ are gases, Br₂ is a red-brown liquid and I₂ is a grey-black solid that sublimes to a purple vapour.AQA
Colours of silver halide precipitates: Silver chloride is a white precipitate, silver bromide a cream precipitate and silver iodide a pale yellow precipitate.OCR
Colours of transition metal hydroxide precipitates: The identity of the metal ion is inferred from the colour of the hydroxide precipitate that forms when aqueous sodium hydroxide or aqueous ammonia is added.OCR
column chromatography: A column is packed with a solid and a solvent moves down the column.AQA
Combined C=O and O–H evidence: Both the C=O peak and the broad O–H peak must be present together to identify a carboxylic acid, since a single C=O peak suggests an aldehyde, ketone, ester…OCR
Combining absorptions to identify a functional group: To identify an alcohol, locate a broad absorption in that region, read its wavenumber in cm⁻¹, and confirm that no other feature contradicts the assignment.OCR
combining half-equations: Scale the two half-equations so the electrons cancel and then add them, never multiplying E⦵ by that scaling factor.AQA
Combining half-equations into a cell equation: The half-equations are combined so that the electrons cancel, giving the overall cell equation.OCR
Combining standard electrode potentials: Writing the two half-equations for the storage cell and combining the standard electrode potentials to give E°cell.OCR
combining techniques: A range of analytical techniques which together enable the structures of new compounds to be confirmed.AQA
combustion data: Arrows point down to combustion products, so the enthalpy change is the sum of the reactants' enthalpies of combustion minus the products'.AQA
Combustion of halogenated plastics: Disposal by burning of plastics containing a halogen, which releases that halogen as a toxic hydrogen halide.OCR
Combustion products: The carbon and hydrogen in the polymer oxidise to carbon dioxide and water, releasing energy in an exothermic reaction.OCR
common oxidising agents: Powerful oxidising agents include manganate(VII), dichromate(VI) and the halogens.AQA
comparing force strengths: Comparisons follow a fixed order: identify every force present in each substance, decide which is strongest, and only then compare.AQA
comparing rates: The stronger acid gives the steeper curve in the initial rate of gas evolution with a carbonate, while the amount of acid fixes the same final volume or mass…AQA
Comparing reaction efficiency measures: A reaction can have high atom economy yet low percentage yield if it is slow or incomplete, and vice versa.OCR
Comparison of solid, liquid and aqueous states: Ions are fixed in the solid and cannot move, but when molten or dissolved in water they become free to move, so the liquid and aqueous states conduct.OCR
Comparison of solid, liquid and gas: In the solid, particles vibrate about fixed positions and energy is localised, giving low entropy; in the gas, particles move freely throughout the container…OCR
comparison to several decimal places: Comparing the calculated value to the same number of decimal places the question quotes, keeping every decimal place through the working.AQA
comparison with a database: The spectrum of the unknown is compared with spectra of known compounds held in a database, and a match across the fingerprint region identifies the compound.AQA
Comparison with carboxylic acids and the carbonate test: Phenol is a weaker acid than carboxylic acids, which do liberate carbon dioxide from carbonates, so the absence of effervescence shows phenol is too weak an…OCR
Comparison with literature data: The measured range is compared with a data book value for the compound synthesised, and a melting point well below the literature figure or a broad range…OCR
Comparison with literature values: The measured value is matched to the literature value within a narrow range to identify the original compound.OCR
comparison with standards: Running a known sample alongside on the same plate, because an Rf value is only fixed for a given solvent and stationary phase.AQA
competing reactions: The concurrent substitution and elimination reactions of a halogenoalkane, such as 2-bromopropane with potassium hydroxide, both happening at the same time.AQA
complementary base pairing: Adenine always hydrogen bonds to thymine, and cytosine always to guanine, so the sequence of one strand fixes the sequence of the other.AQA
complementary colour: The colour you see is the complement of the colour absorbed, because the absorbed wavelengths have been removed from white light.AQA
Complementary colour observed: The light not absorbed is transmitted, so the solution shows the complementary colour of the light absorbed.OCR
complementary filter: The filter that transmits the light the solution absorbs most strongly, being the complementary colour.AQA
complete combustion: With plenty of oxygen an alkane burns so every carbon becomes CO₂ and every hydrogen becomes H₂O.AQA
Complete combustion in excess oxygen: Combustion is complete, so carbon forms CO₂ and hydrogen forms H₂O, with oxygen as the other reactant in excess.OCR
Complete combustion products: In a plentiful supply of oxygen, the alkane reacts with O₂ to give carbon dioxide and water only.OCR
Complete combustion products of alcohols: Burning an alcohol in excess oxygen gives carbon dioxide and water only, releasing energy.OCR
complete dissociation: For a strong monoprotic acid every mole of acid delivers a mole of H⁺, so [H⁺] is simply the acid concentration.AQA
Completing outer shells: The number of single bonds an atom forms depends on how many electrons it needs to complete its outer shell.OCR
Complex formation: The process in which ions that are small and highly charged accept lone pairs from ligands.AQA
Complex ion: A central transition-metal ion surrounded by ligands bonded by coordinate (dative covalent) bonds, with each ligand donating a lone pair into a vacant orbital…OCR
composition of white light: White light contains all the visible wavelengths.AQA
compound ions: Groups of covalently bonded atoms carrying an overall charge, which must be learned as complete units.AQA
Compromise conditions: Industrial conditions selected to balance yield, rate, safety and cost.AQA
compromise pressure: A pressure chosen because the cost of the plant or of compression and the safety risks set a practical ceiling, balancing an acceptable yield against an…AQA
compromise temperature: A temperature chosen because the conditions giving the best yield give an unacceptably slow rate, balancing an acceptable yield against an acceptable rate and…AQA
computer modelling: Using computers to shorten the search when designing an inhibitor, instead of synthesising and testing thousands of compounds by trial and error.AQA
concentrated sulfuric acid: Added to the solid sodium halides, it shows the difference in reducing power, being reduced further by bromide and furthest by iodide.AQA
Concentration and mass concentration: Dividing the amount of the unknown by its volume in dm³ gives its concentration; multiplying by its molar mass gives mass concentration if needed.OCR
Concentration and pressure equivalence: Gas pressure increases act via increased concentration, measured as particles per unit volume, so pressure is treated as gas concentration.AQA
Concentration and rate: Increasing the concentration of a reactant in solution packs more particles into a given volume, so the rate of reaction increases.OCR
concentration changes: Increasing the concentration of a reactant shifts the equilibrium to the right, and removing a product does the same.AQA
Concentration effects: Changing concentrations from standard values alters the actual cell potential, so feasibility under non-standard conditions may differ.OCR
Concentration in mol dm⁻³: The amount in moles divided by the volume in dm³ gives the concentration, so moles equals concentration multiplied by volume in dm³.OCR
Concentration–time graphs: A plot of the concentration of the reactant against time, from which the order is deduced rather than from a rate–concentration graph.OCR
Concentration, volume and amount in solution: Concentration multiplied by volume in dm³ gives the amount in moles in solution.OCR
conclusion from observation: A conclusion that follows directly from the observations recorded and is justified by the observation that produced it, not asserted.AQA
Concordant: Repeat titres that agree closely are treated as concordant, while a titre lying well away from that cluster is flagged as anomalous and excluded from the mean.OCR
concordant titres: Titres that agree within 0.10 cm³ of each other, which are the only ones averaged when calculating the mean.AQA
condensation: The joining of two amino acids in which the -COOH of one reacts with the -NH₂ of the next and a molecule of water is lost.AQA, OCR
condensation polymer: The result of repeating the link thousands of times, with water eliminated at every link.AQA
Condensation polymerisation: Two functional groups react, for example a carboxylic acid with an alcohol or with an amine, and a small molecule such as water is eliminated each time a link…AQA, OCR
Condensation reaction at the carbonyl group: The reaction releases water as the C=O group is converted to C=N-NH-Ar.OCR
Condensed notation with brackets and subscripts: A compact form in which repeated identical groups are enclosed in brackets with a subscript showing how many units repeat, as in CH₃(CH₂)₂CH₃ for butane.OCR
condenser water direction: Cooling water enters the condenser at the lower end and leaves at the upper end so the jacket stays full.AQA
Condenser water flow: Cooling water enters at the lower inlet and leaves at the upper outlet.OCR
Condenser water flow and safety: The condenser carries water in at the lower inlet and out at the upper outlet, and the open top of the condenser is left unsealed.OCR
Conductivity requiring mobile ions: Conduction depends on ions being mobile and free to move and carry charge, not on electrons.OCR
conductivity when molten: Melting or dissolving frees the ions to move, which is why ionic compounds conduct only when molten or in solution, never as solids.AQA
Configuration of d-block cations: Remove electrons from the highest-energy occupied shell first, which for d-block cations means losing 4s electrons before 3d electrons.OCR
confirmation with ammonia: Ammonia is added afterwards to confirm which halide is present, because the three precipitates differ in how readily they dissolve to form the…AQA
Confirmatory tests with ammonia: Silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide is insoluble in ammonia.OCR
confirming a halide: A white precipitate that clears with dilute ammonia is chloride, a cream one that clears only with concentrated ammonia is bromide, and a yellow one that…AQA
Confounding factors: Factors that could influence the measurement and must be held constant, chosen only where they could realistically affect the result.OCR
conjugate acid: The species formed when a base gains a proton, such as NH₄⁺ formed when ammonia takes a proton from water.AQA, OCR
Conjugate acid–base pair: Two species that differ by a single proton, H⁺.OCR
Conjugate acid–base pair buffering: The undissociated CH₃COOH and its conjugate base CH₃COO⁻ form a conjugate acid–base pair that resists pH change.OCR
Conjugate acid–base pairing: The salt must contain the conjugate base of the same weak acid, giving a solution holding both the weak acid and its conjugate base in appreciable amounts.OCR
conjugate base: The particle remaining after the proton has been donated.AQA, OCR
conjugate pair recognition: Looking for two species that differ by one H⁺, which is the conjugate pair.AQA
conjugate pair reservoir: A weak acid HA together with a large reservoir of its conjugate base A⁻, both present in substantial amounts.AQA
conjugate pairs: Each acid is paired with the base that came from it by losing the proton, and the members of a pair differ by one H⁺.AQA
Consequences for alkane reactivity: Alkanes react with few reagents and mainly by combustion or free-radical substitution rather than addition or substitution by polar reagents.OCR
conserving crude oil: The carbon in the polymer comes from carbon dioxide fixed by a plant rather than from a finite fossil resource.AQA
consistency with the drawn shape: Quote an angle consistent with the shape you drew to avoid contradicting yourself.AQA
Consistent unit conversion: Converting units consistently before substitution, for example changing cm³ to dm³ so that concentration in mol dm⁻³ is compatible.OCR
constant area under the curve: The total number of molecules is unchanged when a gas is heated, so the area under the curve must stay the same.AQA
constant concentrations: The concentrations of reactants and products do not change, even though particles are still reacting in both directions.AQA, OCR
Constant half-life: The time for the concentration of the reactant to fall from any value to half that value stays the same as the reaction proceeds, and does not depend on…AQA, OCR
constant pressure: The condition under which enthalpy change is measured, as in an open beaker or a polystyrene cup at constant atmospheric pressure.AQA
Constant temperature and constant titre: Keeping temperature constant because Kc depends on temperature, and repeating the measurement at intervals until the titre is constant to confirm equilibrium…OCR
Constructing the rate equation from individual orders: Combining the individual orders to state the overall order and the rate equation for the reaction.OCR
Construction of enthalpy cycles: Drawing a cycle in which the direct route from reactants to products is set equal to an alternative route through intermediate substances, with arrows showing…OCR
continuation bonds: Two bonds drawn out through the brackets to show where the next unit joins.AQA
continuous monitoring: Following one mixture throughout, by collecting gas in a syringe, recording mass loss on a balance, or tracking colour with a colorimeter, then plotting the…AQA, OCR
Continuous monitoring method: A method that records a changing quantity repeatedly or continuously while a process runs, rather than taking one reading at the end.OCR
Continuous monitoring with data loggers: A pH probe connected to a data logger records pH continuously and can log changes over time.OCR
Continuous operation with fuel supply: The cell keeps producing a voltage while fuel and oxygen are supplied, unlike a secondary cell that must be recharged.OCR
continuous process: The process is continuous, fast and gives a pure product with an atom economy of one hundred per cent.AQA
continuous supply of reactants: Fuel and oxygen are fed in continuously from outside, so the electrode reactions never run down and the cell needs refuelling.AQA
Contrast with addition polymerisation: Alkene monomers join with no by-product and the repeat unit keeps the same atoms, unlike condensation polymers.OCR
Contrast with heterolytic fission: In heterolytic fission one atom takes both electrons and ions form, whereas homolytic fission gives one electron per atom and two radicals.OCR
Contrast with homolytic fission: In homolytic fission each atom receives one electron and radicals form, rather than two oppositely charged ions.OCR
contrast with ketones: Ketones lack the hydrogen on the carbonyl carbon and so are not oxidised in this way, which is why the test distinguishes the two families.AQA, OCR
Contribution of one electron per atom: In the shared pair each atom contributes one electron, shown in a dot-and-cross diagram using one dot and one cross.OCR
Control of variables: Temperature and concentrations of other reactants must be kept constant for valid comparisons.OCR
control variables: Everything except temperature must be held constant: the concentrations, the total volume, the depth of liquid, the same cross, and ideally the same observer.AQA
Controlled comparison of experiments: Selecting two experiments in which only the concentration of the reactant under test changes, so that a single reactant is isolated.OCR
Controlled variables: Factors that could affect the outcome but are deliberately kept the same across all trials so that any change in the dependent variable can be attributed to…OCR
Controlling oxidation products: The product of oxidising a primary alcohol is controlled by the reaction conditions, giving an aldehyde with distillation or a carboxylic acid with reflux and…OCR
controlling the dose: The amount added must be carefully controlled, because chlorine is itself toxic and in excess would poison the people drinking it.AQA
controlling the product: The product ratio is set by the conditions: excess ammonia favours the primary amine, excess halogenoalkane the quaternary salt.AQA
Controlling variables: Other reactant concentrations are held constant so the graph isolates the effect of one reactant.OCR
Controlling variables in equilibrium experiments: Total volume, concentrations of other species and observation time are kept constant so that only the chosen factor changes.OCR
Controlling variables to isolate concentration effects: Varying the concentration of one reactant while keeping others constant shows how initial rate depends on that concentration.OCR
controlling volume and temperature: Repeat runs vary one concentration at a time, keeping the total volume and the temperature constant.AQA
Conversion between concentration and pH: Use Kw to find [H⁺] from a known [OH⁻] in a strong base, or to find [OH⁻] from a known [H⁺] in a strong acid, then convert to pH.OCR
Conversion between moles, concentration and partial pressure: Moles are divided by volume to give concentration, or mole fraction is multiplied by total pressure to give partial pressure.OCR
Converting between cm³ and dm³: Converting consistently using the factor 1000, remembering that 1 dm³ = 1000 cm³, so 25.0 cm³ = 0.0250 dm³.OCR
Converting between Ka and pKa: The relationship is pKa = −log₁₀Ka, and rearranging gives Ka = 10^(−pKa).OCR
converting between mass and moles: Use n = m ÷ M, rearranged as m = n × M or M = m ÷ n, checking the unit asked for.OCR
Converting between moles and number of particles: Multiplying an amount in moles by N_A gives the number of particles, and dividing a particle count by N_A gives the amount in moles.OCR
Converting cm³ to dm³: The mean titre in cm³ is converted to dm³ by dividing by 1000 before use.OCR
Converting energy to enthalpy change per mole: Dividing the calculated energy q by the number of moles of reactant to obtain the enthalpy change per mole.OCR
Coordinate (dative covalent) bonding: A bond that forms when one atom donates both electrons of the shared pair, so a shared pair need not come from one electron per atom.OCR
coordinate bond: Formed from a lone pair on a nitrogen atom of guanine to the platinum.AQA
coordinate bond to nitrogen: A bond formed when a lone pair on a nitrogen atom of guanine attacks the platinum, replacing a platinum to chlorine bond.AQA
Coordination number: The number of ligands attached to the metal in a complex, such as six-coordinate with four ammonia and two water ligands.OCR
Coordination number of copper(II) complexes: [Cu(NH₃)₄(H₂O)₂]²⁺ is six-coordinate because two water ligands remain, while [CuCl₄]²⁻ is four-coordinate.OCR
Correct operating technique: Operating equipment so the reading is valid, covering rinsing, zeroing, reading at eye level and clamping safely.OCR
Correct reading technique for scales: Reading instruments correctly, for example at eye level and with the scale appropriately positioned, so that a reliable measurement is obtained.OCR
Corrosive substances: Substances handled with eye protection, gloves and minimal contact, and spills are diluted and washed away.OCR
cosmetic and food buffers: Shampoos and skin creams are buffered near pH 5.5 to match skin, and foods are buffered with citric acid and a citrate salt to control flavour and inhibit…AQA
Counting bonding and lone pairs: The bonding pairs and lone pairs around the central atom are counted to determine the total number of electron pairs.OCR
Counting carbon neighbours of the –OH-bearing carbon: Drawing the structure and counting the carbon atoms attached to the carbon bonded to the hydroxyl group gives a reliable classification.OCR
Counting groups bonded to nitrogen: Every group bonded to the nitrogen is counted, and the carbonyl carbon counts as one of those groups because it is directly attached to nitrogen.OCR
covalent backbone: The bridging bonds are ordinary covalent bonds, so the backbone does not come apart during normal cell chemistry.AQA
Covalent bond formation: Shown by an arrow from a lone pair on one atom to the atom it attacks.OCR
covalent bonds unbroken: When a molecular substance melts or boils, the covalent bonds inside each molecule are untouched.AQA
Covalent bonds within molecules: Strong bonds that hold the atoms together within each molecule.OCR
Covalent character: Indicated when the experimental lattice enthalpy is significantly more exothermic than the theoretical value, as the extra strength shows the cation polarises…AQA
covalent versus intermolecular: A disulfide bridge is a covalent bond, so it is far stronger than a hydrogen bond, which is not covalent.AQA
Cr and Cu exceptions: Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹, because a half-filled or full 3d subshell is more stable.OCR
cross-links: Links formed when the platinum binds to adjacent guanine bases on the same DNA strand, distorting the double helix.OCR
crude oil composition: Petroleum is a mixture consisting mainly of alkane hydrocarbons of many chain lengths.AQA
Curly arrow conventions: One curly arrow runs from the lone pair to the δ⁺ carbon, and a second shows the C–X bond breaking with both electrons moving onto the halogen.AQA, OCR
curly arrow mechanism: An arrow runs from inside the hexagon to the nitrogen or to the acylium carbon, then a second arrow runs from the C-H bond at that carbon back into the…AQA
Curly arrow notation: A symbol for the movement of one electron pair, with the tail marking the source and the arrowhead pointing at the atom that receives the pair.OCR
Curly-arrow conventions and intermediates: The alkoxide intermediate is shown with an arrow from the nucleophile's lone pair to the carbonyl carbon and an arrow from the C=O π bond onto the oxygen,…OCR
cyanide as nucleophile: The cyanide ion has a lone pair on its carbon and attacks the δ⁺ carbonyl carbon.AQA
Cyanohydrin formation with HCN: The cyanide ion CN⁻ attacks the carbonyl carbon, the π bond breaks to give an alkoxide, and H⁺ protonates the oxygen to give a hydroxynitrile.OCR
cysteine R groups: Each ends in -CH₂SH, and two of them are oxidised so their sulfur atoms bond directly.AQA

D

d subshell splitting by ligands: The d subshell splits into slightly different energy levels when ligands bond to the transition metal ion.OCR
d-block: Elements whose highest-energy electron enters a d subshell, occupied by the transition metals plus scandium and zinc.OCR
damage to buildings and lakes: Aqueous sulfur dioxide is acidic enough to attack carbonate stonework such as limestone and marble, and to acidify lakes so that fish die.AQA
dative covalent bonding: A bond containing a shared pair of electrons with both electrons supplied by one atom.AQA, OCR
Decrease in entropy: A reaction that decreases the number of gas molecules usually decreases the entropy of the system.OCR
Decrease means reduction: A fall in oxidation number is reduction.OCR
Deducing formulae from stated oxidation numbers: Read the numeral, assign that oxidation number to the element, then deduce the formula or the ion charge from the other element present.OCR
Deducing functional groups from supplied data: List the absorptions present, assign each using the table, then combine the assignments into a structure.OCR
deducing monomers: Breaking each link and putting an -OH on the carbonyl carbon and an -H on the oxygen or nitrogen, which is adding the water back.AQA
Deducing order from initial-rate data: Finding the order with respect to each reactant from experimental initial-rate data, not from the coefficients in the balanced equation.OCR
Deducing order with respect to a reactant: Reading the shape and the half-life pattern together, where a straight line confirms zero order and equal successive half-lives confirm first order, with the…OCR
deducing oxidation states: A standard task in which oxygen counts as −2 and the sum equals the overall charge, with positive values always given an explicit positive sign.AQA
Deducing structure from combined spectral evidence: The molecular formula is combined with infrared absorptions, carbon-13 NMR chemical shifts and proton NMR integration, splitting and chemical shift to decide…OCR
Definition of a buffer solution: A system that minimises pH changes on addition of small amounts of an acid or a base.OCR
Definition of atomic number: Atomic number is the same as proton number, that is, the number of protons in the nucleus.OCR
Degree of dissociation and acid strength: A larger Ka and a greater degree of dissociation make a ‘stronger’ weak acid, where the amount dissociated becomes a significant fraction of the initial…OCR
dehydration of alcohols: A reaction in which an alcohol is heated with an acid catalyst so that a whole water molecule is lost and a double bond is left.AQA
delocalisation: Electrons are no longer tied to any one pair of atoms but are spread in a ring of electron density above and below the plane.AQA
Delocalisation and reduced basicity of the amide nitrogen: The lone pair on the amide nitrogen is delocalised into the carbonyl group, so an amide is much less basic than an amine and does not readily accept a proton…OCR
delocalisation energy: The difference between the predicted enthalpy of hydrogenation, calculated as three times the cyclohexene value, and the measured value.AQA
delocalisation into the ring: In phenylamine the nitrogen's lone pair lies next to the delocalised ring and overlaps with it, so part of the electron density is drawn into the ring and…AQA
Delocalisation of the lone pair in phenylamine: The lone pair is delocalised into the aromatic ring, reducing its availability for proton acceptance and lowering basicity.OCR
Delocalisation stabilising the phenoxide ion: The negative charge on the phenoxide ion is delocalised into the aromatic ring, which stabilises the ion.OCR
delocalised electrons: The six π electrons of the benzene ring are shared over all six carbons, above and below the ring, rather than forming three fixed alternating C=C bonds.AQA, OCR
Delocalised versus Kekulé models: The delocalised model is supported by evidence, rather than the Kekulé model with alternating single and double bonds.OCR
Delocalised π system and electron density: Donating the oxygen lone pair into the delocalised π-system increases the electron density, especially at the 2- and 4-positions, and a higher electron…OCR
Delocalised π system in benzene: Benzene's six π electrons are delocalised over all six carbon atoms of the ring, giving lower π electron density at any one carbon.OCR
delta scale: The scale on which chemical shift is quoted, in parts per million, increasing to the left.AQA
denaturing: Loss of the three-dimensional shape of the active site when heating or a large change in pH breaks the hydrogen bonds and ionic attractions between R groups.AQA
Density and layer order: The denser layer collects at the bottom and the less dense layer floats above, so the lower layer is run out through the tap first while the upper layer…OCR
density anomaly: Ice is less dense than water and floats, and on melting some hydrogen bonds break and the structure collapses inwards.AQA
Density as mass per unit volume: Density is mass divided by volume, so the lower density of ice follows directly from its larger volume.OCR
denticity: The number of donor atoms each ligand supplies, so the number of ligands multiplied by their denticity gives the co-ordination number.AQA
Dependence of colour on metal, oxidation state and ligands: The size of the splitting, and so the colour, depends on the metal, its oxidation state, the identity of the ligands and their arrangement.OCR
Derivative formation: The carbonyl compound is converted to a solid derivative, typically the 2,4-dinitrophenylhydrazone, before a melting point is measured.OCR
deriving the rate equation: Combine the orders found for each reactant to write the rate equation, then substitute one set of data to find k and its units.AQA
Deshielding: Carbons near electronegative atoms or in unsaturated groups are deshielded and appear at higher ppm.OCR
detection and current: At the detector each ion gains an electron from the negatively charged plate, producing a current whose size is proportional to the abundance of that ion.AQA
detector current: The current generated as ions arrive at the detector, whose size gives the relative abundance.AQA
Determination of units by unit cancellation: Found by cancelling the units of the numerator against those of the denominator, recognising when the expression is dimensionless.OCR
Determining n from masses: Find n by dividing the relative molecular mass by the empirical formula mass.OCR
Deuterated solvent: A solvent such as CDCl₃ in which hydrogen is replaced by deuterium, so it gives no signal in the ¹H NMR region and is effectively invisible.OCR
Deuterium: Replaces hydrogen in the solvent and does not give a signal in the ¹H NMR region.OCR
development of catalysts: Catalysts were developed that let ethene polymerise at low pressure and gave straight chains.AQA
diamminesilver(I) complex: The soluble complex ion formed when the precipitate reacts, shown as AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq).AQA
Diamond: Each carbon atom bonds tetrahedrally to four others, giving a rigid three-dimensional network.OCR
diamond and graphite: Both are giant covalent, but each carbon in diamond bonds to four others tetrahedrally at 109.5°, while in graphite each carbon bonds to only three, leaving…AQA
Diatomic molecular structure of halogens: Halogens exist as diatomic molecules because two halogen atoms share one pair of electrons in a single covalent bond, giving each atom a full outer shell.OCR
Dibasic acid: An acid such as H₂SO₄ that can release two H⁺, giving HSO₄⁻ and then SO₄²⁻.OCR
Dicarboxylic acid and diamine monomers: The two monomer types that react together to form a polyamide.OCR
Dicarboxylic acid and diol: These two difunctional monomers react repeatedly to form a polyester, with water eliminated as each ester bond forms.OCR
Dicarboxylic acid and diol monomers: Each monomer has two functional groups, so the molecules link repeatedly, and each monomer must carry two reactive functional groups for chain growth.OCR
Dicarboxylic acid and diol or diamine monomers: The carbonyl side rebuilds the dicarboxylic acid and the oxygen or nitrogen side rebuilds the diol or diamine, adding –OH and –H respectively.OCR
Difference between strong and weak acids: A weak acid gives a smaller magnitude because some energy is used in ionising it.OCR
difference of CH₂: Successive members differ by this, so relative molecular masses rise in steps of 14.AQA
Different mass number and mass: Because the neutron number differs, the mass number changes and therefore the masses differ.OCR
Different neutron number: The difference in the number of neutrons in the nucleus, which carry mass but no charge.OCR
different phase: The catalyst is in a different phase from the reactants, typically a solid catalyst with gaseous or aqueous reactants.AQA
different speeds of travel: Each component travels at its own speed because each divides differently between the two phases.AQA
Differentiating electron: The electron whose entry into a subshell in the ground-state atom determines the block to which an element belongs.OCR
difunctional monomers: Monomers that each carry two functional groups and join repeatedly to form a condensation polymer.AQA, OCR
dilute and concentrated ammonia: Silver chloride dissolves in the dilute form, silver bromide does not but dissolves in the concentrated form, and silver iodide is insoluble in both.AQA
Dilution and conservation of solute amount: When a solution is diluted by adding solvent, the amount of solute stays the same, so c₁V₁ = c₂V₂.OCR
dimensionless ratio: A quantity that has no units because it is a ratio.AQA
Diol and dicarboxylic acid monomers: These two monomers join by condensation to give a polyester with ester links.OCR
diprotic acids: A diprotic strong acid supplies two protons per molecule, so 0.0200 mol dm⁻³ H₂SO₄ gives [H⁺] = 0.0400 mol dm⁻³ and pH = 1.40.AQA
Directing effects: Effects that let a chemist predict which substitution product will dominate when an electrophile attacks a substituted benzene ring.OCR
Directing effects of OH, NH₂ and NO₂: OH and NH₂ push electron density into the ring so the next electrophile arrives mainly at the 2- and 4-positions, while NO₂ pulls electron density out so…OCR
Directing effects of the –OH group: The –OH group directs incoming electrophiles to the 2- and 4-positions because its oxygen lone pair donates electron density into the ring by resonance.OCR
direction not extent: The principle predicts direction, never how far or how fast, and the shift never fully cancels the change imposed.AQA
Direction of error: The way a named limitation affects the measured quantity, where it is known, such as an enthalpy change being less exothermic than the true value.OCR
Direction of error from approximations: Using [H⁺] ≈ √(Ka × [HA]₀) overestimates [H⁺] and therefore underestimates pH for a stronger weak acid.OCR
discovery of the neutron: Chadwick added this particle to explain the missing mass that protons and electrons alone could not account for.AQA
Disorder and number of arrangements: A more disordered system has more ways to arrange its particles and their energy, so its entropy is greater.OCR
Dispersal of energy: Entropy is a measure of the dispersal of energy in a system, and it is greater the more disordered a system.OCR
displacement reactions: A halogen displaces the ions of any halogen below it, so bromine displaces iodine but not chlorine, and iodine displaces neither.AQA
Displacement reactions of halogens with halide ions: A more reactive halogen displaces a less reactive halide ion from solution, so chlorine displaces bromide and iodide ions, bromine displaces iodide ions, and…OCR
Displayed and general formulae: The general forms RCONH₂ and RCONHR′ can be used as quick checks, and the R and R′ groups may be alkyl or aryl.OCR
Displayed formula: A formula that shows every atom and every bond, and for enantiomers also shows the arrangement in space with two bonds in the plane, one on a wedge and one…AQA, OCR
displayed formulae: A representation in which each shared pair is drawn as a single straight line.AQA
disposal and toxicity: Cells contain toxic or corrosive materials, including cadmium, lithium compounds and concentrated alkali, which can leach from landfill into groundwater.AQA
Disproportionation: A reaction in which the same element is simultaneously oxidised and reduced, with one portion increasing its oxidation number while another portion decreases…AQA, OCR, Pearson Edexcel
Disproportionation of chlorine: Chlorine is both oxidised and reduced in the same reaction, as one chlorine atom rises and one falls in oxidation state.OCR
Disproportionation of Cu⁺: Simultaneous oxidation and reduction of the same species, here producing Cu²⁺ and Cu with one Cu⁺ oxidised and another Cu⁺ reduced.OCR
Disproportionation with alkali: A reaction in which chlorine is both oxidised and reduced when it reacts with cold, dilute aqueous sodium hydroxide.OCR
Distance from origin and solvent front: Both distances are measured from the same origin line along the direction of travel to keep the ratio valid.OCR
Distillation: A technique in which the vapour of the lower-boiling component leaves the flask, condenses and is collected, separating a product from the reaction mixture by…OCR
distillation against reflux: Distilling the aldehyde off as it forms stops at the aldehyde, while heating under reflux with excess oxidising agent reaches the carboxylic acid.AQA
Distillation apparatus: A set-up with a still head and thermometer between flask and condenser, where vapour is led away and condensed into a receiver.OCR
Distillation of the purified liquid: After the drying agent is filtered off, the pure liquid is obtained by distillation.OCR
Distillation setup and purpose: Boiling a mixture and condensing the vapour so a more volatile component is collected separately, with a thermometer at the still head monitoring the vapour…OCR
Distillation versus reflux: Distillation removes the lower-boiling aldehyde as it forms, preventing further oxidation, whereas heating under reflux with excess oxidising agent keeps the…OCR
Distinction between strength and concentration: Strength describes extent of dissociation, whereas concentration describes amount per volume, so a dilute strong acid can have a lower H⁺ concentration than a…OCR
Distinction from mass number and relative atomic mass: Proton number, nucleon number and relative atomic mass are different quantities, and only proton number gives the modern sequence.OCR
Distinction from molecular formula: A structural formula shows the arrangement of atoms rather than only the numbers of atoms, which is all a molecular formula such as C₄H₁₀ shows.OCR
Distinguishing alcohols from carboxylic acids: A broad absorption around 3200–3600 cm⁻¹ indicates an alcohol O–H bond, whereas a very broad absorption around 2500–3300 cm⁻¹ indicates the O–H of a…OCR
Distinguishing aldehydes and ketones by chemical tests: Aldehydes and ketones share the same C=O absorption, so further tests such as Tollens' or Fehling's reagent distinguish an aldehyde from a ketone.OCR
Distinguishing aldehydes from ketones: An aldehyde turns the reagent from orange to green, whereas a ketone is not oxidised so the reagent stays orange.OCR
Distinguishing aldehydes from ketones by oxidation: Aldehydes are readily oxidised, changing Fehling's solution from blue to a red precipitate and giving a silver mirror with Tollens' reagent, while ketones…OCR
Distinguishing alkenes from alkanes: An alkane leaves the bromine water colour unchanged, whereas decolourisation indicates unsaturation.OCR
Distinguishing amount from mass: Amount of substance is not the same as mass, since equal amounts of different substances can have different masses.OCR
Distinguishing aromatic rings from saturated rings: An aromatic ring has a delocalised π system, whereas a saturated cycloalkane ring such as cyclohexane C₆H₁₂ has no π electrons in the ring.OCR
Distinguishing capacity from filling order: The third and fourth shells are not always filled to their maximum before the next shell begins, because the 4s sub-shell fills before 3d, but the capacity of…OCR
Distinguishing compounds of the same element: Species of the same element that carry different oxidation numbers, such as iron(II) and iron(III), are told apart by the Roman numeral.OCR
Distinguishing exchangeable from non-exchangeable protons: A peak that vanishes belonged to an exchangeable O–H or N–H proton, while a peak that remains belongs to a C–H proton, which does not exchange.OCR
Distinguishing formula types: A displayed formula draws every bond as a line and shows every hydrogen, unlike a structural formula such as CH₃CH₂OH, which groups atoms, or a skeletal…OCR
Distinguishing geometric from optical isomerism: Geometric (E/Z) isomerism is tied to the restricted C=C bond, whereas optical isomerism needs a chiral centre.OCR
Distinguishing metals from non-metals: Metals lose electrons to form positive ions and non-metals gain electrons to form negative ions.OCR
Distinguishing oxidation states by precipitate colour: Fe²⁺ is distinguished from Fe³⁺ by the green colour of Fe(OH)₂ against the reddish-brown colour of Fe(OH)₃.OCR
Distinguishing polar bonds from polar molecules: A molecule with polar bonds can be non-polar overall, and a molecule with an overall dipole is described as polar.OCR
Distinguishing polyamides from polyesters: The amide linkage is –CO–NH–, whereas the ester linkage found in polyesters is –CO–O–.OCR
Distinguishing σ-bonds from π-bonds: σ-bonds form by direct, end-on overlap of orbitals between the bonding atoms, whereas sideways overlap would give a π-bond.OCR
disulfide bridge: Two cysteine R groups, each ending in -CH₂SH, are oxidised so their sulfur atoms bond directly, giving -CH₂-S-S-CH₂-.AQA
dividing by the smallest: All the relative amounts are divided by the smallest value to reach the ratio.AQA
Donor and acceptor: The donor is the atom with a lone pair and the acceptor is the atom or ion with a vacant orbital.OCR
dose determines risk: At the few parts per million used in a public supply, chlorine presents a very small risk, while the disease risk it removes is very large.AQA
dot-and-cross diagrams: A diagram in which the shared pair sits in the overlap of the two circles, with one dot and one cross, and electrons not involved are drawn as lone pairs.AQA, OCR
Double and triple bonds: A double bond contains two shared pairs and a triple bond contains three shared pairs.OCR
double arrow notation: The way the equation for a reversible reaction is written, showing that the reaction can proceed in both directions.AQA
double bond: Contains two shared pairs of electrons, four electrons in all.AQA
Drawing and interpreting polymer structures: The repeat unit is drawn with the two free bonds shown through the brackets and the subscript n outside.OCR
Dropwise addition near the end point: Run titrant in, swirling continuously, and add it dropwise as the end point approaches.OCR
Drying agent: An anhydrous substance, such as anhydrous sodium sulfate or calcium chloride, that removes residual water from the organic layer and does not react with the…OCR
drying agents: A drying agent such as anhydrous sodium sulfate or calcium chloride is added to remove residual water.AQA, OCR
drying and washing the product: The distillate is usually washed in a separating funnel, releasing pressure often if carbon dioxide is produced, then dried with anhydrous calcium chloride.AQA
durability and disposal: All three hydrolyse slowly, so they eventually degrade.AQA
dynamic equilibrium: A state in a closed system where the forward and reverse reactions continue at the same time with equal rates and constant concentrations.AQA, OCR
Dynamic equilibrium and equal forward and reverse rates: In a system at equilibrium the same pathway is available to the forward and the reverse reaction, so both rates rise by the same factor and the two rates…OCR
Dynamic equilibrium at electrodes: The forward and reverse reactions reach a dynamic equilibrium at the electrode surface, and this provides the half-cell potential.OCR
Dynamic equilibrium in closed systems: A reversible reaction reaches dynamic equilibrium in a closed system when the rates of the forward and reverse reactions become equal and the concentrations…OCR

E

E and Z assignment: Z means both higher priority groups on the same side and E means on opposite sides.AQA
E and Z configuration: The Z isomer has the higher-priority groups on the same side of the double bond, while the E isomer has them on opposite sides.OCR
E and Z isomers: Stereoisomers that share a structural formula but differ in spatial arrangement, labelled using CIP priorities rather than appearance.AQA
E-Z isomerism: Arises where rotation is restricted, most often about a C=C double bond, so two groups fixed on the same side differ from two fixed on opposite sides.AQA
E/Z geometric isomerism: Two stereoisomers exist where each doubly bonded carbon carries two different groups: the E isomer has the higher-priority groups on opposite sides, the Z…OCR
E/Z isomerism: A form of isomerism in which each carbon of the C=C group carries two different substituent groups, so the two ends can be ranked by priority.OCR
E/Z stereoisomerism: Z is assigned when the two higher-priority groups lie on the same side of the double bond and E when they lie on opposite sides.OCR
economic consequences: The cost runs in both directions: purifying the feedstock costs money, and so does buying replacement catalyst and shutting the plant down while it is changed.AQA
Economic importance of catalysts: Energy is a major cost in industrial processes, so reduced energy demand lowers energy costs.OCR
Effect of a catalyst on rate versus position of equilibrium: A catalyst only shortens the time taken to reach equilibrium, while the position of equilibrium is unchanged.OCR
Effect of atomic size on electron gain: The atoms get larger down the group, so the incoming electron is gained less readily.OCR
Effect of branching on boiling point: A more branched isomer has a smaller surface area for contact between molecules, so the London forces are weaker and the boiling point is lower than that of…OCR
Effect of carbon-chain length on boiling point: A longer carbon chain gives more electrons and a larger surface area, so the induced dipoles are stronger and more energy is needed to overcome them, raising…OCR
Effect of catalysts on activation energy: Catalysts provide an alternative route with a lower peak, so the activation energy falls.OCR
Effect of catalysts on equilibrium: The catalyst lowers the activation energy for both the forward and reverse reactions equally, so it speeds the approach to equilibrium but does not change the…OCR
Effect of catalysts on rate and equilibrium: A catalyst speeds the forward and reverse reactions equally, so equilibrium is reached sooner but Kc stays the same.OCR
effect of concentration: Lowering [Cu²⁺] pulls the equilibrium to the left, so the electrode accepts electrons less readily and E becomes less positive.AQA
Effect of concentration change on equilibrium position: After an initial observation or measurement, a small volume of a solution containing one of the species is added and the new position is observed or measured…OCR
Effect of concentration, pressure, temperature and catalysts: Raising concentration or pressure increases collision frequency, raising temperature also increases the proportion of particles with energy at or above the…OCR
effect of conditions: Cold, dilute alkali gives sodium chlorate(I), whereas warm, concentrated alkali drives the reaction further to sodium chlorate(V), NaClO₃.AQA
Effect of conditions on products: Cold, dilute sodium hydroxide gives the chlorate(I) product, whereas hot, concentrated alkali gives chlorate(V).OCR
Effect of excess ammonia on product distribution: Excess ammonia favours the primary amine and limits further substitution, because ammonia outnumbers the haloalkane and the newly formed amine is less likely…OCR
Effect of hydrocarbon chain length on solubility: As the hydrocarbon chain lengthens, the non-polar part cannot hydrogen bond, so solubility falls.OCR
Effect of hydrogen bonding on peak shape: Hydrogen bonding in the carboxyl group makes the O–H absorption broad, much wider than the sharper O–H stretch of an alcohol.OCR
Effect of hydrogen bonding on physical properties: Hydrogen bonding explains properties such as the relatively high boiling point of water.OCR
Effect of impurities on melting point: An impure solid melts lower and over a wider range because impurities disrupt the crystal lattice.OCR
Effect of ionic charge and radius on lattice enthalpy: Magnitude rises with higher ionic charges and with smaller ionic radii, since the electrostatic attraction between ions increases.OCR
effect of lone pairs on tetrahedral shapes: For a tetrahedral base, each lone pair closes the angle by about 2.5°.AQA
Effect of temperature change on equilibrium position: Portions of the same mixture are placed in water baths at different temperatures, each is allowed to reach thermal equilibrium, and the colour or reading is…OCR
Effect of temperature on equilibrium position: Raising the temperature shifts the position of equilibrium to the left for an exothermic forward reaction and to the right for an endothermic forward reaction.OCR
Effect of temperature on rate: Raising the temperature flattens and broadens the curve, so a greater proportion of molecules have energy at or above Ea, increasing the rate.OCR
Effect on collision frequency and successful collisions: Because the activation energy is lower, a greater proportion of collisions has energy at or above the activation energy, so more frequent successful…OCR
Effervescence: Visible bubbling caused by the evolution of a gas from a liquid reaction mixture.AQA
Electrical conductivity: Simple molecular substances do not conduct electricity because they contain no free-moving charged particles.OCR
Electrical conductivity and ion mobility: The solid does not conduct because ions are fixed, whereas the molten or aqueous form conducts because ions are mobile.OCR
electrical neutrality: Because the proton and electron charges are equal and opposite, a neutral atom contains equal numbers of each.AQA
Electrical neutrality of ionic compounds: The total positive charge must equal the total negative charge, so the charges cancel before the formula is finalised.OCR
Electrochemical cells: Built from two half-cells, each made by dipping a clean metal electrode into a solution of its own ions, and joined so the circuit is complete.OCR
electrochemical series: A list of standard electrode potentials in which every half-equation is written as a reduction and ordered by E⦵.AQA
electrode half-equations: At the negative electrode hydrogen is oxidised and at the positive electrode oxygen is reduced, each written with OH⁻.AQA
Electrode handling and rinsing: Rinsing the electrode with distilled water and blotting it between buffers and before each measurement.OCR
Electrode potential and equilibrium: Equilibrium between the two oxidation states sets the electrode potential, which depends on the ratio of the two ion concentrations.OCR
Electron affinity: Terms for the non-metal are added, with the first electron affinity of chlorine given a negative sign and the second electron affinity of oxygen endothermic…AQA, OCR
electron balance: Balance the charge by adding electrons to the more positive side.AQA
Electron capacity of s-, p- and d-sub-shells: An s-sub-shell holds a maximum of two electrons, a p-sub-shell a maximum of six electrons and a d-sub-shell a maximum of ten electrons.OCR
Electron configuration: This fills the lowest available energy sub-shell first, so 4s fills before 3d, giving potassium 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.OCR
Electron configuration of Group 17 atoms: The outer shell of a halogen atom holds seven electrons and is written as s²p⁵, one electron short of a full octet.OCR
Electron configuration of Period 4 d-block atoms: Fill orbitals in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, using the atomic number to count electrons.OCR
Electron count from atomic number and ionic charge: Start from the neutral atom's electron count, which equals the atomic number, then adjust for the ionic charge.OCR
Electron impact ionisation: A vaporised organic sample is bombarded with high-energy electrons, and each molecule that loses one electron forms a radical cation.OCR
Electron loss and gain: Oxidation is loss of electrons and reduction is gain of electrons, summarised by OIL RIG.OCR
Electron loss and gain to reach a noble-gas configuration: Ions form when atoms gain or lose electrons to reach a stable noble-gas configuration.OCR
Electron pair acceptance: The defining feature of an electrophile, which accepts an electron pair rather than donating one.OCR
Electron pair donation from an oxygen p-orbital: A lone pair in an oxygen p-orbital of the –OH group is donated into the ring's π-system, increasing the electron density around the ring.OCR
Electron pair movement: An arrow starts at a genuine electron-pair source and ends at the receiving atom or bond region.OCR
Electron pair repulsion: Every pair of electrons around a central atom acts as a charge cloud that pushes other pairs away, so the pairs spread out as far as possible.AQA, OCR
Electron pair repulsion between bonding pairs: The four bonding pairs repel one another and arrange themselves as far apart as possible, and the repulsion acts between bonding pairs of electrons rather…OCR
Electron pair repulsion theory: The shape of a simple molecule or ion is predicted from the number of electron pairs around the central atom using this theory, arranging them so that…OCR
Electron shielding: Each successive element has more inner shells, so electron shielding increases and further reduces the effective nuclear pull on the incoming electron.OCR
Electron transfer: The process underlying redox, in which an oxidising agent accepts electrons and a reducing agent donates electrons.OCR
Electron transfer in redox: Each half-equation is multiplied so the number of electrons lost equals the number gained before the half-equations are added.OCR
electron-deficient carbon: The carbon carrying a small positive charge, δ⁺, is the site every reaction of a halogenoalkane starts from.AQA
Electron-deficient carbon in haloalkanes: The carbon bonded to the halogen carries a partial positive charge because the halogen is more electronegative, so nucleophiles attack that carbon.OCR
Electron-donating alkyl groups and base strength: Alkyl groups release electron density towards nitrogen and stabilise the protonated cation, so aliphatic amines are generally stronger bases than ammonia.OCR
Electron-donating group: A group such as OH or NH₂ that sends the next electrophile mainly to the 2- and 4-positions.OCR
Electron-pair donation: The nitrogen lone pair is donated to an H⁺ ion, as in the formation of NH₄⁺.OCR
Electron-withdrawing group: A group such as NO₂ that sends the next electrophile mainly to the 3-position.OCR
electron-withdrawing neighbours: An oxygen, a halogen, a carbonyl group or a benzene ring pulls electron density away, leaving the nucleus less shielded so it resonates further left at a…AQA
Electronegativity: Measures how strongly an atom attracts the bonding electrons in a covalent bond.OCR
Electronegativity difference: Found by subtracting the smaller Pauling value from the larger; a bigger difference means a more polar bond.OCR
electronegativity down a group: Falls from fluorine at 4.0 to iodine at 2.5, because the atom has more shells so the bonding pair lies further from the nucleus and is screened by more inner…AQA
electronegativity of the donor atom: When two molecules both hydrogen bond, the one with the more electronegative donor atom has the higher boiling point, so ethanol beats ethylamine because…AQA
electrophile: A species that is short of electrons and is attracted to the delocalised ring, which is a region of high electron density above and below the plane.AQA, OCR
Electrophilic addition: A reaction in which the pi bond breaks and two new sigma bonds form, converting the unsaturated alkene into a saturated product.OCR
Electrophilic addition mechanism: The π electrons of the C=C bond attack an electrophile, the electrophile bond breaks heterolytically, and a carbocation intermediate forms before a…OCR
Electrophilic addition of H–X to alkenes: Addition reactions of H–X to unsymmetrical alkenes such as propene proceed through a carbocation intermediate, with the more stable carbocation forming faster.OCR
Electrophilic addition of halogens: A halogen such as bromine adds across the C=C bond to form a dihaloalkane.OCR
Electrophilic addition to alkenes: The reaction in which an alkene reacts with a hydrogen halide, with both atoms of HX adding across the double bond to form a haloalkane.OCR
Electrophilic substitution mechanism: The electrophile attacks a carbon of the delocalised benzene ring, forming a positively charged intermediate with the charge delocalised around the ring,…OCR
Electrophilic substitution of arenes: A reaction in which the electrophile attacks the ring, a delocalised intermediate (a carbocation) forms, and a proton is lost to restore aromaticity.OCR
Electrophilic substitution of aromatic compounds: The ring attacks the electrophile, an unstable arenium intermediate forms, then a proton is lost to restore the delocalised ring.OCR
Electrophilic substitution of benzene: A reaction in which a hydrogen atom on the benzene ring is replaced by a nitro group, attacked by the nitronium ion on the electron-rich ring.OCR
electrostatic attraction: The strong force acting between cations (positive ions) and delocalised electrons, acting in all directions to hold the lattice together.AQA, OCR
Electrostatic attraction in all directions: Each ion is surrounded by ions of opposite charge and is strongly attracted in all directions, not just towards one neighbour.OCR
electrostatic attraction to both nuclei: Both positive nuclei are attracted to the same negative pair of electrons sitting between them, holding the atoms at a fixed separation.AQA
Electrostatic nature of the covalent bond: The electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.OCR
Elements in standard states as reactants: A formation equation always starts from the elements themselves in their standard states, not other compounds.OCR
Eliminating competing functional groups: Confirm that no broad O–H absorption of a carboxylic acid appears around 2500–3300 cm⁻¹ and no O–H of an alcohol appears around 3230–3550 cm⁻¹.OCR
Elimination of a small molecule: The release of a small molecule such as water or hydrogen chloride at every linkage when condensation monomers join.OCR
elimination of water: The reaction type in which water is removed across two adjacent carbons, leaving a double bond, and which is classed as elimination.AQA
Elimination of water or hydrogen chloride: A small molecule is expelled at each new linkage as the two difunctional monomers react repeatedly.OCR
Elimination Reaction: A reaction in which a small molecule (like HBr or $H_2O$) is removed from a larger molecule, leaving a double bond.AQA
Elimination reactions: Reactions in which water is lost from an alcohol and a C=C double bond forms, with H and OH removed from adjacent carbon atoms.OCR
elimination to an alkene: As a base the hydroxide removes a hydrogen from a carbon next to the C–Br carbon, and those bonding electrons swing in to form a C=C bond while bromide…AQA
EMF: Fixed by the difference between the two electrode potentials, so making a cell physically larger does not raise it.AQA
EMF calculation: Subtract the less positive E⦵ from the more positive E⦵, since EMF = E⦵(more positive) − E⦵(more negative).AQA
EMF of a fuel cell: Calculated as +0.40 − (−0.83) = +1.23 V from the E⦵ values of the two electrodes.AQA
Empirical formula: The simplest whole-number ratio of atoms in a compound, obtained by dividing every mole value by the smallest to obtain the ratio.AQA, OCR
Empirical formula calculation: Dividing each mole value by the smallest to reach the simplest whole-number ratio, which gives the empirical formula.OCR
empirical formula mass: Worked out by adding the relative atomic masses in the empirical unit.AQA
Empirical formula mass and molecular formula multiplier: Dividing the relative molecular mass by the empirical formula mass and multiplying the empirical formula by that whole number to give the molecular formula.OCR
enantiomers: Optical isomers that exist as non-superimposable mirror images and differ in their effect on plane polarised light.AQA, OCR
End point versus equivalence point: The end point must coincide with the equivalence point, which is contained in the steep vertical section of the titration curve.OCR
Endothermic: A reaction in which the system gains enthalpy from the surroundings, so the surroundings cool and ∆H carries a positive sign.OCR
Endothermic bond breaking: Breaking bonds absorbs energy, so the average bond enthalpy is positive and the process is endothermic.OCR
Endothermic nature of bond breaking: Bond breaking is endothermic, so average bond enthalpy is always positive.OCR
Endothermic reaction: The products lie higher than the reactants, so ΔH is positive and the arrow points upwards.OCR
Energy balance in reactions: The total energy absorbed in breaking bonds in the reactants is compared with the total energy released in making bonds in the products to classify the…OCR
energy of products: An addition reaction gives a product higher in energy than the starting material, while substitution restores the delocalised ring so the product remains low…AQA
Energy recovery: Burning waste polymers releases heat that can generate electricity or heat buildings, so the material is used as a fuel rather than sent to landfill.OCR
Energy recovery by combustion: Uses the high energy content of polymers to generate electricity or heat, displacing some fossil fuel that would otherwise be burned.OCR
energy requirement: The particles must arrive with at least a minimum combined kinetic energy, enough to start breaking the existing bonds and reach the transition state.AQA
enthalpy against entropy: The balance between the two determines the feasibility of a reaction, with the enthalpy term favouring change that releases energy and the entropy term…AQA
enthalpy change: The heat energy transferred at constant pressure, whose sign shows direction: negative for exothermic, positive for endothermic.AQA, OCR
Enthalpy change (∆H): A measure of the heat energy exchanged with the surroundings at constant pressure, recorded with a sign fixed relative to the system.OCR
Enthalpy change of combustion: The enthalpy change when one mole of a substance burns completely in excess oxygen under standard conditions, written as ∆cH.OCR
Enthalpy change of formation: The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions.OCR
Enthalpy change of hydration: The enthalpy change when 1 mol of gaseous ions dissolves in water to form a dilute aqueous solution of ions.OCR
Enthalpy change of hydrogenation: The value for benzene is less exothermic than the value predicted for a hypothetical cyclohexatriene with three isolated double bonds, so benzene is more…OCR
Enthalpy change of neutralisation: For strong acid with strong base it is about −57.1 kJ mol⁻¹ because the reaction is essentially H⁺ + OH⁻ → H₂O.OCR
Enthalpy change of reaction: The enthalpy change associated with a stated equation, given the symbol ∆rH.OCR
Enthalpy change of solution: The enthalpy change when one mole of an ionic solid dissolves in water to form a very dilute solution.OCR
Enthalpy change of the system: A negative ∆H favours feasibility because it lowers ∆G.OCR
Enthalpy changes of bond breaking and bond making: Breaking a covalent bond requires energy from the surroundings and is endothermic with a positive enthalpy change, while making a bond releases energy to the…OCR
Enthalpy cycle: A cycle built from changes you can measure, such as standard enthalpies of formation or combustion, to find an enthalpy change that is hard to measure directly.OCR
enthalpy cycles: A target reaction is drawn across the top, then a triangle is built down to a common set of substances.AQA
Enthalpy of atomisation: Used to atomise each element starting from the elements in their standard states, with bond enthalpy used for a diatomic such as Cl₂.OCR
enthalpy of formation: One of the relevant energy terms, alongside atomisation, bond dissociation, ionisation and electron affinity, used with the lattice enthalpy of a simple ionic…AQA, OCR
Enthalpy of formation and combustion cycles: If formation enthalpies are given for the reactants and products of an unfamiliar reaction, the reaction enthalpy equals the sum of the products' formation…OCR
enthalpy of hydration: The enthalpy released when each gaseous ion is surrounded by water molecules attracted to its charge, which is negative and larger in magnitude for small,…AQA
enthalpy of hydrogenation: The energy released when hydrogen is added to a C=C, as when cyclohexene releases 120 kJ mol⁻¹.AQA
enthalpy of solution: The sum of the lattice dissociation enthalpy and the hydration enthalpies of each ion present, giving the enthalpy change when an ionic solid dissolves.AQA
Enthalpy profile diagram: Plots enthalpy on the vertical axis against reaction progress on the horizontal axis, with reactants and products at different enthalpy levels.OCR
Enthalpy profile diagrams: These show the different route with lower activation energy, with reactants and products at the same enthalpy levels whether or not a catalyst is present.OCR
enthalpy profiles: On these the products sit below the reactants for an exothermic change and above them for an endothermic one, and both still show an activation energy hump.AQA
Enthalpy sign and exothermic formation: Energy is released as oppositely charged ions pack together, so the value is usually a large negative value for a stable lattice.OCR
entropy: Increases on dissolution, so a solid can dissolve even when the enthalpy change of solution is positive or slightly endothermic.AQA, OCR
entropy as the missing term: The increase in the dispersal of energy and matter when an ordered solid becomes free moving particles is the missing factor pushing these processes forward.AQA
Entropy change and the T∆S term: The entropy term is written T∆S, so its sign follows ∆S: a positive ∆S gives a positive T∆S, and a negative ∆S gives a negative T∆S.OCR
Entropy of the system: Changes in the entropy of the system can be predicted qualitatively by comparing the number of gaseous molecules on each side of a balanced equation.OCR
Environmental and ethical issues: Issues linked to decision making about biofuel use, such as land used for biofuel crops causing deforestation, habitat loss, and raising food prices by…AQA
Environmental benefits: A reduced accumulation of waste and less harm to wildlife and habitats that persistent plastics cause.OCR
Environmental consequences of polymer disposal: The halogen content of a plastic determines which toxic waste product forms on burning, and this product must be captured rather than emitted.OCR
Environmental impact of polymer waste: Waste polymers persist in landfill and can pollute land and water, and burning polymers releases carbon dioxide and can release toxic gases unless controlled.OCR
enzyme sensitivity to pH: A shift of only a few tenths of a pH unit disrupts the shapes of enzymes and of haemoglobin, so the margin is narrow.AQA
enzyme-substrate complex: Formed when the substrate binds to the active site, holding it in exactly the orientation the reaction needs.AQA
equal bond lengths: All six carbon-carbon bonds are the same length, 0.139 nm, between a single bond at 0.154 nm and a double bond at 0.134 nm.AQA
equal effect on both directions: A catalyst lowers the activation energy of the forward and the reverse reactions by the same amount, so both are speeded up equally.AQA
equal forward and reverse rates: The forward and reverse reactions proceed at equal rates, so every molecule of product formed is matched by one converted back.AQA, OCR
equations using [O]: Equations showing [O] as oxidant are acceptable, with water shown only where it is formed.AQA
equations using 2[H]: Overall equations for reduction reactions use 2[H] as the reductant, and writing [H] once instead of 2[H] leaves the equation unbalanced.AQA
Equilibrium and reversibility: The reaction is an equilibrium, so it is reversible and does not go to completion unless a product is removed.OCR
Equilibrium concentrations after partial neutralisation: In a buffer made from a weak acid partly neutralised by strong base, [HA] and [A⁻] come from the amounts present after reaction, divided by the total volume.OCR
Equilibrium concentrations and units: Each concentration in the expression is the equilibrium concentration in mol dm⁻³, and pure solids and pure liquids are omitted because their concentration is…OCR
Equilibrium constant: Expresses the ratio of products to reactants at equilibrium, and the system adjusts the position of equilibrium so that this ratio is restored whenever it is…OCR
Equilibrium constant and temperature dependence: The equilibrium constant Kc depends only on temperature.OCR
Equilibrium constant and yield unaffected by catalysts: The catalyst does not alter the equilibrium yield or the value of the equilibrium constant.OCR
Equilibrium constant calculations: Equilibrium values are substituted into the correct Kc or Kp expression to obtain the required quantity.OCR
Equilibrium constant expressions for Kc and Kp: Written as products over reactants, with each concentration or partial pressure raised to its stoichiometric coefficient.OCR
Equilibrium constant Kc: Depends only on temperature for a given reaction, being the ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power.AQA, OCR
equilibrium constant unchanged: The equilibrium constant is unaffected by a catalyst.AQA
Equilibrium expression for water self-ionisation: Kw is the equilibrium constant for the self-ionisation of water, H₂O ⇌ H⁺ + OH⁻, written Kw = [H⁺][OH⁻].OCR
Equilibrium expressions from balanced equations: Written with product terms in the numerator and reactant terms in the denominator, each raised to its stoichiometric coefficient.OCR
equilibrium position: An electrode potential is the position of an equilibrium, so anything that shifts that equilibrium changes E.AQA
Equilibrium position and yield versus rate: A catalyst speeds up the approach to equilibrium but does not change the position of equilibrium or the equilibrium yield.OCR
equilibrium reached sooner: Both reactions are speeded up equally, so equilibrium is reached sooner.AQA
Equivalence point: Marked by a near-vertical steep section on a pH curve, which spans pH 3-11 for strong acid-strong base.AQA, OCR
equivalence point pH: Above pH 7 for a weak acid with a strong base, below pH 7 for a strong acid with a weak base.AQA
equivalent protons: Protons in identical surroundings give a single peak, so symmetry cuts the number of peaks actually seen.AQA
Equivalent protons and signal number: The twelve protons of TMS are equivalent, so TMS produces a single reference peak rather than a cluster.OCR
errors in enthalpy of combustion: An experimental enthalpy of combustion from a calorimeter comes out less exothermic than the data book value, and incomplete combustion is a standard mark…AQA
Ester and amide formation: Reaction of acyl chlorides with an alcohol to give an ester, or with ammonia or an amine to give an amide, under mild conditions.OCR
Ester and amide link recognition: Locating each –COO– ester group in a polyester or each –CONH– amide link in a polyamide before splitting the chain there.OCR
Ester and amide linkages: A diol plus a dicarboxylic acid gives a polyester with ester links, while a diamine plus a dicarboxylic acid gives a polyamide with amide links.OCR
ester and amide links: A polyester repeat unit contains one residue of each monomer joined by two ester groups, while a polyamide repeat unit has –CONH– amide links.AQA, OCR
Ester functional group: The linkage broken in hydrolysis, namely the carbon–oxygen single bond between the carbonyl carbon and the alkoxy oxygen.OCR
Ester functional group structure: The ester link is written as –COO–, in which a carbonyl carbon is bonded to an oxygen that is itself bonded to a carbon chain.OCR
ester linkage: A structure in which the acidic hydrogen is replaced with an alkyl group, giving R-COO-R′, with the C=O staying put and the second alkyl group hanging off the…AQA, OCR
Ester linkage structure in polyesters: The ester groups in polyesters are the –COO– linkages repeated along the chain, formed by condensation between a dicarboxylic acid and a diol.OCR
Esterification: A carboxylic acid and an alcohol react, in the presence of an acid catalyst, to give an ester and water.AQA, OCR
Esterification of amino acids: With an alcohol and an acid catalyst the carboxylic acid group reacts reversibly to give an ester and water, the ester link forming between the carboxyl…OCR
Estimates from mean values: Because the values are averages, the calculated ΔH is an estimate and may differ from a measured value.OCR
Estimating reaction enthalpy: Adding the mean bond enthalpies of the bonds broken and subtracting the mean bond enthalpies of the bonds formed for a gas-phase reaction.OCR
Ethanol as a solvent for mixing: Ethanol is present as a solvent so the haloalkane mixes with the aqueous silver nitrate.OCR
Ethene as an industrial feedstock: The feedstock is valuable because the double bond can be opened and new single bonds formed, building larger or functionalised molecules.OCR
evaluating a treatment: Giving both sides and a conclusion, here the benefits to health of water treatment by chlorine against its toxic effects.AQA
Even heat distribution: Spreading heat around a vessel for higher, more even temperatures, as a sand bath does.OCR
evidence for mechanism: The rate equation is a window into the mechanism, but it is evidence consistent with one rather than proof of a single unique mechanism.AQA
evidence into legislation: Replication by independent groups, not the authority of any one scientist, is what turned evidence into legislation.AQA
Exact versus approximate Ka calculations: The exact treatment solves the quadratic from Ka = x²/([HA]₀ − x), where x = [H⁺], instead of using the approximation.OCR
Examples of iodine and ice: In iodine, I₂ molecules pack in a lattice held together by weak intermolecular forces; in ice, hydrogen bonds between water molecules hold the lattice…OCR
excess ammonia: A large excess of ammonia favours the primary amine when a halogenoalkane is heated with ammonia dissolved in ethanol in a sealed tube.AQA
excess of one reagent: A large excess of methane keeps most molecules at the mono-substituted stage, while an excess of chlorine drives the mixture towards the more highly…AQA
Exchangeable O–H and N–H protons: These are protons on oxygen or nitrogen whose signal disappears from the ¹H NMR spectrum after D₂O is added.OCR
Exothermic: A reaction in which the system loses enthalpy to the surroundings, so the surroundings warm and ∆H carries a negative sign.OCR
Exothermic and endothermic forward reactions: For an exothermic forward reaction, raising the temperature decreases the equilibrium constant, while for an endothermic forward reaction, raising the…OCR
Exothermic and endothermic reactions: A reaction is exothermic when bond making releases more energy than bond breaking absorbs, and endothermic when bond breaking absorbs more energy than bond…OCR
exothermic combustion: Burning an alkane is strongly exothermic because the bonds made in the products are stronger overall than the bonds broken in the fuel and in oxygen.AQA
exothermic lattice formation: Lattice enthalpy is exothermic, so a stable giant ionic lattice has a negative value.OCR
Exothermic nature of ion–dipole attraction: Energy is released as ion–dipole attractions form between the ion and the δ⁻ oxygen or δ⁺ hydrogen ends of water, so the process is always exothermic for a…OCR
Exothermic reaction: The products lie lower than the reactants, so ΔH is negative and the arrow points downwards from reactants to products.OCR
Exothermic sign convention: The sign is kept negative for an exothermic neutralisation because the reaction releases energy to the surroundings.OCR
Experimental control: Keeping the concentrations of all other reactants constant so the deduction applies to the chosen reactant.OCR
Experimental limitations: Factors that cannot be controlled, such as ambient room temperature, which should be acknowledged rather than ignored.OCR
Experimental rate equations: Reflect the species involved up to and including the rate-determining step, or in a fast equilibrium before it.OCR
Exponential dependence of k on temperature: The rate constant k rises exponentially as temperature T increases, because T appears in the exponent –Ea/RT of the Arrhenius equation.OCR
exponential increase: A plot of rate against temperature is not a straight line but a curve that steepens, an exponential rise.AQA
Extension to Ka and Kw: The same construction transfers to Ka for a weak acid, written as [H⁺][A⁻]/[HA] from the dissociation equation, and to Kw = [H⁺][OH⁻].OCR
Extent of dissociation and acid strength: A larger Ka means the acid dissociates to a greater extent, so Ka measures acid strength.OCR
extrapolation to time of mixing: Extending the cooling section back to the time of mixing to obtain a corrected temperature change with no heat loss.AQA

F

factor of ten per unit: One whole pH unit corresponds to a factor of ten in hydrogen ion concentration, because the logarithm is to base 10.AQA
Faster, catalyst-free route compared with carboxylic acids: The anhydride reacts faster and does not need a strong acid catalyst, and it avoids the reversible equilibrium seen with a carboxylic acid and alcohol.OCR
feasibility: An oxidising agent will oxidise the reduced species of any half-equation with a less positive E⦵, and a positive overall EMF is the test.AQA
Feasibility and the sign of ∆G: A process is feasible when ∆G has a negative value, and a positive ∆G indicates the process is not feasible under those conditions.OCR
feasibility criterion: A reaction is feasible when the value of ∆G is zero or negative.AQA
Feasibility from the sign of E°cell: A positive E°cell indicates the discharge reaction is feasible under standard conditions.OCR
Feasibility of redox reactions: The cell emf is the difference between the two E⦵ values and is positive for a feasible reaction.OCR
feasibility prediction: Compare E⦵ values so that the more positive half-equation runs forwards as a reduction and the more negative one is forced backwards as an oxidation.AQA, OCR
Feedstock recycling: Breaks polymer chains back to monomers or useful small molecules, so the carbon can re-enter the chemical industry instead of being lost.OCR
Fehling's red precipitate: A blue copper(II) complex gives a red precipitate with an aldehyde and stays blue with a ketone.AQA
Fehling's solution: A blue solution containing Cu²⁺ complexes in alkaline tartrate, which an aldehyde reduces to a brick-red precipitate of copper(I) oxide, Cu₂O, when warmed…AQA, OCR
Fermentation: The breakdown of sugars by enzymes in yeast in the absence of oxygen to produce ethanol and carbon dioxide.AQA
fermentation of glucose: Ethanol is produced industrially by this process, in which enzymes in yeast convert glucose to ethanol in the absence of air.AQA
fermentation then dehydration: Sugar from a crop is fermented to ethanol, then the ethanol is dehydrated over an acid catalyst to ethene.AQA
fibres and mouldings: Terylene can be drawn into a fine fibre that resists stretching and does not crease easily, and the same polymer moulded rather than spun becomes the drinks…AQA
filter selection: Choosing the filter that transmits the light the solution absorbs most strongly, which is the complementary colour to the one you see.AQA
Filtrate: The liquid that passes through the filter medium, collected separately from the solid left behind.OCR
Filtration under reduced pressure: Separates a solid from a liquid quickly by pulling air out of the receiving flask with a water pump or vacuum line, so liquid is drawn through faster than…OCR
Filtration under reduced pressure and washing: Crystals are collected using a Büchner funnel and washed with a little cold solvent to remove soluble impurities from the crystal surface.OCR
fingerprint region: Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon…AQA
Fingerprint region and compound matching: The region below about 1500 cm⁻¹ gives a pattern useful for identifying a specific compound by comparison with a database.OCR
fingerprint region and database matching: The complex region below about 1500 cm⁻¹, used for matching a whole spectrum to a database rather than for assigning individual bonds.OCR
Fingerprint region and matching to reference spectra: The fingerprint region below about 1500 cm⁻¹ is used to confirm a match with a reference spectrum.OCR
First and second ionisation energies: Both decrease down Group 2, meaning less energy is needed to remove the two outer electrons.OCR
First electron affinity: The enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1⁻ ions.OCR
First ionisation energy: The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.AQA, OCR
first ionisation energy dips: Rises overall across the period but dips twice, at aluminium and at sulfur, for reasons of sub-shell energy and electron repulsion.AQA
First order kinetics: A reaction in which the half-life is constant and independent of concentration, so the time for the reactant concentration to fall from any value to half that…OCR
First order: constant half-life: The concentration falls exponentially, giving a curve whose gradient decreases steadily, and each successive halving of concentration takes the same time.OCR
first point of difference: If the two attached atoms are the same element, moving one atom further out along each branch and comparing the sets of atoms found.AQA
Fixed activation energy: The activation energy itself does not change with temperature; what changes is the proportion of molecules exceeding it.OCR
Fixed number of orbitals per sub-shell: Each sub-shell is built from a fixed number of orbitals, with an s-sub-shell containing one, a p-sub-shell containing three and a d-sub-shell containing five.OCR
fixed values and exceptions: Group 1 is +1, Group 2 is +2 and fluorine is always −1, with hydrogen and oxygen having stated exceptions.AQA
Flammable liquids: Liquids kept away from naked flames and hot surfaces, with lids on containers and good ventilation.OCR
flammable organic liquids: Because the liquids are flammable, heating is done in a water bath rather than over a flame.AQA
flight time and mass: Since kinetic energy is ½mv², equal energy means a heavier ion must be slower, so the flight time t = d/v separates ions by mass.AQA
flue gas desulfurisation: A slurry of powdered calcium oxide or calcium carbonate in water is sprayed into the rising gases, and the solid produced is calcium sulfite.AQA
fluoridation: Fluoride reduces tooth decay, but at higher doses there is dental fluorosis and the objection that nobody can opt out of what comes from their tap.AQA
Fluted filter paper: Paper folded into a concertina and opened into a cone, increasing the exposed surface so liquid passes faster than a plain cone.OCR
Forces broken on melting: Melting point reflects the forces broken on melting: strong metallic bonds in the metals, many strong covalent bonds in silicon, and only weak intermolecular…OCR
forces overcome on melting: Melting means overcoming strong electrostatic attraction, breaking covalent bonds, or separating molecules held by weak intermolecular forces.AQA
Formation by mixing acid with its salt: Route one mixes a weak acid with a salt of that weak acid, for example CH₃COOH with CH₃COONa.OCR
Formation by partial neutralisation with excess weak acid: Route two makes the conjugate base in situ by partial neutralisation: an excess of weak acid reacts with a limited amount of strong alkali, so some weak acid…OCR
formation data: Arrows point up from elements, so the enthalpy change is the sum of the products' enthalpies of formation minus the reactants'.AQA
Formation of 1⁻ ions: A halogen atom gains an electron into its outer shell, and the ease of this gain depends on the attraction between the incoming electron and the nucleus.OCR
Formation of 2,4,6-tribromophenol: Substitution occurs at all three available 2-, 4- and 6-positions, each substitution replacing one ring hydrogen with one bromine atom, so three bromine…OCR
Formation of 2⁺ ions: Lower ionisation energies mean easier removal of two electrons to form a 2⁺ ion, making the metal more reactive down the group.OCR
Formation of a chiral centre and racemic mixture: A new chiral centre can be created at the carbon bearing OH and CN, so a racemic mixture may form.OCR
Formation of alcohols from haloalkanes: With a primary haloalkane the main organic product is a primary alcohol, as the hydroxide supplies the new O–H group, for example bromoethane with aqueous…OCR
Formation of alkaline hydroxide solutions: The resulting solutions are alkaline because the hydroxide ion is released.OCR
Formation of amides: Amides are formed when a carboxylic acid derivative such as an acyl chloride or acid anhydride reacts with ammonia or an amine.OCR
Formation of ammonium salts with dilute acids: When an amine reacts with dilute hydrochloric acid, the lone pair forms a dative bond to H⁺ and the product is the substituted ammonium salt.OCR
Formation of bleach: Sodium chlorate(I), NaClO, is the active ingredient of household bleach, which oxidises stains and microorganisms.OCR
Formation of chlorinated hydrocarbons: Chlorine reacts with organic matter to form chlorinated hydrocarbons, some of which are suspected carcinogens.OCR
Formation of copper(II) ammine and chloride complexes: [Cu(NH₃)₄(H₂O)₂]²⁺ forms from [Cu(H₂O)₆]²⁺ with excess ammonia, and [CuCl₄]²⁻ forms from [Cu(H₂O)₆]²⁺ with concentrated hydrochloric acid.OCR
Formation of dihaloalkanes: The product has one halogen atom on each of the two adjacent carbon atoms, as when ethene with bromine gives 1,2-dibromoethane.OCR
Formation of esters from carboxylic acids and alcohols: Esters are compounds formed when a carboxylic acid reacts with an alcohol, releasing water.OCR
Formation of esters, carboxylic acids and amides: Water hydrolyses an acyl chloride to the carboxylic acid, an alcohol gives an ester, ammonia gives a primary amide and a primary amine gives a secondary amide.OCR
Formation of halide ions by electron gain: In many redox reactions each halogen atom gains one electron, filling the p subshell to s²p⁶ and forming a halide ion with a single negative charge.OCR
Formation of hydroxynitriles: The product contains both a hydroxyl group and a nitrile group on the same carbon skeleton, which is why it is called a hydroxynitrile.OCR
Formation of insoluble metal hydroxides: A precipitation reaction forms an insoluble solid when two aqueous solutions are mixed, with the hydroxide of the metal separating as a coloured precipitate.OCR
Formation of metal hydroxides and hydrogen: The metal is oxidised to a 2⁺ ion while hydrogen in water is reduced from 1⁺ to 0 in H₂, and two hydroxide ions balance one 2⁺ metal ion.OCR
Formation of metal oxides MO: Heating a Group 2 metal in oxygen gives a solid metal oxide with the general formula MO, such as magnesium oxide MgO or calcium oxide CaO.OCR
Formation of one mole of water: The definition fixes the amount of water at 1 mol, so values are quoted per mole of water.OCR
Formation of saturated alkanes: The product is the saturated alkane, with the same carbon skeleton and the C=C double bond converted to a C–C single bond.OCR
formation of sulfur dioxide: Sulfur in a fuel is oxidised during combustion, forming sulfur dioxide by the equation S + O₂ → SO₂.AQA
Formula of a hydrated salt: Found by determining the moles of anhydrous salt and the moles of water separately, then dividing both by the smaller to obtain the ratio.OCR
formula units: The simplest unit of an ionic compound, whose ions' relative atomic masses are added to give the relative formula mass.AQA
Formulae of common acids: The common acids named in the specification are HCl, H₂SO₄, HNO₃ and CH₃COOH.OCR
Formulae of common alkalis: The common alkalis named in the specification are NaOH, KOH and NH₃.OCR
Formulae of ionic compounds: An ionic compound such as MgCl₂ is represented by its empirical formula.OCR
four co-ordinate complexes: Complexes with a co-ordination number of 4, usually allowed by bulky ligands such as Cl⁻.AQA
four crystal types: Ionic, metallic, macromolecular (giant covalent) and molecular are the four types of crystal structure.AQA
four different groups: What a carbon must be bonded to for a molecule to be chiral, making that carbon an asymmetric or chiral centre.AQA
four hydrogen bonds per water molecule: Each water molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen.AQA
Four-fold coordination: A central metal ion is bonded to four donor atoms, which can lie in one plane or sit at the corners of a tetrahedron.OCR
Fractional abundance: The weighting of each isotope in the weighted mean, so abundant isotopes influence the value more.OCR
fractional distillation: A separation method that works because ethanol boils at 78 °C and water at 100 °C, so the vapour above the heated mixture is richer in ethanol.AQA
fractionating column: A column in which the vapour repeatedly condenses and re-evaporates, each cycle enriching it further.AQA
Fragmentation: The process in which the radical cation breaks into a charged fragment and a neutral radical, with only charged fragments appearing as peaks.OCR
fragmentation and bond breaking: Fragmentation follows the weakest bond, so the spectrum reveals structural pieces as pieces lost from the molecular ion.OCR
fragmentation fingerprint: A pattern that acts as a fingerprint, which software matches against a database of known spectra to identify the component.AQA
Free rotation about σ-bonds: Because the σ-bond electron density is symmetrical about the bond axis, rotation about a C–C σ-bond does not break the bond, so the groups can rotate freely…OCR
Freedom of motion of particles in each state: Particles in a gas move freely through a large volume, particles in a liquid can slide past one another but remain close together, and particles in a solid…OCR
Friedel–Crafts acylation: An acyl chloride with a halogen carrier gives an acylium ion that attacks the ring to form a ketone, useful because the C=O can be reduced to an alkyl chain.OCR
Friedel–Crafts acylation and the acylium ion: An acyl chloride with aluminium chloride generates an acylium ion, RCO⁺, giving a ketone such as phenylethanone.OCR
Friedel–Crafts alkylation: A haloalkane with a halogen carrier generates an electrophile, here a carbocation, which attacks the electron-rich ring to give an alkyl substituent.OCR
Friedel–Crafts reactions: Reactions in which a benzene ring gains a new carbon–carbon bond to a substituent, either by alkylation using a haloalkane or by acylation using an acyl…OCR
from evidence to structure: Putting the molecular formula, the functional groups and the environments together so that usually only one structure survives.AQA
Fuel cell: Uses the energy from the reaction of a fuel with oxygen to create a voltage, producing a voltage while fuel and oxygen are supplied.OCR
fuel cells: Devices used to generate an electric current that do not need to be electrically recharged.AQA
Full dissociation of soluble salts: A soluble ionic salt such as CH₃COONa dissociates fully to give CH₃COO⁻ ions and Na⁺ ions.OCR
full equations: Shows every substance taking part, with correct formulae and, where required, state symbols.AQA
fume cupboard: Working in one removes any hydrogen cyanide, and it is a named precaution paired with the release of toxic hydrogen cyanide gas as its reason.AQA
Functional group: The atom or group of atoms responsible for the characteristic reactions of a compound.AQA, OCR
Functional group identification: Using the position and shape of absorption bands to distinguish functional groups, for example a broad absorption around 3200–3600 cm⁻¹ indicating an O–H…OCR
Functional group interconversion: An ester is converted into a carboxylic acid and an alcohol, the two products identified by functional group.OCR
Functional group interconversion and equation balancing: The equation is RCOOH + SOCl₂ → RCOCl + SO₂ + HCl, balanced by counting O, S and Cl as well as C and H.OCR
functional group isomers: Isomers that have a different functional group altogether while keeping the same formula.AQA
Functional group recognition: Naming the functional group or groups present in the given structure before predicting behaviour.OCR
Functional group to reaction mapping: Using the functional groups you have identified to say how a molecule will behave, linking each group to its characteristic chemistry.OCR
Functional groups: The groups kept explicit in a skeletal formula, such as the OH group placed on the correct carbon of the chain.OCR
Functional groups of aldehydes and carboxylic acids: The aldehyde functional group is –CHO and the carboxylic acid group is –COOH.OCR
Functional groups: amine and carboxylic acid: An amine group (NH₂) and a carboxylic acid group (COOH) carried on the same carbon, the α-carbon.OCR
further substitution: The sequence runs on because the amine still has a lone pair, so it attacks another halogenoalkane, through the secondary and tertiary amine to the quaternary…AQA
Further substitution giving polyhalogenated products: The halogenoalkane first formed can react again so that the halogen substitutes at the same carbon or elsewhere, giving di- and poly-substituted products.OCR

G

gas chromatography: A column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.AQA
gas chromatography with mass spectrometry: A combination in which the components separated by gas chromatography are fed into a mass spectrometer, which records a spectrum for each component as it…AQA
Gas confirmation: The gas is confirmed by bubbling it through limewater, which turns milky or cloudy.OCR
Gas detection using litmus: The gas is detected by its pungent smell and by turning damp red litmus paper blue, because NH₃ is an alkaline gas that dissolves to form OH⁻ ions.OCR
Gas volume measurement: Collect gas over water in an inverted measuring cylinder or use a gas syringe, reading the volume at room temperature and pressure.OCR
Gaseous molecules: Gases have far more freedom of motion and far more accessible microstates than liquids or solids.OCR
gaseous phase assumption: The method assumes all species are gaseous, ignoring intermolecular forces in liquids or solids.AQA
gaseous state: The state specified in the definition because breaking a bond in a liquid would also have to pay for the intermolecular forces.AQA
Gaseous-state condition: The value applies to gaseous molecules, not to liquids or solids.OCR
General formula: The formula of a homologous series into which the value of n is substituted to predict the formula of any member of the series.AQA, OCR
General formula and relative molecular mass: The general formula stays the same, and the CH₂ step means relative molecular mass rises by 14 each time.OCR
General formula of alkyl groups: For n carbon atoms there are 2n + 1 hydrogen atoms, so the formula is CₙH₂ₙ₊₁.OCR
General formula of α-amino acids: Written as RCH(NH₂)COOH, showing the α-carbon bonded to R, H, NH₂ and COOH.OCR
Generation of a halogen electrophile: The halogen carrier polarises the halogen molecule and helps generate a halogen electrophile, for example Br⁺, which attacks the electron-rich ring.OCR
Generation of electrophiles by catalysts: The electrophile is often generated from a reagent by a catalyst such as FeBr₃, AlCl₃ or concentrated H₂SO₄.OCR
Generation of the nitronium ion: The nitronium ion, NO₂⁺, is generated by protonation and loss of water from nitric acid, with concentrated sulfuric acid acting as catalyst and dehydrating…OCR
Geometric (cis–trans) isomerism: Two identical ligands can be either adjacent (cis) or opposite (trans), as in square planar [Pt(NH₃)₂Cl₂] or octahedral [Co(NH₃)₄Cl₂]⁺.OCR
Giant covalent lattice: A continuous three-dimensional network of atoms joined by strong covalent bonds, with no discrete molecules.OCR
Giant Covalent Structure (Macromolecular): A massive 3D lattice of atoms held together entirely by strong covalent bonds.AQA
giant ionic lattice: The arrangement the ions build rather than a molecule, with the attraction acting in all directions.AQA, OCR
Giant ionic lattice structure: A regular three-dimensional array of alternating oppositely charged ions, as in sodium chloride where Na⁺ and Cl⁻ alternate.OCR
Giant lattice structure: Cations arranged in a regular lattice with a sea of delocalised electrons that are free to move throughout the structure.OCR
Giant metallic structure: A lattice of positive metal ions held together by electrostatic attraction to delocalised electrons.AQA
Giant metallic versus giant covalent versus simple molecular: Sodium, magnesium and aluminium form giant metallic lattices; silicon forms a giant covalent (macromolecular) lattice; phosphorus, sulfur, chlorine and argon…OCR
giant, simple molecular and monatomic structures: Melting point follows structure: giant metallic and giant covalent elements, then simple molecular elements, then monatomic argon.AQA
Gibbs equation and free energy change: The free energy change, ∆G, is calculated as ∆G = ∆H – T∆S, where ∆H is the enthalpy change, ∆S the entropy change and T the temperature in kelvin.OCR
Gibbs free energy: Combines the two competing factors in one number, given by the relationship ∆G = ∆H – T∆S.AQA
Gibbs free energy and feasibility: The relation ∆G = ∆H − T∆S links the enthalpy change and the entropy term, and a process is feasible when ∆G is negative, equivalently when ∆H − T∆S < 0.OCR
glycerol: Propane-1,2,3-triol, given as the second product of the alkaline hydrolysis.AQA
glycerol backbone: Propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon, each of which can be esterified.AQA
Gradient: Measured by picking two points far apart on the straight line of best fit, reading their coordinates from the axes, and dividing the change in the y-value by…OCR
gradient and activation energy: The activation energy comes from multiplying the gradient by −R.AQA
Gradient of concentration–time graph: The rate of reaction at a particular moment equals the gradient of the concentration–time graph at that moment.OCR
Graph reading and comparison: Halve the concentration value, read off the corresponding time, and repeat from a lower concentration to compare the two time intervals.OCR
Graphene: A single layer of graphite, one atom thick.OCR
Graphical determination of activation energy: Measuring k at several temperatures, converting each T to kelvin, plotting ln k against 1/T and drawing a line of best fit, then calculating Ea = –gradient × R.OCR
Graphical determination of rate constant: The gradient of a rate against concentration graph equals the rate constant k for a first order reaction.OCR
Graphite: Each carbon atom bonds to three others in a hexagonal layer, and the layers are held by weak intermolecular forces, with delocalised electrons between layers.OCR
Greenhouse gases and their bonds: Atmospheric gases containing C=O, O–H and C–H bonds, such as CO₂, H₂O and CH₄, absorb infrared radiation at characteristic wavenumbers.OCR
ground state and excited state: An electron in the lower group absorbs a photon and jumps to the upper group, which is the ground state to excited state transition.AQA
Group 17 trends: The larger radius and greater shielding outweigh the rising nuclear charge, so the tendency to form a 1⁻ ion decreases and reactivity falls.OCR
Group 2 compounds as bases: Magnesium hydroxide and calcium carbonate act as antacids because they are bases that neutralise excess hydrochloric acid in the stomach.OCR
Group 2 metal reactions with water: Group 2 metals react with water to give the metal hydroxide and hydrogen gas.OCR
Group 2 oxides as basic anhydrides: Group 2 oxides react with water to form the metal hydroxide, for example CaO(s) + H₂O(l) → Ca(OH)₂(aq).OCR
Group 2 reactions with dilute acids: The assessed skill is predicting and describing how Group 2 metals react with dilute acids, naming the salt formed from the acid used and identifying the gas…OCR
Group 2 reactivity increasing down the group: Reactivity increases down the group as the first and second ionisation energies fall and the atoms become larger, so the metals react more vigorously with…OCR
Group 2 reactivity trends: Reactivity increases down the group because the first and second ionisation energies decrease as atomic radius increases and shielding grows, so the outer…OCR
Group 2 sulfate solubility: The sulfates of the elements Mg–Ba become less soluble in water down the group, the opposite way round from the hydroxides.AQA
Group 2 trend: Values fall down the group as radius and shielding increase.AQA
Group 7 reactivity decreasing down the group: Reactivity decreases down the group because the outer shell is further from the nucleus and more shielded, so oxidising power falls.OCR
Group number prediction: The number of electrons in the outermost shell equals the group number for main-group elements.OCR
Group trends in bond strength: A stronger bond needs more energy to break, so the rate trend is C–F < C–Cl < C–Br < C–I.OCR
guanine as the target: The base whose ring nitrogen keeps a lone pair not used for bonding, which attacks the platinum.AQA

H

H⁺ as the reacting acid species: In aqueous solution an acid supplies H⁺(aq), and this ion is the species that reacts in each case.OCR
Haber process: Makes ammonia in the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), an exothermic reversible reaction run at roughly 450 °C and about 200 atm over solid iron.AQA
haemoglobin structure: The whole assembly, four haem groups held within one globin protein, so a single haemoglobin can carry up to four O₂ molecules.AQA
Half-cell construction: A metal or non-metal in contact with its own ions in aqueous solution, where the aqueous ions are supplied by a solution of a soluble salt of that element.OCR
Half-cells and electrodes: Each half-cell is prepared by dipping a clean metal electrode into a solution of its own ions, for example a copper electrode in copper(II) sulfate solution.OCR
Half-equation writing and electron balance: Each half-equation is written with atoms, charge and electrons balanced before combining them.OCR
Half-equations: Separate oxidation and reduction equations in which atoms other than O and H are balanced first, then O with H₂O, H with H⁺ in acidic conditions, and charge…AQA, OCR
Half-equations and electron balance: The anode half-equation is H₂ + 2OH⁻ → 2H₂O + 2e⁻ and the cathode half-equation is O₂ + 2H₂O + 4e⁻ → 4OH⁻.OCR
Half-equations and electron transfer: One electron is transferred per iron ion, so the half-equation Fe²⁺ → Fe³⁺ + e⁻ and the reverse reduction half-equation are written and balanced for charge…OCR
half-equations in acid: Equations balanced with H⁺ and H₂O as well as electrons, such as VO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.AQA
halide ion test: The use of acidified silver nitrate solution to identify and distinguish between halide ions.AQA
Halide tests with silver nitrate: Acidifying with dilute nitric acid and adding silver nitrate gives a white precipitate for chloride, cream for bromide and yellow for iodide.OCR
Haloalkanes: The product formed when the –OH group of an alcohol is replaced by a halide, with the carbon skeleton unchanged.OCR
Haloalkanes as substrates: The haloalkane is attacked at its carbon by the nucleophile, with the halide ion displaced as the leaving group.OCR
Halogen carrier: A Lewis acid catalyst such as aluminium chloride or iron(III) chloride that accepts a chloride ion from the haloalkane or acyl chloride to generate a reactive…AQA, OCR
Halogen carrier in benzene bromination: Benzene needs a halogen carrier such as FeBr₃ or AlBr₃ to generate a strong electrophile, whereas an alkene reacts directly.OCR
Halogen displacement reactions: Reactions in which a more reactive halogen oxidises a halide ion of a less reactive halogen.OCR
Halogen reactions with alkali: A reaction such as chlorine with cold dilute aqueous sodium hydroxide, in which chlorine is both oxidised and reduced, forming chloride ions and chlorate(I)…OCR
Halogen reactions with water: A reaction in which chlorine with water forms hydrochloric acid and chloric(I) acid, with chlorine both oxidised and reduced.OCR
Halogenation of benzene: A halogen such as Br₂ needs a halogen carrier such as FeBr₃ or AlCl₃ to polarise the molecule and generate Br⁺, followed by the same attack, intermediate and…OCR
Halogenation products and equations: Benzene plus the halogen forms a halogenobenzene such as bromobenzene plus hydrogen halide, shown in the overall equation.OCR
Halogenation with a halogen carrier: A halogen carrier such as a iron(III) halide or aluminium chloride polarises the halogen and produces the electrophile, giving a halogenoarene.OCR
Halogenoalkane: An alkane in which one or more hydrogen atoms have been replaced by a halogen atom.AQA
Halogens as oxidising agents: Because the halogen gains electrons in the reaction, the halogen itself is the oxidising agent.OCR
hazard reduction: Non-hazardous starting materials cut the risk to workers and to the environment.AQA
heat change q: Given by the equation q = mc∆T, where m is the mass of the substance that has a temperature change ∆T and c is its specific heat capacity.AQA
Heating under reflux: A technique in which a vertical condenser returns condensed vapour to the flask, so the mixture can be boiled for a long period without losing solvent or…OCR
helium as an exception: It is placed above the noble gases due to its inert chemical behaviour, but its 1s² configuration makes it an s-block element.AQA
Hess cycle: A route that starts from the reactants and products of the reaction and takes both down to the same combustion products, such as CO₂(g) and H₂O(l).OCR
Hess’ law: The enthalpy change of a reaction is independent of the route taken, provided the initial and final states are the same.OCR
Hess's law: Used with standard enthalpy changes of formation to find an enthalpy change of reaction, following the route reactants → elements → products.AQA, OCR
Hess's law and route independence: The total enthalpy change is independent of route, so the sum of the steps one way equals the sum of the steps the other way.OCR
Hess's law for indirect determination: Used when a reaction cannot be measured directly, for example combining enthalpy of combustion or formation data, or measuring a related reaction and…OCR
Hess's law rearrangement: Setting the enthalpy of formation equal to the sum of the steps on the indirect route, then rearranging for the unknown term before substituting values.OCR
heterogeneous and homogeneous: Heterogeneous catalysts are in a different phase from the reactants, typically a solid surface with gases passing over it, while homogeneous catalysts share…AQA
Heterogeneous and homogeneous catalysis: Heterogeneous catalysts provide a surface on which reactant molecules adsorb, weaken their bonds and react, while homogeneous catalysts form intermediate…OCR
Heterogeneous catalysis: A catalyst is in a different physical state from the reactants, typically a solid catalyst with gaseous or liquid reactants.OCR
heterogeneous catalyst: A catalyst that is in a different phase from the reactants.AQA
Heterolytic fission: Shown by an arrow starting on a bond and pointing to one of the two atoms, which takes both electrons and becomes a negative ion while the other becomes…AQA, OCR
high atom economy: Because nothing else is formed, the atom economy is one hundred per cent.AQA
High bond enthalpy of C–C and C–H bonds: The C–C and C–H bonds are strong σ-bonds with high bond enthalpies, so a lot of energy is needed to break them.OCR
high electron density: A region of the double bond where π electrons are exposed and held less tightly than σ electrons.AQA
high resolution: A property of an instrument that measures the mass to four or five decimal places, so formulae that appear identical at whole-number resolution separate…AQA
high temperature and pressure: The severe conditions that break C–C bonds homolytically, giving a free-radical mechanism.AQA
high-resistance voltmeter: A high-resistance voltmeter connects the two electrodes and reads the e.m.f.AQA, OCR
High-resolution ¹H NMR: A technique whose spectra show spin–spin splitting because a proton's signal is influenced by the magnetic states of non-equivalent protons on adjacent carbon…OCR
High-resolution mass spectrometry and molecular formula: Used to fix the molecular formula and degree of unsaturation from the data before candidate structures are tested.OCR
Homogeneous and heterogeneous catalysis: In homogeneous catalysis the catalyst is in the same phase as the reactants; in heterogeneous catalysis the catalyst is in a different phase, typically a…OCR
homogeneous catalysis: A catalyst is in the same physical state as the reactants, so it is usually dissolved in the same solution or present in the same gas mixture.AQA, OCR
homogeneous catalyst: A catalyst that shares a phase with the reactants.AQA
homogeneous equilibria: Equilibria where all species are in the same phase, and the principle is applied only to these.AQA, OCR
Homogeneous equilibrium: An equilibrium in which all species share one phase, so their concentrations can appear together in a single Kc expression.OCR
Homogeneous versus heterogeneous equilibria: In a homogeneous equilibrium all species are in the same phase and all appear in the expression, whereas in a heterogeneous equilibrium a pure solid or pure…OCR
Homologous series: A family of organic compounds with the same functional group, the same general formula and similar chemical properties, where successive members differ by a…AQA, OCR
Homolytic fission: Breaking of a covalent bond in which the shared pair is split evenly, so each bonding atom receives one electron from the bonded pair.AQA, OCR
homolytic fission by ultraviolet: Breaking of a bond by ultraviolet radiation with one electron going to each atom.AQA
horseshoe intermediate: The species formed when the ring attacks the electrophile, carrying its positive charge.AQA
Hydrated: Contains water of crystallisation within the crystal lattice.OCR
Hydration: An addition reaction in which water is added to a molecule.AQA
Hydration mechanism: The mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst, ending with the loss of H⁺ to…AQA
hydration of alkenes: Alcohols are produced industrially by this process in the presence of an acid catalyst.AQA, OCR
Hydration of an anhydrous salt to remove water: The anhydrous salt removes residual water by hydrating and forming a hydrated solid, and enough drying agent is used to leave some free-flowing solid.OCR
Hydration ordering: Water molecules become highly ordered around ions with high charge density, which can outweigh the disordering of the solid lattice breaking down.AQA
hydride and cyanide nucleophiles: The nucleophile is shown as H⁻ for NaBH₄ and as CN⁻ for the reaction with KCN, with the lone pair and charge drawn before any arrow.AQA
Hydride as a nucleophile: Hydride ions, H⁻, supplied by NaBH₄, act as the nucleophile that attacks the δ⁺ carbonyl carbon.OCR
hydride as nucleophile: The hydride's lone pair forms a bond to the δ⁺ carbon while the π electrons of the C=O shift onto the oxygen.AQA
hydrocarbon: A molecule that contains only carbon and hydrogen.AQA
Hydrochloride: The product is described as this when the –NH₂ group becomes –NH₃⁺ with HCl and a chloride salt forms.OCR
hydrofluorocarbons and alkanes: Hydrofluorocarbons such as CH₂FCF₃ and simple alkanes such as pentane are used as replacements because they contain no C–Cl bonds at all.AQA
hydrogen bonding: A strong intermolecular attraction between molecules containing N, O or F and the H atom of –NH, –OH or HF, stronger than ordinary permanent dipole–dipole…AQA, OCR
hydrogen bonding between bases: Each bond runs from an N-H on one base to a lone pair on an oxygen or a nitrogen of its partner, holding the two strands together.AQA
hydrogen bonding between chains: Nylon 6,6 has an N-H on every amide link, so its chains hydrogen bond to one another strongly.AQA, OCR
Hydrogen bonding in ice: Each water molecule forms up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on oxygen.OCR
Hydrogen bonding in water: Each water molecule forms hydrogen bonds, as the polar O–H bonds leave hydrogen partially positive and oxygen partially negative, so lone pairs attract…OCR
Hydrogen bonding requirements: Hydrogen bonding requires a hydrogen atom covalently bonded to N, O or F, as in –NH, –OH or HF groups.OCR
Hydrogen chloride by-product: HCl released in every reaction of an acyl chloride, which fumes in moist air and is often described as steamy.OCR
hydrogen cyanide released by acid: Cyanide salts liberate hydrogen cyanide, a volatile and extremely toxic gas, on contact with acid, including the dilute acid added in the second stage of the…AQA
hydrogen flammability: Hydrogen is extremely flammable and is stored at very high pressure.AQA
Hydrogen ion concentration from hydroxide concentration: Rearranging Kw gives [H⁺] = Kw ÷ [OH⁻], and for a diprotic strong base such as Ba(OH)₂, [OH⁻] is twice the base concentration.OCR
hydrogen on the carbonyl carbon: The carbonyl carbon of an aldehyde carries a hydrogen atom, so no carbon–carbon bond need break during oxidation.AQA
Hydrolysis: A reaction in which an aqua ion causes a water ligand to lose H⁺, producing a hydroxo complex and an acidic solution.AQA
Hydrolysis of amides: Predicting that an amide hydrolyses under acid or alkaline conditions to give a carboxylic acid or its salt plus an ammonium ion or amine.OCR
Hydrolysis of condensation polymers: Cleavage of ester or amide linkages by reaction with water, reversing condensation polymerisation.OCR
Hydrolysis of haloalkanes: A substitution reaction in which a haloalkane is heated with aqueous sodium or potassium hydroxide, or with water alone, giving an alcohol and a halide ion.OCR
Hydrolysis of haloalkanes by water: Water acts as the nucleophile and hydrolyses the carbon–halogen bond, releasing a halide ion.OCR
hydrolysis of links: Water, especially with an acid or an alkali present, attacks the carbonyl carbon and cuts the chain, reversing the condensation and returning the monomers.AQA
Hydrolysis of polyesters: Polyesters can be hydrolysed, which matters for recycling and biodegradability.OCR
Hydrolysis of primary haloalkanes: Heating under reflux with aqueous alkali gives an alcohol and a halide ion, with OH⁻ replacing the halogen on the carbon skeleton.OCR
hydrolysis of the peptide link: A molecule of water is put back across every peptide link, cutting the -CO-NH- bond and returning the individual amino acids.AQA
hydrolysis products: Cut every C-N bond of a link, give the carbonyl carbon an -OH and the nitrogen an -H, and adjust for acid or alkaline conditions.AQA
Hydrolysis to carboxylic acids: Heating with hot dilute aqueous acid, or with aqueous alkali followed by acidification, produces a carboxylic acid with the same carbon skeleton.OCR
hydrophobic tail: The non-polar part of the surfactant that is not attracted to water and sticks outwards from the coated surface.AQA
hydrophobic tail and ionic head: A non-polar hydrocarbon tail that mixes with grease and an ionic COO⁻ head that is attracted to water.AQA
Hydroxide precipitate colours: Adding sodium hydroxide solution gives a blue precipitate for Cu²⁺, green for Fe²⁺, reddish-brown for Fe³⁺, pale pink for Mn²⁺ and grey-green for Cr³⁺.OCR
Hydroxide precipitate formation: Aqueous Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺ and Cr³⁺ react with aqueous sodium hydroxide and aqueous ammonia to give a hydroxide precipitate.OCR
hydroxide solubility: Sodium hydroxide is very soluble, whereas magnesium hydroxide is sparingly soluble, so few OH⁻ ions get into solution.AQA
Hydroxyl group: The OH group in ethanol, which makes ethanol behave as an alcohol.OCR
Hydroxynitrile (cyanohydrin) formation: A proton transfers to the negatively charged oxygen, producing a hydroxynitrile with a new carbon–carbon bond.OCR
hydroxynitrile product: The product has the OH and the CN on the same carbon.AQA

I

ice and iodine: Both are molecular: iodine is a lattice of I₂ molecules held by London forces, and ice is held by hydrogen bonds in an open tetrahedral cage.AQA
ICE tabulation method: Tabulating initial moles, the change in moles and the moles at equilibrium, using the stoichiometry of the balanced equation.OCR
identical chemical properties: Explained by the same number of electrons and therefore the same electron configuration, so isotopes react in exactly the same way.AQA
Identical conditions required for valid comparison: Retention time depends on conditions, so comparisons are only valid when conditions are kept the same, such as the same column, carrier gas flow and…OCR
identical once formed: Once formed, the bond is indistinguishable from the others, with all four bonds in the ammonium ion the same length and strength.AQA
identification by colour: Transition metal ions can be identified by their colour.AQA
Identification by comparison with a standard: Running a standard of the suspected compound and comparing its retention time with that of the unknown peak; a match supports identification and a mismatch…OCR
Identification by comparison with standards: Comparing the Rf of an unknown spot with that of a known standard run under the same conditions helps identify the component.OCR
identifying elements: The element is identified from the mass-to-charge values of the peaks and the ratio of the peak heights, using the whole pattern.AQA
Identifying eliminated small molecules: Each time a link forms in a condensation polymer, a small molecule such as water is eliminated and should be named.OCR
Identifying functional groups in unknowns: Selecting a reagent, adding it to a small sample in a test tube, recording what is seen and matching that observation to a functional group.OCR
Identifying ions by precipitate colour: Aqueous sodium hydroxide gives a green precipitate with Fe²⁺ and a red-brown precipitate with Fe³⁺, which helps identify each ion.OCR
identifying method errors: Check the temperature of each step first, give the corrected instruction as well as naming the mistake, and carry the safety reason with you.AQA
Identifying oxidation and reduction electrodes: The more positive electrode potential marks the positive electrode where reduction occurs, and the more negative marks the negative electrode where oxidation…OCR
identifying protein structure: Peptide links mean primary structure, backbone hydrogen bonds mean an α-helix or a β-pleated sheet, and R group interactions mean tertiary structure.AQA
Identifying the carbonate ion with dilute acid and limewater: Adding dilute acid and passing any gas through limewater, which turns milky or cloudy when carbon dioxide is produced.OCR
Identifying the organic layer: Which layer is organic is identified from density data or by adding a little water and seeing which layer grows.OCR
Immiscibility of organic and aqueous layers: The organic liquid and water do not mix, so they settle into two separate layers that can be drawn off separately.OCR
Immiscible layer separation: The mixture is poured into a separating funnel and the two immiscible layers are allowed to settle.OCR
importance of dose: A substance harmful in quantity may be entirely safe and useful at a few parts per million, so risk depends on dose.AQA
inactive trans isomer: Only in the cis isomer are the two leaving groups close enough to reach two bases on the same strand, so the trans isomer is inactive as a drug.AQA
incineration and energy recovery: Destroys the polymer, cuts the volume sharply and the heat released can generate electricity, but carbon dioxide is produced.AQA
Including units in headings: Every column heading names the quantity and gives its unit, for example 'Volume of acid / cm³'.OCR
incomplete combustion: With a limited oxygen supply the carbon is only partly oxidised, giving carbon monoxide, or not oxidised at all, giving carbon as soot, while the hydrogen…AQA
Incomplete combustion in limited oxygen: In limited oxygen, incomplete combustion gives carbon monoxide or soot.OCR
Incomplete combustion products: In a limited supply of oxygen, incomplete combustion occurs, producing carbon monoxide and/or carbon (soot) alongside water.OCR
Increase in entropy: A reaction that increases the number of gas molecules usually increases the entropy of the system.OCR
increase in particles: Two reactant particles become seven product particles, and this increase in the number of particles is the cause of the chelate effect.AQA
Increase means oxidation: A rise in oxidation number is oxidation.OCR
Increased rate of reaction: Because a greater proportion of collisions have at least the activation energy, the rate of reaction increases.OCR
Increased reaction rate: A catalyst brings about a faster rate of reaction while remaining chemically unchanged at the end of the overall reaction.OCR
Increasing orbital energy order: The sequence 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p, where each symbol is lower in energy than the next.OCR
increasing reactivity down the group: Calcium, strontium and barium all react with cold water directly and with increasing vigour, as the ionisation energies fall.AQA
independence of field strength: Dividing by the operating frequency makes delta a ratio, so a sample run on a low-field machine gives the same delta values as one run at high field.AQA
Independent and dependent variables: The variable you change goes on the horizontal axis, and the dependent variable goes on the vertical axis.OCR
independent replication: Separate groups using different methods agreed, which carries more weight than a single result.AQA
Indicator: A weak acid whose two forms are different colours, usable only if its colour change range falls entirely within the steep section of the pH curve.AQA
Indicator added before titration: Add the indicator to the measured analyte in the conical flask before starting the titration.OCR
indicator choice: A suitable indicator changes colour close to the equivalence point; phenolphthalein suits a strong acid with a strong base, while methyl orange suits a strong…AQA, OCR
Indicator pH range: An indicator is a weak acid whose two coloured forms change ratio over a narrow pH range, its end point range.OCR
Indicator selection for titrations: Choose an indicator whose end-point range lies within the steep section of the titration curve.OCR
Indirect determination of enthalpy change: Summing the known enthalpy changes along the alternative route, reversing signs where an arrow is reversed, to obtain the unknown value.OCR
Indirect heating methods: Heating a reaction mixture using a water bath, an electric heater or a sand bath rather than a naked Bunsen flame.OCR
Induced dipole polarisation: The process by which a non-polar molecule such as bromine becomes polarised as it approaches the alkene, the nearer bromine atom becoming δ⁺ and the other δ⁻.OCR
induced dipole-dipole forces: Act between every molecule, as constantly moving electrons create an instantaneous dipole which induces a matching dipole in a neighbour.AQA
Induced dipole–dipole (London dispersion) interaction: An attraction arising in all molecules when an instantaneous uneven electron distribution creates a temporary dipole that induces a dipole in a neighbouring…OCR
Induced dipole–dipole (London) forces: These are the intermolecular forces, also called London forces, that hold simple molecular substances together in the liquid or solid and arise from…OCR
industrial choice of reagent: Industry chooses ethanoic anhydride over ethanoyl chloride because it is cheaper to buy and much less readily hydrolysed by moisture in the air.AQA
Industrial conditions and catalysts: High temperature and high pressure with a solid phosphoric acid catalyst for the hydration of ethene.OCR
Industrial importance in manufacturing chemicals: Catalysis increases reaction rates so that chemicals such as ammonia, sulfuric acid and methanol can be manufactured economically at lower temperatures and…OCR
inert electrodes: A platinum electrode is written in where neither species is a solid metal that can conduct.AQA
inert gas at constant volume: Pumping in an unreactive gas at constant volume does not speed the reaction up, because the partial pressures of the reactants, and so their concentrations,…AQA
Inert platinum conduction: The platinum is inert and only conducts; the reacting species are the gas and its ions.OCR
Inert platinum electrode: Platinum does not react; it provides a conducting surface where electron transfer between the two oxidation states occurs.OCR
Infrared absorption and functional group identification: Infrared spectroscopy identifies functional groups by the wavenumbers at which bonds absorb.OCR
Infrared absorptions and functional groups: Infrared absorptions are used to confirm or exclude functional groups such as C=O or O–H.OCR
Infrared absorptions of O–H, C=O, C=C and C–H bonds: Bonds are identified by the wavenumber at which they absorb, using the supplied data table rather than memory.OCR
Infrared breathalysers measuring ethanol in breath: Ethanol vapour in exhaled breath absorbs infrared radiation at a characteristic wavenumber, and the amount of absorption is proportional to the ethanol…OCR
infrared functional groups: A broad absorption around 3230 to 3550 cm⁻¹ says an alcohol O-H, one near 1680 to 1750 cm⁻¹ says a carbonyl.AQA
Infrared spectroscopy and functional group identification: Absorptions are used to identify or exclude functional groups such as C=O, O–H, N–H and C≡N.OCR
Initial rate from gradients: The rate taken from the steepest part of the graph near the start, used to compare initial rates.OCR
Initial rate method: A method that measures how fast a reaction is at the very start, before concentrations have changed appreciably.OCR
Initial rates method: Carrying out separate experiments with one reactant's concentration varied while others are held constant, measuring the rate at the very start of each…OCR
initiation: The step in which the halogen molecule splits homolytically to form two halogen radicals.AQA, OCR
Instantaneous rate: The rate at that instant is found using the tangent, not a chord between two data points.OCR
insulation and heat loss: A lid, insulation or a polystyrene cup is a design feature that cuts heat exchange and reduces heat loss.AQA
Integration and proton counting: The integration, or relative peak area, gives the number of protons in that environment.OCR
integration ratios: Give the relative numbers of equivalent protons in the molecule, never the absolute number, so a ratio needs the molecular formula before it becomes a…AQA
Integration traces: A stepped line whose step heights are in the same ratio as the peak areas, so the relative numbers of each type of proton are read from the step heights.OCR
Intercepts on axes: Found by reading where the line crosses the chosen axis, giving the y-intercept at x = 0 or the x-intercept at y = 0.OCR
Interchangeable ground-glass joints: Joints of standard sizes that give a good seal and resist attack by organic vapours, so pieces can be joined without corks or rubber bungs.OCR
Interference from carbonate ions: Carbonate ions would also give a white precipitate with barium ions but dissolve with effervescence, so the acid prevents false positives.OCR
intermediate species: The species formed between the first and second steps, when the catalyst is consumed in the first step and regenerated in the second.AQA
intermediate structures: The species formed between reactants and products, such as the carbocation in electrophilic addition, whose correct structure earns a mark.AQA
Intermediates and fast steps: Intermediates formed in one step are consumed in a later step so they do not appear in the overall equation.OCR
Intermolecular forces: Weak forces that hold discrete molecules together in a lattice, and which must be overcome for melting and boiling.OCR
Intermolecular forces and physical properties: Hydrogen bonding raises boiling point in alcohols and carboxylic acids, and larger chains raise boiling point through stronger induced dipole–dipole forces.OCR
Intermolecular forces versus covalent bonds: Melting and boiling require energy to overcome these intermolecular attractions, not to break the covalent O–H bonds.OCR
Intermolecular hydrogen bonding: The δ⁺ hydrogen of one alcohol molecule is attracted to a lone pair on the oxygen of a neighbouring molecule, forming hydrogen bonds between separate molecules.OCR
International environmental regulation: International agreements such as the Montreal Protocol phased out CFCs, replaced by HCFCs and then HFCs.OCR
Interpreting graphs of quantity against time: Plotting readings against time so that a rate can be found, for example from the gradient of a volume–time graph.OCR
Interpreting the magnitude of Kc: A large value indicates the equilibrium lies towards the products, and a small value indicates it lies towards the reactants.OCR
Inverse relationship between Ka and pKa: Because of the negative sign, a larger Ka gives a smaller pKa, so a stronger acid has the lower pKa.OCR
Inverse relationship between time and rate: Because the same visible end-point is used each time, the time taken is inversely related to the initial rate, so a shorter time means a faster reaction.OCR
iodine clock: A method where a limited amount of thiosulfate consumes the iodine until it runs out and the starch suddenly turns blue black, so the same tiny extent of…AQA
Iodine–thiosulfate titration with starch indicator: Iodine is titrated with thiosulfate, and starch indicator gives a blue-black complex whose disappearance marks the end-point as iodine is used up.OCR
Ion charge: The charge on an ion can be predicted from the group number of the element, for example a Group 2 metal forming a 2⁺ ion.OCR
ion configurations: Transition metal ions lose their 4s electrons first, so iron(II) is 3d⁶ with no 4s.AQA
ion drift: The ions drift through a field-free flight tube of known length, so the flight time separates them by mass.AQA
Ion electron configurations: Deduced by adding or removing electrons, noting that 4s electrons are removed before 3d when transition-metal cations form.OCR
Ion formation: The atom receiving both electrons becomes a negative ion and the other atom becomes a positive ion.OCR
ion movement in electrolyte: Inside the cell, ions move through the electrolyte or porous separator, so the circuit is complete; electrons never do.AQA
ion repulsion and high activation energy: Both reactants are negative ions that repel each other, so the uncatalysed reaction is slow because the activation energy is high.AQA
Ionic bonding: The electrostatic attraction between positive and negative ions, produced when a metal atom transfers electrons to a non-metal atom.OCR
ionic charge and radius: Higher ionic charges and smaller ionic radii increase the magnitude of lattice enthalpy by strengthening electrostatic attraction between the oppositely…OCR
Ionic equation for neutralisation: The equation H⁺ + OH⁻ → H₂O, which is the same for any strong acid reacting with any strong alkali.OCR
Ionic equation H⁺ + OH⁻ → H₂O: The ionic reaction for a strong acid with a strong base, in which H⁺ from the acid reacts with OH⁻ from the base to form H₂O.OCR
ionic equations: Equations that show only the species that change, by splitting soluble ionic compounds into their ions and keeping insoluble solids, gases and water together.AQA, OCR
Ionic equations and charge notation: Balanced ionic equations are written for the precipitation reactions, with charges shown as superscript signs such as CO₃²⁻, SO₄²⁻ and Cl⁻.OCR
Ionic equations and complex ion formation: Writing the ionic equation for a precipitation or for complex ion formation, such as the tetraamminecopper(II) ion, that matches the stated observation.OCR
Ionic equations and proton transfer: Hydroxide ions accept protons from the acid, reacting with hydrogen ions to form water, as in Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O.OCR
Ionic equations and spectator ions: The ionic equation shows only the ions that change, so the sodium ion and any spectator ions are omitted.OCR
Ionic equations and state symbols: The ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) is written with correct formulae and state symbols.OCR
Ionic equations with state symbols: The equation is Ag⁺(aq) + X⁻(aq) → AgX(s), where X is the halide, with (aq) for the ions and (s) for the precipitate.AQA, OCR
ionic lattices: Giant lattices that ionic compounds form, rather than collections of molecules, so there is no molecule whose mass could be quoted.AQA
ionic product of water: Water is slightly dissociated, and its concentration is effectively constant, so it is absorbed into the constant to give Kw = [H⁺][OH⁻].AQA
Ionic product of water Kw: Equal to [H⁺][OH⁻], with a value of 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K, and it changes with temperature.OCR
Ionisation and separation: The sample is ionised, the ions are accelerated and separated by mass-to-charge ratio, m/z.OCR
ionisation energy: The removal of electrons from the metal atoms as successive steps in building gaseous ions.AQA
Ionisation energy and atomic radius: As atoms get larger down the group, the outer electrons are further from the nucleus and more shielded, so the first and second ionisation energies fall.OCR
Ionisation enthalpy: The metal is ionised, using first and, for Mg, second ionisation enthalpy, and both first and second steps are included for a 2⁺ ion.OCR
ionisation equations: Equations with state symbols showing one electron removed, for example Na(g) → Na⁺(g) + e⁻.AQA
iron catalyst: A solid catalyst that raises the rate only and does not shift the position of equilibrium, so it does not increase the yield.AQA
iron(II) complex: Haem is described as this, with the iron remaining Fe²⁺ throughout and not oxidised when oxygen binds.AQA
irreversible adsorption: Adsorption that does not reverse, so the impurity never desorbs and the sites are permanently out of use.AQA
Irritant substances: Substances kept off skin and out of eyes when solids and liquids are handled.OCR
isoelectric point: The pH at which the positive and negative centres balance.AQA
isomers: Compounds that share a molecular formula, so a molecular formula alone never identifies a compound.AQA
isomers not distinguished: Two isomers have exactly the same molecular formula and therefore exactly the same precise molecular mass, so the technique cannot separate them.AQA
Isotope: Atoms of the same element with the same number of protons but a different number of neutrons.AQA
isotope notation: The mass number sits above the atomic number to the left of the symbol.AQA
isotope peaks and structure deduction: Small M+1 isotope peaks caused by carbon-13 are distinguished from the molecular ion peak, and m/z values are used to deduce a plausible structural fragment.OCR
isotopes: Atoms with the same number of protons but different numbers of neutrons, because the number of neutrons in a nucleus can vary while the proton count stays…AQA, OCR
Isotopic abundance: The relative amount of each isotope of an element, given as fractions or percentages and used to weight each isotope mass in the calculation.OCR
Isotopic abundance and its effect on the mean: Relative atomic mass allows for the different abundances of the isotopes present, so the mean is weighted rather than a simple average.OCR
IUPAC convention: Every electrode half-equation is written as a reduction, with the electrons on the left-hand side: Ox + ne⁻ ⇌ Red.AQA
IUPAC naming of haloalkanes: Naming the haloalkane products correctly using IUPAC nomenclature for a given alkene and hydrogen halide.OCR
IUPAC nomenclature rules: A set of rules used for systematically naming substituted aromatic compounds, applied to build or interpret a name.OCR
IUPAC rules: A fixed algorithm for naming compounds, using the longest continuous carbon chain that contains the principal functional group and numbering it from the end…AQA

J

justifying a test: Explaining why silver nitrate solution is used, why it is acidified and why ammonia solution is added.AQA
Justifying exclusion: Spotting the outlier and justifying the exclusion by reference to the surrounding data, citing the size of the deviation from the cluster.OCR
justifying with electronegativity: Naming the more electronegative element to justify the labels, since oxygen and nitrogen are both more electronegative than carbon and hydrogen.AQA

K

Kc and Kp conventions: Kc uses equilibrium concentrations in mol dm⁻³, while Kp uses equilibrium partial pressures in Pa, keeping the two conventions distinct.OCR
Kc and Kp expressions: Built from the balanced equation, with products over reactants and each concentration or partial pressure raised to the power of its stoichiometric coefficient.OCR
Kc expression from a balanced equation: Written with product concentration over reactant concentration, each concentration raised to its stoichiometric coefficient, omitting pure solids and pure…OCR
Kekulé model: The model of benzene with three isolated double bonds, which predicts a value three times that of cyclohexene.AQA
ketone product: The ketone that results has no hydrogen on its carbonyl carbon, so oxidising it again would mean breaking a carbon–carbon bond.AQA
killing micro-organisms: Chlorine kills bacteria and other micro-organisms, which is why water-borne diseases such as cholera and typhoid are rare where supplies are treated.AQA
kinetic energy of particles: When a gas or solution is heated the particles move faster, so collisions happen more often, and their energy per collision is greater.AQA
Kinetic limitations: A positive E⦵cell says nothing about rate, so a reaction may be feasible but kinetically very slow because of a high activation energy.OCR
Kw expression and units: The expression is Kw = [H⁺][OH⁻], with units mol² dm⁻⁶, and at 298 K Kw = 1.00 × 10⁻¹⁴.AQA

L

Laboratory safety with flammable substances: Choosing a method by reference to the flammability of the substance and the avoidance of a naked flame.OCR
landfill: Cheap and needs no sorting, but takes up land, polyalkenes stay there for centuries and decomposing waste releases methane and leachate.AQA
Large jumps in successive ionisation energies: These reveal the number of electrons in the outer shell, so a jump after the second value indicates two outer electrons.OCR
Lattice disordering: The entropy increase caused by the solid lattice breaking down when a substance dissolves.AQA
lattice dissociation: The step in which the lattice is pulled apart into gaseous ions, costing energy and being strongly endothermic.AQA
lattice dissociation enthalpy: The exact reverse of lattice formation enthalpy, where one mole of the solid dissociates fully into gaseous ions; it is always positive and numerically equal.AQA, OCR
Lattice enthalpy: A quantity of a simple ionic solid that cannot be measured directly and is found from measurable quantities combined through a Born–Haber cycle.AQA, OCR
Lattice enthalpy as a measure of ionic bond strength: The magnitude of ΔLEH measures how strongly the ions are held in the giant ionic lattice, so a more negative value means stronger ionic bonding.OCR
lattice formation enthalpy: The enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions; it is always negative, because strong electrostatic…AQA
lattice of cations: A regular arrangement of positive ions left behind when each atom releases its outer-shell electrons.AQA
Le Chatelier's principle: When a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to oppose that change.AQA, OCR
Le Chatelier's principle and equilibrium shift: Adding acid raises [H⁺] so equilibrium shifts left towards HA, while adding alkali removes H⁺ so equilibrium shifts right towards A⁻.OCR
Le Chatelier's principle and position of equilibrium: Changing concentration or pressure shifts the position of equilibrium, but the ratio returns to the same value once equilibrium is re-established.OCR
leaving group: The halide ion, which leaves when the carbon-halogen bond breaks heterolytically as ammonia attacks the carbon.AQA
Leaving group and nucleophile: The halogen atom is the leaving group that is replaced by a nucleophile, such as OH⁻ or NH₃.OCR
liberating the free amine: Adding excess sodium hydroxide at the end to release the free amine from its salt, because the amine is a base and was protonated by the excess acid.AQA
lid and saturated vapour: The lid saturates the atmosphere with solvent vapour and stops solvent evaporating off the plate, which would otherwise distort the chromatogram.AQA
Ligand: An ion or molecule that donates one or more lone pairs of electrons to a central metal ion or metal atom.AQA, OCR
ligand effects on colour: Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.AQA
ligand replacement: The substitution of the chloride ligands, which is the first step in the mechanism.AQA
ligand size: The factor that decides the shape, because small ligands pack six around a metal ion while larger ligands allow only four to fit.AQA
Ligand substitution: A reaction in which the ligands bonded to a metal ion are replaced by other ligands, occurring stepwise, and the metal ion's oxidation state does not change.AQA, OCR
ligand substitution with DNA: A chloride ligand is replaced by water, and the resulting complex bonds to a nitrogen atom on a guanine base in DNA, then repeats with a second guanine.AQA
limewater for carbon dioxide: Bubble the gas from the carbonate test through limewater, which turns milky, rather than using a lighted splint.AQA
Limitation: A feature of a procedure that restricts how far the results can be trusted or how widely they apply.OCR
Limitations of ∆G predictions: ∆G predicts whether a process can happen, not whether it will happen at an observable rate.OCR
Limited capacity of buffers: The buffer works until one reservoir is exhausted, after which the pH changes sharply.OCR
Limiting reactant: The reactant that must be identified first, since the theoretical yield depends on it.OCR
Limiting reagent: The strong alkali, which is fully ionised and is neutralised almost completely by the excess weak acid.OCR
Limiting reagents and gas volumes: When two reactants are given, the limiting reagent is used, and gas volume is divided by 24.0 dm³ at room temperature and pressure.OCR
limits of the enthalpy criterion: The sign of ∆H, on its own, is not a reliable test of whether a change will happen.AQA
Line of best fit: The straight line on which two widely separated points are chosen rather than data points, because this reduces the effect of random error.OCR
Line of best fit and gradient calculation: The gradient is found from the coordinates of two well-separated points on the line of best fit, not from a single data point.OCR
linear and trigonal planar: Two pairs go to opposite sides, linear at 180°; three spread into a flat triangle, trigonal planar at 120°.AQA
lines for covalent bonds: A straight line is drawn between the two atoms, one line for each shared pair in an ordinary covalent bond.AQA
Link to general cis–trans isomerism from 4.1.3: The square-planar case links to the broader idea from 4.1.3 that cis–trans isomerism needs restricted rotation or rigid geometry.OCR
Linking frameworks: The two frameworks are linked by showing that electron loss raises the oxidation number.OCR
Linking ratios to structural groups: Steps of 3, 2 and 1 show three types of proton in the ratio 3:2:1, which for a molecule with six protons could be a CH₃ group, a CH₂ group and an OH proton.OCR
Linking reagents and observations to a named ion: Choosing the reagent and the observable change that together confirm a named ion rather than naming an ion without evidence.OCR
ln k against 1/T: A plot that gives a straight line of gradient −Eₐ/R and intercept ln A.AQA
Localised π bond in alkenes: An alkene's π electrons are localised between two carbon atoms, giving a localised electron density.OCR
locants in monomer names: The monomer is named with the lowest locants, and correct locants are needed in the monomer name where the substituent position matters.AQA
locating agent: A chemical such as ninhydrin sprayed onto the dried plate to make colourless amino acid spots visible.AQA
logarithm and antilogarithm: pH = −log₁₀[H⁺] converts concentration of hydrogen ions into pH, and working backwards [H⁺] = 10⁻ᵖᴴ converts pH into concentration.AQA
logarithmic form: The equation ln k = −Eₐ/RT + ln A, which is the usable form of the Arrhenius equation.AQA
Logarithmic form and graphical determination of Ea: Taking natural logs gives ln k = ln A – Ea/(RT), so a graph of ln k against 1/T is a straight line of gradient –Ea/R and intercept ln A.OCR
Logarithmic pH scale: A scale linking [H⁺(aq)] to pH through pH = –log₁₀[H⁺(aq)], so a tenfold change in [H⁺(aq)] shifts pH by one unit.OCR
Logarithmic pKa scale: A logarithmic scale that makes the wide range of Ka values easier to handle.OCR
logarithmic scale: A scale used as a measure of hydrogen ion concentration, compressing a huge range by taking a logarithm.AQA
London forces and chain length: Long chain molecules have more electrons and a greater surface area of contact, so their London forces are stronger and their boiling points higher.AQA
lone pair donation: One species supplies a lone pair and the other must have a vacant orbital to receive it.AQA
Lone pair effects: Among four electron regions, one lone pair gives a pyramidal shape and two lone pairs give a non-linear shape.OCR
lone pair on nitrogen: In an amine it is this that accepts the proton, and it must not be placed on a carbon atom rather than the nitrogen atom.AQA
Lone pair versus bonded pair repulsion: Lone pairs repel more strongly than bonded pairs, so each lone pair compresses the remaining bond angles.OCR
lone pairs: Pairs of electrons that belong to the central atom alone and are attracted by only one nucleus rather than two.AQA
Lone pairs and available electrons: A nucleophile must have a lone pair available for donation, so not every negative ion is automatically a good nucleophile.OCR
Lone pairs as hydrogen-bond acceptors: The δ⁺ hydrogen is attracted to a lone pair on an N, O or F atom of a neighbouring molecule.OCR
lone pairs closing bond angles: Each lone pair squeezes the remaining bond angles by roughly 2.5°, because a lone pair is held by one nucleus only and so spreads closer to the central atom.AQA
Lone pairs versus bonding pairs: Lone pairs are outer-shell electron pairs not shared in bonding, while bonding pairs are the shared pairs drawn between the nuclei.OCR
long-chain carboxylic acids: Acids whose R group is a long hydrocarbon chain of typically 15 to 17 carbons.AQA
longest chain: The longest continuous carbon chain that contains the principal functional group, from which the stem is taken.AQA
Longest-chain selection: Finding the longest carbon chain containing the principal functional group, which gives the stem such as meth-, eth-, prop- or but-.OCR
loss of H⁺ forming the π bond: A base removes a hydrogen from a carbon adjacent to the positive carbon, the arrow running from that C–H bond into the C–C bond, making the π bond of the…AQA
Loss of outer s electrons as oxidation: When a Group 2 atom reacts, it loses both outer s electrons and is itself oxidised.OCR
loss of water: A second molecule of water is eliminated when the phosphate condenses with the OH on carbon three of the next sugar.AQA
low and high density poly(ethene): Branched chains gave a soft, low density material, while straight chains gave the rigid, higher density poly(ethene) used for pipes and crates.AQA
Low polarity of σ-bonds: The σ-bond electrons are shared fairly evenly, so there is no significant δ⁺ or δ⁻ centre for polar reagents or nucleophiles to attack.OCR
Low yield and difficult separation of product mixtures: Separating a mixture of similar organic products is difficult, so the yield of the single desired compound is low.OCR
lower activation energy: The alternative pathway has a lower activation energy, so a greater proportion of collisions between particles have at least the activation energy.AQA, OCR
lower catalysed activation energy: The catalysed barrier is marked lower on the energy axis, to the left of the original activation energy.AQA
Lowering activation energy: A surface or intermediate species provided by the catalyst lowers the activation energy of a reaction.OCR
lowest locants: Numbering the chain from whichever end gives the lower locant to the functional group first and then to the substituents.AQA
Lowest whole-number ratio of ions: The ions are combined in the lowest whole-number ratio, so subscripts are simplified to their smallest whole numbers.OCR
Lowest-locant numbering: Numbering the ring so substituents receive the lowest possible locants, with the principal functional group taking priority for numbering.OCR

M

m = n × M: The relationship used to convert moles back into mass by multiplying by the molar mass.OCR
m/z scale and relative abundance: A mass spectrum plots relative abundance against m/z, the mass-to-charge ratio.OCR
magnesium lattice: A giant metallic structure with 2⁺ ions and two delocalised electrons per atom.AQA
Magnitude and sign of the gradient: The gradient's sign shows whether the quantity rises or falls, and the rate of reaction is the magnitude, so report the positive value.OCR
Magnitude of Kc: Comparing the value of Kc with 1 indicates where a homogeneous equilibrium lies, with a value well above 1 meaning the products dominate and a value well…OCR
major alcohol product: With an unsymmetrical alkene the water adds to give the more substituted alcohol, so hex-1-ene gives mainly hexan-2-ol.AQA
major and minor products: The major product is formed via the more stable carbocation, which forms faster, while the minor product comes from the less stable carbocation.AQA
Malleability and ductility: Properties of metals arising because layers of cations can slide over one another when a force is applied, rather than being brittle.OCR
Manganate(VII) as a self-indicating oxidant: Acidified manganate(VII) is purple and is reduced to almost colourless Mn²⁺, so the end-point is the first permanent pink tinge as excess MnO₄⁻ appears.OCR
marking the solvent front: When the solvent front nears the top, the plate is removed and the front marked at once, before it dries.AQA
Markovnikov's rule and carbocation stability: With an unsymmetrical alkene the hydrogen halide adds to give the more stable carbocation intermediate, so the major product follows this rule.OCR
Markovnikov's rule and major products: With an unsymmetrical alkene such as propene, two products are possible and the major product forms via the more stable carbocation, so propene with HBr gives…OCR
Markownikoff's rule: Used to predict formation of a major organic product in addition reactions of H–X to unsymmetrical alkenes, in terms of the relative stabilities of…OCR
Mass difference: Found by subtracting each fragment m/z value from the molecular ion m/z value to give the mass of the neutral group lost, then matched to a structural fragment.OCR
Mass loss and gas volume methods: When a gas is produced, measuring the mass lost from the reaction vessel on a balance, or measuring the volume of gas collected with a gas syringe.OCR
mass number: Gives the total of protons and neutrons, so neutrons equal the mass number minus the atomic number.AQA, OCR
Mass Number (A): The total number of protons and neutrons in the nucleus of an atom.AQA
mass of solution: The m in q = mc∆T is the mass in grams of the liquid whose temperature you measured, not the mass of the solid added.AQA
mass to moles: Converting a mass to an amount in moles before counting particles.AQA
Mass-to-charge separation of ions: A mass spectrometer separates ions by mass-to-charge ratio and records the relative intensity of each peak.OCR
Matching apparatus to the changing property: Choosing a sensor or measuring device that can log the property that changes without interrupting the reaction.OCR
Matching apparatus to the quantity measured: Identifying the quantity being measured and selecting apparatus designed to record that quantity, such as a balance for mass or a gas syringe for the volume…OCR
Matching indicator to acid and alkali strength: The equivalence pH depends on the acid and alkali strengths, so the indicator's range must suit that pH.OCR
Matching indicator to titration type: An indicator whose range lies within the steep section is selected so the end point matches the equivalence point, for example phenolphthalein for a weak…OCR
Matching units to measured quantities: Recording every measured quantity with a unit that matches the instrument and the quantity itself, never attaching a unit that belongs to a different quantity.OCR
Maximum occupancy of two electrons: A single orbital holds a maximum of two electrons.OCR
Maximum of two electrons per orbital: Each orbital holds a maximum of two electrons.OCR
maximum separation: Pairs of electrons in the outer shell of atoms arrange themselves as far apart as possible.AQA
Maximum two electrons per orbital: Each orbital holds a maximum of two electrons with opposite spins.OCR
Maxwell–Boltzmann distribution: Shows how molecular energies are spread at a given temperature, starting at the origin, rising to a peak and tailing off at higher energies.OCR
mean bond enthalpy: The enthalpy change when one mole of a stated covalent bond is broken in the gaseous state, averaged over a range of different compounds containing that bond.AQA, OCR
mean energy: The average energy, which sits slightly to the right of the peak because the tail drags the average up.AQA
mean energy versus tail: The rate depends on the number of molecules with energy greater than or equal to Eₐ, not on the mean, which rises by only a few per cent.AQA
measure of ionic bond strength: The magnitude of lattice enthalpy measures the strength of ionic bonding in a giant ionic lattice, where a more negative ΔLEH means stronger electrostatic…OCR
Measurement precision: The fineness to which a piece of equipment gives a reading, such as a burette delivering variable, measured volumes to ±0.05 cm³.OCR
Measurement using voltmeter and salt bridge: The half-cell is connected to the standard hydrogen electrode with a high-resistance voltmeter and a salt bridge, keeping the half-cell under standard…OCR
Measuring pH: Carried out using a pH chart, or a pH meter, or a pH probe on a data logger.OCR
Measuring rates of reaction: Following a reaction by measuring a quantity that changes as the reaction proceeds and recording it against time.OCR
Measuring temperature change experimentally: Calculating the temperature change from the initial and final readings and substituting it into q = mc∆T.OCR
Mechanism construction: Building the slow step first, then adding fast steps that consume remaining reactants and produce the products.OCR
Mechanism notation: A dot placed beside the formula of a species in a mechanism to represent its single unpaired electron.OCR
Melting and boiling points: Low for simple molecular substances because only weak intermolecular forces must be overcome, rising with stronger or more extensive intermolecular forces.OCR
melting point and bonding: Follows structure rather than radius, with weak van der Waals forces between simple molecular and monatomic elements.AQA
Melting point and brittleness explained by the model: Many strong electrostatic attractions throughout the lattice must be overcome to melt it, while sliding a layer brings like charges together and causes…OCR
Melting point as a purity indicator: A pure solid melts sharply over a narrow range close to its literature melting point, while an impurity widens the melting range and usually lowers the…OCR
melting point as a purity test: A pure sample melts sharply at the value in the data book, and an impure one melts lower and over a range.AQA
Melting point as a test of purity: A sharp melting point close to the accepted value indicates a pure sample, while a depressed or broad range indicates impurities.OCR
Melting point as an identification tool: The measured range is compared with a data book value to help identify a solid and judge its purity.OCR
Melting point as evidence of structure: Melting point rises from sodium to magnesium to aluminium, peaks sharply at silicon, then falls abruptly to phosphorus and sulfur, with chlorine and argon…OCR
Melting point determination: The melting point of the purified solid derivative is measured and compared with published values for derivatives of candidate carbonyl compounds.OCR
Melting point range versus single value: The range is recorded from the first sign of melting to the point where the solid has just liquefied, rather than recording only one temperature.OCR
Meniscus read at eye level on the graduation mark: Adding solvent dropwise with a dropping pipette until the bottom of the meniscus sits on the mark, with the eye level with the mark.OCR
Metal-aqua ion: A complex ion in which water molecules act as ligands and are co-ordinately bonded to a central metal ion.AQA
metallic bonding and melting point: Melting point generally falls from calcium to barium because the larger 2⁺ ions sit further from the delocalised electrons and the metallic bonding weakens.AQA
metallic bonding strength: Aluminium melts above sodium because Al³⁺ is smaller, more highly charged and contributes three delocalised electrons, making the attraction to the sea of…AQA
Metals and acids: A metal above hydrogen in the reactivity series reacts with a dilute acid to give a salt and hydrogen gas.OCR
methane, ammonia and water: Methane keeps the full 109.5°, ammonia closes to 107° and water closes further to 104.5°.AQA
methyl esters: Compounds of the form RCOOCH₃, where the methyl group sits on the single-bonded oxygen.AQA
Minimising rather than preventing pH change: The pH shifts only slightly because the solution contains reservoirs that remove added H⁺ or OH⁻ ions.OCR
minimising repulsion: Electron pairs take up the arrangement that puts them as far apart as possible because that minimises the repulsion between them.AQA
Minimum hot solvent: The smallest volume of hot solvent needed to dissolve the impure solid, so the solution is saturated on cooling.OCR
Mixing by inversion to give a homogeneous standard solution: Stoppering and inverting the flask repeatedly after making up to the mark to mix thoroughly.OCR
Mobile and stationary phases: Chromatography separates the components of a mixture between a mobile phase and a stationary phase.OCR
mobile phase: A solvent that moves up by capillary action, in which amino acids have relative solubility.AQA, OCR
Molar enthalpy change in kJ mol⁻¹: Divide q by the moles reacting to get ΔH in kJ mol⁻¹, converting joules to kilojoules.OCR
Molar gas volume: The volume occupied by one mole of any gas, quoted in dm³ mol⁻¹.OCR
Molar gas volume at room temperature and pressure: One mole of any gas occupies 24 dm³, so volume in dm³ equals moles multiplied by 24.OCR
molar mass: The mass per mole of a substance, found by adding the relative atomic masses of every atom in the formula.OCR
Molar mass and mass–mole conversion: Mass is converted into moles by dividing by molar mass before any ratio is applied.OCR
Molar volume of a gas at room temperature and pressure: At room temperature and pressure, one mole of any gas occupies approximately 24 dm³.OCR
mole: The SI unit for amount of substance, with the symbol mol.OCR
Mole calculations from concentration and volume: Moles = concentration × volume in dm³, and the balanced equation gives the reacting ratio, allowing calculation of an unknown concentration.OCR
Mole calculations from titration data: Starting from the mean titre in cm³ and the known concentration of the standard solution, the amount in moles is found by n = c × V.OCR
Mole concept and molar mass: Dividing a given mass by the molar mass gives the amount in moles, and multiplying moles by the molar mass of the target gives its mass.OCR
Mole conversion from mass or percentage: Converting the mass or percentage of each element into moles by dividing by its molar mass before comparing ratios.OCR
Mole fraction: The number of moles of a component divided by the total number of moles of all components present, with the fractions in a mixture summing to 1.OCR
Mole ratio and simplest whole-number ratio: Dividing each percentage by the relative atomic mass of that element gives the mole ratio, then dividing through by the smallest value gives the simplest…OCR
Mole ratio calculation: Dividing each element's amount in moles by the smallest amount, then scaling to the lowest whole numbers.OCR
Mole ratios: Obtained by dividing each percentage by the relative atomic mass of that element.OCR
Mole ratios from a balanced equation: The correct mole ratio between the alcohol and oxygen or carbon dioxide is taken from a balanced equation.OCR
Mole ratios from balanced equations: The balanced equation gives the mole ratio between the two reactants, so the amount of the unknown is found.OCR
Mole ratios from balanced redox equations: The balanced equations give the ratios to apply, such as 1 MnO₄⁻ to 5 Fe²⁺ and the 1:2 mole ratio in the iodine–thiosulfate equation.OCR
Mole-ratio calculations from balanced equations: The balanced equation is used to establish the mole ratio, for example five moles of Fe²⁺ react with one mole of MnO₄⁻, before calculating concentration.OCR
Mole–volume relationship: Moles = volume in dm³ ÷ 24, and volume = moles × 24 dm³.OCR
molecular docking: Candidate molecules are docked into the model on screen and scored for how well they fit and how strongly they would bind.AQA
Molecular formula: The actual number of atoms per molecule, found by dividing the relative molecular mass by the empirical formula mass and multiplying the empirical formula by…AQA, OCR
Molecular formula from Mr: Combining the empirical formula with the relative molecular mass from the mass spectrum to obtain the molecular formula.OCR
Molecular formula from relative molecular mass: The empirical formula combined with the relative molecular mass from mass spectrometry gives the molecular formula.OCR
Molecular formula prediction: Identifying the series from the functional group first, then applying the correct general formula and substituting n carefully to give the molecular formula.OCR
Molecular ion: Forms when a molecule loses one electron, so its m/z equals the relative molecular mass for a singly charged ion.OCR
molecular ion and molecular mass: The molecular ion peak sits at the highest m/z of the main cluster, and the relative molecular mass is read from it.OCR
molecular ion peak: The peak at the highest mass to charge ratio, ignoring small isotope peaks beyond it, which gives the relative molecular mass directly.AQA, OCR
Molecular shape and symmetry: Determines whether bond dipoles cancel or add to give an overall dipole, as in linear, trigonal planar, tetrahedral and bent shapes.OCR
Molecular shape names: Linear, non-linear, trigonal planar, pyramidal, tetrahedral and octahedral, describing the atom positions rather than the lone pairs.OCR
Molecular structure interpretation: Drawing the structure and marking equivalent carbons with the same label before counting signals helps relate the number of ¹³C NMR signals to molecular…OCR
Molecular symmetry: Symmetry such as a plane of symmetry can make two separate groups equivalent, so their hydrogens count as one environment.OCR
Molecular versus ionic substances: Relative molecular mass is used only where discrete molecules exist, while relative formula mass is used for ionic or giant structures.OCR
molecules closer together: The reasoning depends on the molecules being brought closer together, which raises the concentration of every gaseous reactant.AQA
moles from concentration and volume: Convert the known solution to moles with n = cV/1000, using the volume in cm³.AQA
moles from mass: The amount is found by dividing the mass by the molar mass.AQA
Moles of gas: The change in the number of moles of gas is the main influence on ∆S in chemical reactions.AQA
moles rather than concentrations: Because both species share the same solution, the volumes cancel and moles can be used directly in the ratio.AQA
Monitoring air pollutants such as CO and NO: Carbon monoxide and nitrogen monoxide from car emissions absorb infrared radiation at their own characteristic wavenumbers, so an infrared detector can…OCR
Monobasic: One mole of acid supplies one mole of H⁺, so no factor of two appears and the one-to-one mole ratio is used.OCR
Monobasic acid: An acid such as HCl or CH₃COOH that supplies one H⁺ per molecule, with only the carboxyl hydrogen acidic in CH₃COOH.OCR
Monodentate and bidentate ligands: Water, ammonia and chloride are monodentate and contribute 1 each, while ethanedioate (oxalate) and ethane-1,2-diamine (en) are bidentate and contribute 2.OCR
Monodentate ligands: Water and ammonia both act as these, bonding through a single atom, so the coordination number stays six and the complex remains octahedral.AQA, OCR
Monomer residues and eliminated small molecules: The –OH from the acid and the –H from the diol or amine are removed at each joining point, and the eliminated atoms are never left inside the unit.OCR
monomer to polymer: Take the monomer, open the double bond, and enclose the two carbons.AQA
Monomer-to-polymer deduction: The repeat unit is deduced from a given monomer by converting the C=C double bond into two single bonds and retaining every substituent and hydrogen atom.OCR
more than one alkene: The result when an unsymmetrical alcohol reacts, because the hydrogen can be taken from either neighbouring carbon, giving products such as but-1-ene and both…AQA
most probable energy: The energy at the peak of the curve, where the largest number of molecules have that energy.AQA, OCR
motor fuels: The main product of catalytic cracking, which burns smoothly in an engine.AQA
multi step mechanisms: Reaction pathways in which the overall change happens through more than one step, and the rate depends on the slowest step.AQA
Multi-step reactions: Reactions that proceed through a sequence of steps, one of which is the slowest and limits the overall rate.OCR
multidentate ligand: The porphyrin ring is this, donating four lone pairs from four nitrogen atoms to occupy four co-ordination sites.AQA
multidentate substitution: A reaction in which a bidentate or multidentate ligand displaces all the monodentate ligands from a complex, as when one EDTA⁴⁻ ion displaces all six water…AQA
Multiple covalent bonding: Occurs when two atoms share more than one pair of electrons, forming double bonds or triple bonds.OCR
Multiple functional groups in one molecule: A molecule may carry several groups, so a correct answer names all of them and does not stop at the first.OCR
Multiple oxidation states of transition elements: The existence of more than one oxidation state for each element in its compounds.OCR
multiplier from relative molecular mass: The relative molecular mass divided by the empirical formula mass, by which every subscript is multiplied.AQA
multiply charged ions: A single peak could belong to several different elements or to a multiply charged ion, so the whole pattern must be used.AQA

N

n = m ÷ M: The relationship used to convert a mass in grams into moles by dividing by the molar mass.OCR
n+1 rule: A rule used to predict splitting based on hydrogens on adjacent carbon atoms.OCR
named species in buffer action: Added H⁺ is removed by the ethanoate ion and added OH⁻ by the ethanoic acid molecule, and naming those two species earns the marks.AQA
naming from a structure: To name a polymer from a drawn structure, work back to the monomer first, name that alkene correctly, and only then wrap it in poly and brackets.AQA
naming from the nitrile carbon: The nitrile carbon is carbon one, so the hydroxyl group ends up on carbon two.AQA
Naming methyl, ethyl and propyl groups: A methyl group has n = 1 and so CH₃, an ethyl group has n = 2 and so C₂H₅, and a propyl group has n = 3 and so C₃H₇.OCR
Naming of polymers: Adding the prefix 'poly' to the monomer name in brackets, for example poly(propene).OCR
naming reagents fully: Giving the reagent's full name, including acidified or ammoniacal where required.AQA
naming shapes from atoms: The shape is named from the positions of the atoms alone, not from all the electron pairs.AQA
naming structures: The structure must be named as giant metallic, giant covalent, simple molecular or monatomic, with the force overcome named alongside it.AQA
need for a log scale: A linear scale cannot conveniently display such a range, so a logarithm is used.AQA
Negative sign: The negative sign converts the negative logarithm of a dilute acid into a positive pH, so omitting it would report a negative pH for an ordinary acid.OCR
Neutral atom: The electron count equals the atomic number.OCR
Neutralisation of acids by bases: An acid reacts with a base to give a salt and, in most cases, water.OCR
Neutralisation of phenol with sodium hydroxide: Phenol reacts with aqueous sodium hydroxide in a neutralisation, forming sodium phenoxide and water, because hydroxide is a strong enough base to remove the…OCR
Neutralisation reactions: Reactions in which a base neutralises stomach acid, raising the pH of stomach contents towards neutral and relieving indigestion.OCR
Neutralising acid soils: Calcium hydroxide is added to acidic soil to neutralise the acid and raise pH.OCR
neutrality and temperature: Neutrality means [H⁺] = [OH⁻], and the concentration at which that happens depends on Kw, which rises as temperature rises, so pH 7 is neutral only at 298 K.AQA
neutron count: Obtained as mass number minus atomic number.AQA
new chiral centre: In the product, that carbon carries four groups: hydroxyl, nitrile, and the two original groups, and if these two are different, the carbon is chiral.AQA
Nickel as a suitable catalyst: Nickel is a suitable catalyst for the reaction between an alkene and hydrogen.OCR
ninhydrin: A developing agent that can be sprayed on a plate to make the spots of amino acids visible.AQA
nitrate ion: The nitrate ion is NO₃⁻, one of the four negative ions whose name and formula must be recalled.OCR
Nitration and the nitronium ion: Concentrated nitric acid with concentrated sulfuric acid generates the nitronium ion, NO₂⁺, which substitutes a hydrogen to give nitrobenzene.OCR
nitration conditions: A mixture of concentrated nitric acid and concentrated sulfuric acid, warmed to about 50 °C, with the sulfuric acid acting as the catalyst.AQA
Nitration conditions and products: Carried out under reflux at about 50–60 °C, with benzene plus nitric acid forming nitrobenzene plus water and sulfuric acid regenerated.OCR
Nitration of benzene: Concentrated HNO₃ with concentrated H₂SO₄ generates the nitronium ion NO₂⁺, which attacks the ring and, after loss of H⁺, gives nitrobenzene.OCR
Nitrile formation and hydrolysis: Addition of a carbonyl compound to a cyanide ion gives a hydroxynitrile, and acid hydrolysis converts the nitrile group into a carboxylic acid, adding one…OCR
Nitrile formation from haloalkanes: A haloalkane reacts with KCN in aqueous ethanol under reflux to give a nitrile, adding one carbon to the chain.OCR
nitrile reduction: Reducing a nitrile avoids the mixture, using LiAlH₄ in dry ether, or hydrogen over a nickel catalyst, with RCN + 4[H] giving RCH₂NH₂.AQA
Nitrile synthesis: Warming a haloalkane with a solution containing CN⁻ in ethanol produces a nitrile, for example propanenitrile from bromoethane.OCR
Nitriles as synthetic intermediates: The nitrile is the product of the carbonyl-plus-HCN addition step and is treated as the starting material for further reactions.OCR
Nitrogen oxide radicals: •NO radicals that work in a similar catalytic cycle to chlorine radicals, breaking down ozone.OCR
nitronium ion: The species NO₂⁺ that forms when the sulfuric acid protonates the nitric acid.AQA
Nitronium ion as electrophile: The electrophile is the nitronium ion, NO₂⁺, generated from nitric acid.OCR
nitronium ion generation: The two concentrated acids react, HNO₃ + H₂SO₄ giving NO₂⁺ + HSO₄⁻ + H₂O, with the positive charge sitting on the nitrogen atom.AQA
no by-product: Nothing else is formed, so the polymer has the same empirical formula as the monomer.AQA
no C–Cl bond: The absence of a carbon–chlorine bond means ultraviolet light cannot release a chlorine radical, so no catalytic cycle can start.AQA
no colour change: The mixture stays orange because the dichromate is not reduced to chromium(III), and that negative result is itself a useful test.AQA
no curly arrows required: Curly arrows are not required for radical mechanisms, and omitting them does not earn a mark; they are simply ignored if absent.AQA
No effect on equilibrium position or enthalpy change: The catalyst does not alter the equilibrium position or the overall enthalpy change, because it accelerates the forward and reverse reactions equally.OCR
no functional group: There is nothing for a reagent to grip, so the polymer survives acids, alkalis and the enzymes of decomposer organisms.AQA
no further oxidation: Acidified dichromate cannot oxidise the ketone further, because that would mean breaking a carbon–carbon bond.AQA
no hydrogen on the hydroxyl carbon: The carbon bearing the hydroxyl group has no hydrogen atom attached, so oxidation cannot remove hydrogen from that carbon.AQA
No reaction of phenol with carbonates: Phenol does not react with sodium carbonate or sodium hydrogencarbonate, so no effervescence of carbon dioxide is seen, because these ions are not strong…OCR
noble gas configuration: In each case the ion reaches the electron configuration of the nearest noble gas.AQA
Noble-gas electron configurations: The electronic configuration of a noble gas that the ions formed are recognised to have.OCR
Noble-gas shorthand: A configuration written using a noble-gas core, such as [Ar] 4s² 3d⁶, matching the noble gas to the electrons already accounted for.OCR
Non-aromatic rings: One way in which the carbon and hydrogen of an aliphatic compound are joined together.OCR
Non-biodegradability of addition polymers: Addition polymers are chemically inert and non-biodegradable because their saturated carbon backbones resist attack, so waste polymers persist in landfill and…OCR
non-biodegradable: Because there is no functional group for enzymes to act on, the polymer persists in landfill.AQA
Non-equivalent adjacent protons: Protons that are non-equivalent to the observed proton and are on adjacent atoms, and only these cause splitting.OCR
non-polar C–C and C–H bonds: Carbon and hydrogen have almost the same electronegativity, so these strong bonds are essentially non-polar.AQA
non-superimposable: However you rotate one, it will not superimpose on the other.AQA
non-superimposable mirror images: The relationship between the two forms of a molecule with a single chiral centre, in the same way that a left hand will not fit into a right glove.AQA
Non-volatile impurities: Impurities that stay in the flask while the more volatile product distils over.OCR
not disproportionation: Chlorine is only reduced, from 0 in Cl₂ to −1 in Cl⁻, so it ends in a single oxidation state.AQA
not the greenhouse effect: Ozone in the stratosphere is filtering ultraviolet, not infrared, so it is not acting against global warming.AQA
nuclear atom: A model in which almost all the mass and all the positive charge sit in a tiny central nucleus, because a diffuse positive charge could not make alpha…AQA
nuclear model: An atom consists of a nucleus containing protons and neutrons surrounded by electrons.AQA
Nuclei of bonded atoms: The two positively charged nuclei that are both attracted to the shared pair of electrons.OCR
nucleophile: A species that is an electron pair donor, supplying a lone pair of electrons to an electron-deficient centre to form a new covalent bond.AQA, OCR
Nucleophile identity and reagents: The nucleophile depends on the reagent: with aqueous sodium or potassium hydroxide the nucleophile is the hydroxide ion, OH⁻, while with water alone it is a…OCR
Nucleophile versus electrophile: The nucleophile supplies the electron pair as the donor, while the electrophile is the acceptor, so the donor and acceptor roles are kept clearly separate.OCR
Nucleophilic addition: Aldehydes and ketones undergo this reaction, in which nucleophiles attack the δ⁺ carbon; with HCN in the presence of a trace of alkali the product is a…OCR
Nucleophilic addition mechanism: A curly arrow is drawn from the nucleophile's lone pair to the carbonyl carbon and a second arrow from the C=O π bond onto the oxygen, followed by protonation…OCR
Nucleophilic addition to carbonyl compounds: The carbonyl carbon is attacked by CN⁻, the C=O π bond breaks and the oxygen takes the electron pair, giving an alkoxide.OCR
Nucleophilic addition to carbonyls: CN⁻ attacks the δ⁺ carbonyl carbon, the π bond of C=O breaks and the electrons move onto the oxygen, giving a negatively charged intermediate.OCR
nucleophilic addition-elimination: A reaction in which the nucleophile's lone pair attacks the δ⁺ carbonyl carbon, and the chloride ion or a carboxylate is then eliminated, so the carbonyl…AQA
Nucleophilic addition–elimination at the carbonyl carbon: The reaction in which a nucleophile attacks the carbonyl carbon and the chloride leaves, fast at room temperature and often needing no catalyst.OCR
nucleophilic attack: The δ⁺ charge on the carbonyl carbon leaves it open to attack by a nucleophile, such as water or the hydroxide ion.AQA
Nucleophilic substitution: A one-step mechanism in which an arrow goes from the nitrogen lone pair to the δ⁺ carbon while an arrow goes from the carbon-halogen bond to the halogen,…AQA, OCR
Nucleophilic substitution mechanism: A one-step mechanism in which the hydroxide ion's lone pair attacks the δ⁺ carbon bonded to the halogen, and the halogen departs with the bonding electron pair.OCR
Nucleophilic substitution of haloalkanes: The haloalkane undergoes nucleophilic substitution in which the silver ion assists departure of the halide and the released halide ion is captured by silver…OCR
Nucleophilic substitution versus elimination: Haloalkanes undergo nucleophilic substitution and elimination, with aqueous hydroxide favouring substitution and ethanolic hydroxide with heat favouring…OCR
Number of accessible microstates in each state: The gas has a greater number of accessible microstates than the liquid or solid, because particles in a gas move freely through a large volume and can occupy…OCR
Number of orbitals in each subshell: A p subshell contains three orbitals and a d subshell contains five orbitals.OCR
Number of orbitals per shell: The nth shell contains n² orbitals, each of which holds two electrons.OCR
Number of peaks gives number of components: Each peak on a gas chromatogram represents a separate component leaving the column, so the number of peaks indicates the number of separable components.OCR
Number of signals and environments: The number of signals equals the number of carbon environments, so count signals, not atoms.OCR
Number of signals and non-equivalent environments: In ¹H NMR, the number of signals equals the number of chemically non-equivalent proton environments in the molecule.OCR

O

O–H bond absorption in the infrared: An absorption of infrared radiation by the O–H bond of an alcohol, located as a broad peak in the region around 3200–3600 cm⁻¹.OCR
Observable colour change as evidence of redox: With acidified potassium dichromate(VI) the orange solution stays orange on heating, showing no reduction to green Cr³⁺ ions.OCR
observations and smells: Misty white fumes of hydrogen chloride with sodium chloride, brown bromine vapour and choking sulfur dioxide with sodium bromide, and hydrogen sulfide…AQA
observed colours: Chlorine added to aqueous sodium bromide gives an orange solution of bromine, and with aqueous sodium iodide gives a brown solution, or a black solid, of…AQA
Obtaining a stable, precise reading: Placing the electrode in the sample, stirring gently and waiting for a stable reading before recording the pH to the precision the meter allows.OCR
Octahedral complexes: Complexes in which six monodentate ligands surround the metal ion, giving a shape that is unchanged when water ligands are replaced by ammonia ligands.OCR
Octahedral geometry with bidentate ligands: A complex in which three bidentate en ligands each bind through two N donor atoms, giving a coordination number of six.OCR
Octahedral shape: The arrangement of six donor atoms as far apart as possible, with the metal at the centre and the six ligands at the vertices of an octahedron.OCR
one curve, two barriers: A single distribution curve is used and a second, lower activation energy is marked to the left of the original.AQA
one mark per curly arrow: Each arrow is judged on where it starts, with a valid origin and a clear destination.AQA
One mole of compound formed: A feature of a formation equation is that exactly 1 mol of the compound is formed as the product.OCR
One mole of substance combusted: Exactly 1 mol of the substance is combusted, so the fuel coefficient in the equation must be 1.OCR
open lattice in ice: When water freezes, hydrogen bonds hold the molecules in a rigid, open tetrahedral cage, keeping them further apart on average than in the liquid.AQA
Open rigid lattice structure: Directional bonds hold the molecules in a rigid, open hexagonal lattice with large spaces between them.OCR
opposing the imposed change: When a system at equilibrium is subjected to a change, the position of equilibrium shifts in the direction that opposes, and so partially counteracts, that…AQA
Opposite spins: The two electrons in one orbital have opposite spins, shown as one arrow pointing up and one arrow pointing down.OCR
Optical isomerism: A form of stereoisomerism in which isomers differ only in their spatial arrangement, being non-superimposable mirror images about a chiral centre.AQA, OCR
Optical isomers: Non-superimposable mirror images that rotate plane-polarised light in opposite directions.OCR
Optically active: Capable of rotating the plane of plane-polarised light.AQA
Optically inactive: Having no overall effect on plane-polarised light.AQA
optimum temperature: Held near 30 to 40 °C because below this the enzymes work too slowly, and much above it they are denatured and stop working altogether.AQA
orange to green: The colour change seen when the orange dichromate ion is reduced to the green Cr³⁺ ion.AQA
orange to green colour change: Acidified dichromate turns from orange to green as it works.AQA
Orbital: A region of space where the probability of finding an electron is high, holding a maximum of two electrons.AQA, OCR
orbital capacities: An s sub-shell holds 2 electrons, p holds 6 and d holds 10, because each orbital takes two electrons of opposite spin.AQA
orbitals replacing orbits: The modern model replaces orbits with regions of probability.AQA
Order: The superscript m or n with respect to a reactant, taking a value of 0, 1 or 2 and found by experiment, not from the balanced equation.OCR
order of base strength: Primary aliphatic amine, then ammonia, then primary aromatic amine.AQA
Order of reaction: Revealed by the shape of the line: a horizontal line means zero order, a straight line through the origin means first order, and an upward curve of increasing…AQA, OCR
Order of reaction steps: Planning the sequence so that a group needed later in the route is not removed early and survives to the target molecule.OCR
order of reactivity: Reactivity falls along the series acyl chloride, anhydride, ester, amide, because the group leaving the carbonyl carbon becomes harder to displace.AQA
order of repulsion strengths: Lone pair–lone pair repulsion is greater than lone pair–bond pair repulsion, which is greater than bond pair–bond pair repulsion.AQA
order of strength: Hydrogen bonding is strongest and induced dipole forces weakest for small molecules.AQA
Ordered rules for assigning oxidation numbers: A short set of rules applied in order is used for assigning and calculating oxidation number for atoms in elements, compounds and ions.OCR
Ordering by atomic (proton) number: Elements are arranged in order of increasing atomic (proton) number, which is the ordering principle of the modern periodic table.OCR
Ordering of reaction steps: The two reactions are named in order, and routes that skip a step are rejected.OCR
orders and molecularity: A species to the first power contributes one molecule to the rate determining step, one to the second power contributes two, and a zero order species is…AQA
orders are experimental: They are determined experimentally and cannot be deduced from the equation, so the rate equation has to be measured.AQA
orders of reaction: The values m and n in the rate equation, restricted to 0, 1 and 2, which tell you how the rate responds.AQA
orientation: Even an energetic collision can fail if the reacting groups are pointing the wrong way.AQA
Outer shell s² configuration of Group 2 elements: Each Group 2 atom has two electrons in its outer s subshell, giving an s² configuration such as [Ne] 3s² for magnesium.OCR
Outer-shell electron count: The number in the outermost shell is found by counting the electrons removed before the first big jump.OCR
Outer-shell electron counting: Counting the outer-shell electrons contributed by each atom before pairing any electrons, and checking the total against the number of electrons shown in the…OCR
outlining mechanisms: Drawing the structures of the species involved and curly arrows to represent the movement of electron pairs.AQA
Overall balanced equation: Shows the net change, so any species in it but absent from the rate equation must enter in a fast step after the slow step.OCR
overall cell reaction: Adding the two half-equations cancels the electrons, one water and the hydroxide to give H₂ + ½O₂ → H₂O.AQA
Overall charge: The charge of a complex ion, determined from the oxidation state of the metal and the charges of its ligands.OCR
Overall charge of complex ions: Found by adding the oxidation state of the central ion to the charges of all ligands, giving 2⁺ for [Cu(NH₃)₄(H₂O)₂]²⁺ and 2⁻ for [CuCl₄]²⁻.OCR
overall equation: The balanced equation 2O₃ → 3O₂ that remains after the chlorine species cancel from the added steps.AQA
Overall order: The sum of the individual orders, which fixes the units of k.AQA, OCR
overcoming intermolecular forces: Melting or boiling a molecular substance separates whole molecules, so only the intermolecular forces are overcome.AQA
Oxidation: The process of electron loss, in which a species gives electrons away and its oxidation state rises.AQA, OCR, Pearson Edexcel
Oxidation Number: A bookkeeping value that tracks electron control in a species, also called the oxidation state.OCR, Pearson Edexcel
Oxidation Number (State): A number assigned to an element in chemical combination that represents the number of electrons lost or gained by an atom of that element in the compound.Pearson Edexcel
Oxidation number change: Oxidation is an increase in oxidation number and reduction is a decrease.OCR
Oxidation number changes: These can confirm which species is oxidised and which is reduced: the oxidising agent's oxidation number falls, and the reducing agent's oxidation number rises.OCR
Oxidation number rules: An uncombined element has oxidation number 0, a monatomic ion has its charge, oxygen is usually −2 and hydrogen usually +1 in compounds.OCR
Oxidation numbers: Values assigned to each atom so that the elements which change can be identified and the total increase equalised with the total decrease.OCR
Oxidation numbers from group position and rules: The known oxidation number assigned to each element using its group and the rules, such as oxygen usually −2, hydrogen usually +1, group 1 metals +1 and group…OCR
Oxidation of alcohols: A primary alcohol is oxidised to an aldehyde on distillation or to a carboxylic acid on reflux, while a secondary alcohol is oxidised to a ketone.OCR
Oxidation of alcohols and aldehydes: Alcohols undergo oxidation to aldehydes, ketones or carboxylic acids depending on their class and the conditions, while aldehydes are oxidised by Tollens' or…OCR
Oxidation of aldehydes: The hydrogen attached to the carbonyl carbon allows the aldehyde to be oxidised to a carboxylic acid, while the ketone resists this oxidation under the same…AQA, OCR
Oxidation of aldehydes to carboxylic acids: Aldehydes are oxidised to carboxylic acids by acidified dichromate(VI), with the carbonyl carbon oxidised from +1 to +3.OCR
Oxidation of metals to 2⁺ ions: Each metal is oxidised by losing its two outer s electrons to form a 2⁺ ion, while another species is reduced.OCR
Oxidation of primary, secondary and tertiary alcohols: Primary alcohols oxidise to aldehydes and then carboxylic acids, secondary alcohols to ketones, and tertiary alcohols resist oxidation under these conditions.OCR
oxidation of water: The oxygen in water is oxidised from −2 to 0 in O₂.AQA
oxidation state and colour: Oxidation state matters too, since MnO₄⁻ is purple while Mn²⁺ is almost colourless.AQA
Oxidation state and overall charge: Both ligands are neutral, so the overall charge stays 3⁺ and the chromium oxidation state remains +3 during ligand substitution.OCR
Oxidation states of chlorine: Chlorine is in oxidation state 1⁺ in HClO, 1⁻ in HCl, and 0 in Cl₂.OCR
Oxidation states of chromium: Chromium is +3 in Cr³⁺ and +6 in each chromium of Cr₂O₇²⁻, a rise of three per chromium atom.OCR
Oxidation states of iron: Fe²⁺ is the +2 oxidation state of iron and Fe³⁺ is the +3 oxidation state, and in each interconversion the oxidation state of iron changes by one.OCR
oxidation states of sulfur: Fall from +6 to +4 with sodium bromide, and from +6 to −2 with sodium iodide.AQA
Oxidation to ketones: On oxidation with acidified dichromate(VI), a secondary alcohol is oxidised to form a ketone.OCR
Oxide character: Changes from basic through amphoteric to acidic as metallic character falls and non-metallic character rises across a period.OCR
oxidising ability: A halogen acts as an oxidising agent by gaining electrons, and this power falls from F₂ to I₂ down the group.AQA
Oxidising Agent: An electron acceptor that takes the electrons in, so it is itself reduced and its oxidation state falls.AQA, OCR, Pearson Edexcel
oxidising agent strength: The species on the left of the most positive half-equations are the strongest, because they take electrons most readily.AQA
Oxidising and reducing agents: An oxidising agent accepts electrons and is itself reduced, while a reducing agent donates electrons and is itself oxidised.OCR
Oxidising power down Group 17: Decreases as the halogen gets larger and less able to gain an electron.OCR
Oxidising power of halogens: The halogen is a weaker oxidising agent down the group because the incoming electron is gained less readily.OCR
Ozone depletion and ultraviolet radiation: One radical can destroy many ozone molecules before it is removed, thinning the ozone layer that screens ultraviolet radiation.OCR
Ozone layer: The layer that absorbs harmful UV and is depleted because one chlorine radical destroys many ozone molecules.OCR

P

p orbital overlap: Sideways overlap of the six p orbitals, each pointing perpendicular to the plane of the ring, all the way round.AQA
p-block: Elements whose highest-energy electron enters a p subshell, formed by groups 13 to 18.OCR
p-orbital: Dumbbell-shaped, with two lobes on opposite sides of the nucleus and a nodal plane through the nucleus where electron density is zero.OCR
Parent ring and substituent identification: Identifying the parent ring as benzene and naming substituents as prefixes (methyl, chloro, nitro, hydroxy) or, for some groups, as a suffix.OCR
Partial charge: In a polar bond the more electronegative atom takes δ⁻, as its higher Pauling value means a stronger attraction for the bonding electrons.OCR
partial charge labelling: Writing δ⁺ and δ⁻ at the two ends of each polar bond on a drawn molecule, rather than labelling the molecule as a whole.AQA
partial charges: The carbon carries a small positive charge, δ⁺, and the halogen a small negative charge, δ⁻.AQA
Partial charges δ⁺ and δ⁻: The more electronegative atom gains a partial negative charge (δ⁻) while the less electronegative atom gains a partial positive charge (δ⁺).OCR
partial dissociation: Only a small proportion of the molecules dissociate in aqueous solution, so the equilibrium lies to the left.AQA
Partial dissociation of weak acids: A weak acid such as CH₃COOH is only partially dissociated, so the mixture contains a substantial reservoir of undissociated CH₃COOH molecules.OCR
partial neutralisation: Adding the strong base to an excess of the weak acid, so some of the acid is converted into the salt and the rest is left unreacted.AQA
Partial pressure: The pressure a gas would exert if it alone occupied the container, equal to its mole fraction multiplied by the total pressure.OCR
Partial pressure from mole fraction: Calculated as mole fraction multiplied by total pressure before substituting into Kp.OCR
Partial pressure from mole fraction and total pressure: Calculated using p = mole fraction × total pressure.OCR
Partially filled d sub-level: Required for coloured ions, variable oxidation states and catalytic activity, and lacking in zinc and scandium.AQA
particle counting: A three-step routine in which protons equal the atomic number, neutrons equal mass number minus atomic number, and electrons equal protons minus the charge.AQA
particles at lattice points: Ions, atoms, molecules, or ions and electrons occupy the lattice points, depending on the crystal structure.AQA
particles per unit volume: Concentration counts particles per unit volume, so doubling the concentration of a reactant in solution gives twice as many of its particles in every cubic…AQA
partition: The retention mechanism when a liquid stationary phase is used, where components are retained by dissolving.AQA
parts per million: The unit in which chemical shift is measured on the delta scale.AQA
Pauli exclusion principle: Two electrons in one orbital cannot have the same spin, so opposite spins are required.OCR
Pauling electronegativity values: Comparative numbers on a scale, not measured in units, that let you compare atoms and judge how polar a bond will be.OCR
Peak area and spot size as measures of amount: The amount of each component present controls how large a signal it gives, shown by the size of its spot or the area of its chromatographic peak.OCR
Peak area relates to amount of component: The area under a peak relates to the amount of that component in the mixture, and peak areas are compared to judge relative proportions.OCR
peak lower and shifted right: The curve for the higher temperature has its peak lower and shifted to the right.AQA
peak patterns: Every element produces a characteristic pattern of peaks, so a mass spectrum acts as a fingerprint.AQA
peer review: The work was published and checked in this way, so governments treated the conclusion as reliable.AQA
penalised representations: Arrows that start on a negative charge, on an atom, or in mid-air gain no credit and are penalised.AQA
pencil baseline: A line ruled in pencil about a centimetre from the bottom of the plate, because ink would dissolve and travel with the solvent.AQA
peptide link: The -CO-NH- left when the -COOH of one amino acid reacts with the -NH₂ of the next and a molecule of water is lost.AQA
per mole quantities: The basis on which every enthalpy value is quoted, relative to a defined starting point and per mole of a specified substance, in kJ mol⁻¹.AQA
Per-mole basis for a single ion type: Because the definition fixes the amount at 1 mol of gaseous ions, ∆hydH is quoted per mole of that ion, so a compound such as MgCl₂ involves two separate…OCR
percentage composition: Data giving composition by mass or percentage by mass, where any missing percentage is found by subtraction.AQA, OCR
Percentage composition by mass: The percentage by mass of each element in a compound, determined by elemental analysis.OCR
Percentage error: The uncertainty divided by the measured value, multiplied by 100, so a small reading gives a larger percentage error.OCR
percentage uncertainty: A value in the temperature change calculated from the resolution of the thermometer, which more solid in the same volume of water lowers.AQA
Percentage yield: The actual yield divided by the theoretical yield and multiplied by 100, comparing the amount actually obtained with the maximum predicted.OCR
Percentage yield and purity checks: Calculating the percentage yield from the mass of purified product and the theoretical mass from the balanced equation, and checking purity against a quoted…OCR
Percentage yield calculation: Compares the amount actually obtained with the theoretical maximum: actual yield divided by theoretical yield, multiplied by 100.OCR
Percentage yield versus atom economy: Atom economy depends only on the equation, not on the actual yield obtained, so a reaction can have high atom economy but low percentage yield.OCR
Perfect ionic model: A theoretical model that assumes undistorted spherical ions, used to calculate a theoretical lattice enthalpy.AQA
Period 3 trend: Values rise overall as nuclear charge increases while shielding stays similar, but there are two dips.AQA
Periodicity: The repeating pattern in physical and chemical properties as you move across a period and then down to the next period.OCR
permanent dipole: Arises from a difference in electronegativity between bonded atoms, with the bond dipole pointing towards the more electronegative atom.AQA, OCR
Permanent dipole and molecular polarity: Exists only when the individual bond dipoles do not cancel by symmetry, so polarity is decided by combining electronegativity difference with shape, not by…OCR
permanent dipole-dipole forces: Act in addition wherever the molecule has a permanent dipole, with the δ⁺ of one molecule attracted to the δ⁻ of the next.AQA
permanent dipole–dipole forces: Forces by which chains attract each other because the C–Cl bond is polar, acting as well as van der Waals forces.AQA
Permanent dipole–dipole interaction: An attraction between molecules that already have a permanent dipole, where the δ⁺ region of one polar molecule attracts the δ⁻ region of another.OCR
permanent positive charge: A quaternary ammonium ion has four groups bonded to the nitrogen and this charge, because there is no lone pair left and no N-H bond to lose.AQA
peroxides and metal hydrides: Hydrogen is −1 in metal hydrides and oxygen is −1 in peroxides, where these differ from their usual values.AQA
Persistence and mobility of organohalogens: Organohalogen compounds persist in soil and water and accumulate in fatty tissue, and their mobility creates environmental problems.OCR
Persistent plastic waste: Unreactive polymers that persist in landfill for a very long time, and whose accumulation is lessened by both breakdown routes.OCR
pH calculation: pH = −log₁₀[H⁺], reported to an appropriate number of decimal places.OCR
pH colour-change ranges: Each indicator changes colour over a characteristic pH range, such as roughly pH 8.2–10.0 for phenolphthalein or roughly pH 3.1–4.4 for methyl orange.OCR
pH curve: Plots pH against volume of titrant added, and its shape depends on the strengths of the acid and base.AQA
pH definition: The negative logarithm to base 10 of the hydrogen ion concentration, with the concentration in mol dm⁻³.AQA
pH expression: pH = –log[H⁺] and its rearrangement [H⁺] = 10^(–pH) are the two forms used to convert between pH and hydrogen ion concentration.OCR
pH meter calibration: Carried out against buffer solutions of known pH before starting, because the absolute readings depend on it.AQA
pH of the product: The sodium product ends up strongly alkaline, around pH 13-14, while the magnesium product reaches only about pH 9-10.AQA
pH titration curves: Plots pH against the volume of titrant added during a titration, with a shape that depends on whether the acid and base are strong or weak.OCR
pH, pKa and logarithmic relationships: pH is the negative logarithm to base ten of the hydrogen ion concentration, pKa = −log Ka, and pH = pKa + log([A⁻]/[HA]).OCR
phase boundaries: Marked by a single vertical line, these separate two phases within a cell diagram.AQA
phosphate bridge between sugars: One phosphate joined to the CH₂OH on carbon five of its own sugar and condensing with the OH on carbon three of the next sugar.AQA
phosphate ester link: The link formed when the phosphate joins through the CH₂OH group on carbon five of the sugar.AQA
phosphate ion: Drawn with phosphorus bonded to four oxygens and carrying the negative charge, and given in the Chemistry Data Booklet.AQA
phosphoric acid catalyst: Concentrated phosphoric acid is used as the catalyst, so it is regenerated and does not appear in the overall equation.AQA
Photodegradable polymers: Polymers that contain groups which absorb ultraviolet light, so sunlight breaks the polymer chains and the material fragments.OCR
photon energy matches the gap: An electron in the lower group can absorb a photon whose energy is exactly ΔE and jump to the upper group.AQA
pKa: The logarithmic form pKa = −log₁₀Ka, where a smaller value means a stronger acid.AQA
planar carbonyl carbon: The carbonyl carbon is trigonal planar, so the cyanide ion can approach from above or below the plane with equal probability.AQA
planar double bond: The two carbon atoms and the four atoms attached to them all lie in one plane.AQA
planar geometry: The three groups around each double-bond carbon lie in a plane at about 120°.AQA
planar intermediate: A flat species such as a trigonal planar carbonyl carbon under attack or a planar positive carbon in a carbocation.AQA
planar ring: A flat regular hexagon in which all bond angles are 120°.AQA
Planarity of the alkene unit: The two carbons and their attached atoms lie in the same plane, with the π-electron density above and below that plane.OCR
Plane-polarised light: Light whose vibrations occur in one plane only.AQA
planning a test sequence: Ordering a set of tests so that one result does not interfere with the next, testing for a carboxylic acid before using dichromate.AQA
Planning organic synthesis routes: Choosing the order in which groups are introduced so a desired substituent can be placed where it is needed rather than relying on chance.OCR
plasticiser between chains: A small molecule added between the chains that pushes them further apart, so the intermolecular forces act over a greater distance and are weaker.AQA
plasticisers: Substances mixed into a polymer such as PVC, where the ester molecules sit between the chains and hold them apart, weakening the forces between chains so the…AQA
plotting a smooth curve: Plotting pH against total volume added and drawing a smooth curve rather than joining the points with straight lines.AQA
poisoning by sulfur: Sulfur impurities poison the iron, so the gases are purified first.AQA
polar backbone faces water: Being a chain of negative charges, the backbone is strongly attracted to water, which is why it faces outwards while the flat, far less polar bases point…AQA
Polar bond: Arises when two covalently bonded atoms have different electronegativities, so the shared electron pair is pulled towards the more electronegative atom.OCR
Polar carbon–halogen bond: A bond that leaves the carbon electron-poor and open to attack by a nucleophile.OCR
polar carbonyl group: The carbonyl group is polar because oxygen is more electronegative than carbon, so the carbon is δ⁺ and is the point of attack.AQA
Polarisability and number of electrons: As the molecules get larger down the group they have more electrons and a larger, more polarisable electron cloud, so the induced dipoles are stronger.OCR
Polarity and molecular symmetry: A permanent dipole depends on molecular shape, since symmetrical molecules such as CO₂ cancel their bond dipoles.OCR
Polarity of the C=O bond: The C=O bond is polar, so the carbonyl carbon is electron-deficient (δ⁺) and the oxygen is δ⁻.OCR
Polarity of the carbonyl group: The C=O bond is polar because oxygen is more electronegative than carbon, leaving the carbon δ⁺ and oxygen δ⁻.OCR
Polarity of the carboxyl group: The carboxyl group contains a polar O–H bond, so its hydrogen carries a partial positive charge, and its C=O oxygen carries a partial negative charge.OCR
Polarity of the O–H bond: The O–H bond is polar because oxygen is far more electronegative than hydrogen, so oxygen carries a partial negative charge (δ⁻) and hydrogen carries a…OCR
Polarizing power: The ability of a charged ion to distort or draw electron density towards itself in a nearby bond.AQA
poly(chloroethene): A polymer that carries a chlorine atom on every second carbon, commonly known as PVC.AQA
poly(monomer) form: The IUPAC name of an addition polymer is poly followed by the monomer name in brackets, and the brackets are kept even when the monomer name is a single word.AQA
Polyamide: A condensation polymer whose repeat unit is joined by amide linkages, with each amide group retained in the chain rather than lost as a small molecule.OCR
polyamide linkage: An amide link between the repeating units of a polyamide, with the nitrogen bonded to the carbonyl carbon.AQA
polyester linkage: An ester link between the repeating units of a polyester such as Terylene.AQA
Polyfunctional organic molecules: Organic molecules containing several functional groups, which can react at more than one site.OCR
Polymer naming: Naming follows by adding 'poly' to the monomer name.OCR
polymer to monomer: Find the shortest length that repeats, cut the backbone at equivalent points, and put the double bond back between the two backbone carbons.AQA
porphyrin ring: The multidentate ligand that donates four lone pairs from four nitrogen atoms and occupies four of the six co-ordination sites, all in one plane.AQA
Position isomerism along a carbon chain: Substitution can occur at more than one carbon along a carbon chain, so a longer alkane yields structural isomers such as 1-chloropropane and 2-chloropropane.OCR
position isomers: Isomers that keep the skeleton and move the functional group.AQA
position of equilibrium: In water, ethylamine sets up an equilibrium with water, and that equilibrium lies well to the left, so only a small fraction of the molecules are protonated…AQA, OCR
position of equilibrium unchanged: The equilibrium is the same, with the position unmoved and the composition at equilibrium identical.AQA
Positive ∆S: A positive ∆S can make an endothermic process feasible by offsetting the enthalpy change.AQA
Positive and negative electrodes: The positive electrode is the half-cell with the more positive E⦵ value and the other half-cell is the negative electrode.OCR
positive and negative results: Recording every result, including the ones where nothing happens, rather than reporting only the positive tube and leaving the other unexplained.AQA
positive inductive effect: The electron-releasing character of alkyl groups, which pushes electron density towards the carbon and spreads the positive charge.AQA
positively charged intermediate: An unstable species in which the delocalisation is broken and only part of the ring, drawn as a horseshoe, is still delocalised.AQA
Practical apparatus selection: Choosing equipment suited to the reaction conditions, such as a round-bottomed flask with a reflux condenser when prolonged heating is needed.OCR
Practical technique for paper and thin layer chromatography: A small spot of mixture is applied near the baseline, the solvent is allowed to rise, and the spot must not dip into the solvent.OCR
Practical technique with capillary tube and heated block: A small dry sample is packed into a capillary tube to a depth of a few millimetres, the tube is placed in the heated block and the temperature is raised…OCR
Practical technique within organic synthesis: Suits organic preparations where a crude solid must be isolated before recrystallisation, isolating a solid product from a liquid mixture rather than…OCR
Pre-equilibrium and intermediates: A fast equilibrium that produces an intermediate before the slow step, so the intermediate's concentration may need to be substituted using the equilibrium…OCR
Pre-exponential factor and intercept: The intercept on the ln k axis equals ln A, so A is found from A = e^(intercept), and a higher intercept means a larger A.OCR
precaution paired with reason: A precaution must be given with the hazard it controls, such as gloves because the solid and its solution are toxic on skin contact.AQA
Precipitate: An insoluble solid formed when solutions react.AQA
Precipitation of silver halides: Aqueous silver nitrate gives a white precipitate with chloride, a cream precipitate with bromide and a pale yellow precipitate with iodide.OCR
Precipitation reactions: Adding silver nitrate solution, AgNO₃(aq), to a sample acidified with dilute nitric acid forms a precipitate of the silver halide.OCR
precise atomic masses: The masses given in the question for individual isotopes, used instead of the weighted average relative atomic mass in the periodic table.AQA
Precision: Describes how close repeated measurements are to one another.OCR
Precision and resolution of instruments: Matching the resolution and range of the chosen instrument to the precision the measurement requires, and checking that the apparatus can cover the full range…OCR
Precision of measurement: Significant figures communicate how precisely a measured or calculated value is known, and recorded data limit the precision of any derived result.OCR
predicting ionic charge: The group number predicts the ion, as Group 1 loses one electron to give 1⁺ and Group 7 gains one to give 1⁻.AQA
Predicting major and minor organic products: Predict the product by placing the halogen on the carbon that can best support the positive charge in the intermediate, giving the major product and the minor…OCR
Predicting rate equations: Writing the slow step, substituting any intermediate from earlier equilibria, and comparing the resulting expression with the experimental rate equation.OCR
Predicting spectra from structures: Work from a proposed structure and state which absorptions it must show and which it must lack.OCR
Prefix and suffix naming: Naming substituents as prefixes, or as a suffix that changes the parent name, such as –COOH giving benzoic acid, –OH giving phenol and –NH₂ giving phenylamine.OCR
Preparation of amines: The heading introduces the preparation routes for amines that the specification requires, with its content in the sub-clauses that follow.OCR
Pressure and concentration effects: Increasing pressure shifts equilibrium towards the side with fewer moles of gas, while adding a reactant or removing a product shifts it to the right.OCR
pressure and gas concentration: Squeezing a fixed amount of gas into a smaller volume packs the same number of molecules into less space, so the concentration of every gaseous reactant rises.AQA
pressure and moles of gas: Raising the pressure shifts the equilibrium towards the side with fewer moles of gas; if both sides have the same number, pressure has no effect.AQA
Pressure of gases: Raising pressure compresses the same amount of gas into a smaller volume, giving an equivalent increase in particle concentration.OCR
Primary alcohols: An alcohol in which the carbon bearing the –OH group is bonded to exactly one other carbon atom.OCR
primary sequence: The sequence of amino acids in the chain, held by the covalent peptide links.AQA
primary structure unchanged: Every peptide link, and therefore the whole primary structure, is still intact even though the chain has unravelled and the enzyme no longer works.AQA
Primary versus secondary alcohols: An aldehyde reacts with NaBH₄ to form a primary alcohol, while a ketone reacts to form a secondary alcohol.OCR
Principal energy levels: These are the levels in an atom that electrons occupy, often called shells, labelled by the principal quantum number n = 1, 2, 3 and so on.OCR
Processes and techniques for identifying ions: Dissolving or suspending the sample, adding the chosen reagent, warming where a gas must be released, and testing any gas with the correct indicator or reagent.OCR
product form and pH: In excess acid every amine group is protonated so the amino acids come out as their cations, while hot aqueous alkali gives the carboxylate salts and free…AQA
Product isolation and purification: Obtaining the crude product using a separating funnel or filtration under reduced pressure, then purifying it by distillation or recrystallisation and…OCR
products minus reactants: The entropy change of a reaction is the difference in dispersal between the two sides, calculated as ΣS(products) − ΣS(reactants).AQA
Products versus reactants predominating: A large Kc, well above 1, means the products dominate at equilibrium, while a small Kc, well below 1, means reactants dominate.OCR
Products: an ester and a carboxylic acid: The ester is formed from the alcohol and acyl group, while a carboxylic acid is the second product rather than water.OCR
propagation: A halogen radical reacts with the alkane to form a hydrogen halide and an alkyl radical, then the alkyl radical reacts with a halogen molecule to form the…AQA, OCR
Properties and disposal of addition polymers: These polymers are saturated and generally unreactive, which raises environmental issues because they are not readily biodegradable.OCR
Properties of a good standard: TMS is chemically inert, volatile so it is easily removed, soluble in organic solvents, and absorbs beyond most sample protons so its peak rarely overlaps…OCR
proportion exceeding Eₐ: The fraction of particles with energy greater than or equal to Eₐ rises steeply, and it is this fraction that multiplies up the number of successful collisions.AQA
Proportion of molecules exceeding activation energy: Raising the temperature means a larger proportion of molecules has energy at or above the activation energy, while lowering it reduces that proportion.OCR
proportion of successful collisions: At the same temperature a much larger share of colliding particles has enough energy to react, so the frequency of successful collisions rises.AQA
proportion unchanged: The energy distribution of the particles is set by temperature alone, so the proportion of collisions that succeed is unaltered.AQA
Proportionality: Rate is directly proportional to concentration for a straight line through the origin, and proportional to the square of the concentration for an upward curve…OCR
Proportions as fractions of the total: Expressing each component's amount as a fraction of the total gives the proportions, found by comparing each component's amount with the total of all…OCR
protection from harmful radiation: The radiation removed is the band that damages DNA, so a thinner layer means more skin cancer, more cataracts and damage to crops and plankton.AQA
Proton acceptance by the nitrogen lone pair: The nitrogen atom carries a lone pair that can accept a proton (H⁺), forming a positively charged alkylammonium or arylammonium ion.OCR
proton acceptor: A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting.AQA
Proton environment: A set of hydrogen atoms that are chemically equivalent, meaning they sit in the same structural position and experience the same magnetic surroundings.OCR
Proton exchange with D₂O: Protons on oxygen or nitrogen exchange rapidly with deuterium when deuterium oxide, D₂O, is added.OCR
proton loss: The final step in which a second molecule of ammonia or amine removes H⁺ from the intermediate salt.AQA
Proton NMR integration and splitting patterns: Relative areas fix how many hydrogens sit in each environment, and splitting patterns confirm the number of hydrogens on adjacent carbon atoms.OCR
Proton NMR peak counting: Each distinct environment gives one peak in a low-resolution proton NMR spectrum, so the count equals the number of peaks.OCR
Proton NMR prediction: Identifying each distinct hydrogen environment, predicting the relative integration from the number of hydrogens, and predicting splitting using the n+1 rule.OCR
proton number ordering: The Periodic Table is built in order of increasing proton number, and each element's position follows from its electron configuration.AQA
proton transfer: Acid–base equilibria involve the transfer of protons from the acid to the base.AQA
Proton transfer from acid to base: The ionic view that O²⁻ and OH⁻ accept H⁺ ions from the acid to form water.OCR
Protonated amine salt and alkaline liberation: The initial product in strong acid is the protonated amine, so alkali is added to liberate the free aromatic amine.OCR
Protonation: The amine group is protonated to form an ammonium-type cation, –NH₃⁺, with a positive charge on nitrogen.OCR
protonation of the alcohol: A lone pair on the alcohol's oxygen attacks H⁺ from the acid, giving a protonated alcohol.AQA
Purification and melting point identification: The precipitate is filtered off, purified by recrystallisation and its melting point measured and compared with known data to identify the specific aldehyde…OCR
Purification of liquid organic products: After a liquid organic product has been separated and purified, traces of water often remain, and the dried liquid is decanted or filtered off after the…OCR
purine with pyrimidine: A two-ring purine always pairs with a one-ring pyrimidine, so every rung is the same width and the helix keeps a constant diameter.AQA
purines: Adenine and guanine, built from two fused rings, as given in the Chemistry Data Booklet.AQA
purines and pyrimidines: Adenine and guanine are the two-ring purines; cytosine and thymine are the single-ring pyrimidines.AQA
Purity and identification of organic solids: Melting-point measurement identifies a solid organic product and tests its purity, with a pure solid melting sharply over a narrow range close to its…OCR
Purity and recrystallisation: The solid is recrystallised to remove impurities, because impurities lower and broaden the melting range, so the melting point is sharp and reliable.OCR
pyrimidines: Cytosine and thymine, with one ring, as given in the Chemistry Data Booklet.AQA

Q

q = mc∆T: The relationship used to find heat energy transferred, where q is energy in joules, m is mass in grams, c is specific heat capacity and ∆T is temperature…OCR
Qualitative analysis: In qualitative analysis the effervescence with carbonate is used alongside other tests to confirm a carboxylic acid.OCR
Qualitative analysis of anions: A positive result is a white precipitate that does not dissolve when excess dilute acid is added, identifying SO₄²⁻.OCR
Qualitative analysis of halide ions: Cl⁻, Br⁻ and I⁻ are identified by acidifying with dilute nitric acid, adding silver nitrate solution and confirming with dilute ammonia.OCR
Qualitative analysis on a test-tube scale: Identifies ions in an unknown compound from observable changes rather than measurements, carried out on a test-tube scale.OCR
Qualitative anion tests: Tests for the anions carbonate, chloride, bromide, iodide and sulfate, learned by reagent, observation and ionic equation.OCR
qualitative buffer action: The conjugate base removes added H⁺ and the weak acid removes added OH⁻.AQA
Qualitative distinction between phenol and carboxylic acid: No reaction with CO₃²⁻ indicates a phenol rather than a carboxylic acid, while a carboxylic acid gives visible bubbles of carbon dioxide.OCR
Qualitative observation: The evidence is the loss of colour, not a measurement.OCR
Qualitative test-tube analysis: Small-scale test-tube chemistry in which the evidence is an observation, not a measurement, used to identify which functional group an unknown organic…OCR
Quantitative transfer of solute with rinsing: Transferring the solution to the flask through a funnel and rinsing the beaker and glassware into the flask so no solute is lost.OCR
Quantitative transfer to a volumetric flask: Transfer the solution and beaker rinsings through a funnel into a volumetric flask, then add distilled water to the graduation mark and invert to mix.OCR
quaternary ammonium salts: A salt with four alkyl groups and no N-H bond, so there is no proton left to lose.AQA
Quenching and sampling: Withdrawing a small aliquot and stopping the reaction quickly so that the equilibrium composition is fixed before measurement.OCR
Quickfit ground-glass apparatus: Glassware with ground-glass joints that fit together to give a closed, leak-free assembly.OCR

R

racemic drugs: A drug supplied as a racemic mixture, which is at best half active because only one enantiomer binds and reacts.AQA
racemic mixture: A mixture of equal amounts of enantiomers, also called a racemate.AQA, OCR
Racemic mixture / racemate: A mixture containing equal amounts of the two enantiomers.AQA
racemic product: Equal approach from both faces produces equal amounts of the two enantiomers, forming an optically inactive racemate.AQA
Radical: A species that contains an unpaired electron, which makes it highly reactive.OCR
Radical cation: Formed when a molecule loses one electron, written M⁺•, with a 1⁺ charge and the same relative isotopic composition as the intact molecule.OCR
Radical chemistry in the stratosphere: Ultraviolet radiation breaks the C–Cl bond to form chlorine radicals, and a chlorine radical catalyses breakdown of ozone.OCR
radical dot notation: The unpaired electron in a radical is represented by a dot placed next to the atom carrying it.AQA
Radical formation: Because each atom keeps one of the two electrons, each fragment is left with an unpaired electron and is therefore a radical.OCR
Radical reactions: Reactions in which radicals such as Cl• and •NO take part, shown with the radical dot included.OCR
Radicals and unpaired electrons: Species with an unpaired electron, shown with a dot, produced when a carbon–halogen bond breaks.OCR
Random slip: A slip made by the operator, which is distinguished from a limitation of the procedure.OCR
range of hydrogen ion concentration: The concentration of hydrogen ions in aqueous solution covers a very wide range, roughly 10 to 10⁻¹⁵ mol dm⁻³.AQA
ranking acid strength: A larger Ka, and so a smaller pKa, means a stronger acid.AQA
Ranking acid strength by pKa: Use the inverse relationship to rank acid strength: lowest pKa means strongest acid.OCR
Ranking entropy of solids, liquids and gases: For a given substance at comparable conditions, the gas has the greatest entropy, the liquid has an intermediate value and the solid has the lowest.OCR
ranking polarity: Comparing electronegativity differences, not just the presence of polar bonds, using both the shapes and the electronegativities of the atoms involved.AQA
rapidly dividing cells: The healthy cells most affected, such as bone marrow, hair follicles and the gut lining.AQA
rate against concentration graphs: The shapes separate the orders directly: zero order gives a horizontal line, first order gives a straight line through the origin, and second order gives an…AQA
Rate constant: The symbol k in the rate equation rate = k[A]ᵐ[B]ⁿ.OCR
Rate Constant (k): The constant of proportionality in the rate equation, which is temperature dependent.AQA
Rate constant determination: Finding the rate constant, k, from the constant half-life, t₁/₂, using the relationship k = ln 2/t₁/₂, with ln 2 taken as 0.693.OCR
Rate equation: An equation of the form rate = k[A]ᵐ[B]ⁿ, linking the rate of reaction to the concentrations of reactants raised to their orders.AQA, OCR
Rate equation and order of reaction: For a first order reaction the rate equation is rate = k[A], and a straight line through the origin confirms first order behaviour with respect to that…OCR
Rate equation and orders: Species that appear in the slow step as reactants before the rate-determining step contribute to the rate equation, with order equal to their number of…OCR
Rate from gradient of a quantity–time graph: Reaction rate is found from the gradient of a graph of a physical quantity against time, where the quantity may be concentration, mass, gas volume or…OCR
rate of reaction: The change in concentration of a reactant or product per unit time.AQA, OCR
rate proportional to 1/t: Since the same amount of sulfur forms each time, the rate is proportional to 1/t.AQA
rate versus position: Rate and position are separate ideas, and only the position sets the potential.AQA
Rate-determining step: The slow step, whose reactants must be the species named in the rate equation, with their orders.AQA, OCR
Rate–concentration graph: Plots the initial rate of a reaction on the vertical axis against the concentration of one reactant on the horizontal axis, with all other reactant…OCR
Rate–concentration relationship: How the initial rate responds to changes in a reactant's concentration when all other concentrations are kept constant.OCR
Rates linked to carbon–halogen bond strength: The time for the precipitate to appear measures the rate of hydrolysis, and a faster precipitate means a weaker, more easily broken carbon–halogen bond.OCR
Rates of reaction and activation energy: The weaker the C–X bond, the lower the activation energy and the faster the hydrolysis, so iodoalkanes hydrolyse fastest and fluoroalkanes slowest.OCR
Rates of reaction and temperature: The lower activation energy allows an acceptable rate at a lower temperature, and less heating is needed.OCR
ratio controls pH: Since [H⁺] = Ka × [HA]/[A⁻], only the ratio matters, and dilution divides both concentrations by the same factor so the pH is unchanged.AQA
Ratio simplification to whole numbers: The ratio is converted to whole numbers by dividing by the smallest value, and the total is checked against the molecular formula.OCR
Reaction conditions: An acid catalyst such as H₃PO₄ or H₂SO₄ is required together with heat.OCR
reaction mechanisms: A mechanism details how a reaction happens: which bonds break, which form, and where electrons go.AQA, OCR
Reaction of acid anhydrides with alcohols: An acid anhydride reacts with an alcohol to form an ester and a carboxylic acid.OCR
Reaction of carboxylic acids with carbonate: A carboxylic acid is strong enough to react with carbonate ions, releasing carbon dioxide.OCR
Reaction of phenol with bromine water: Phenol reacts with bromine water at room temperature, giving 2,4,6-tribromophenol as a white precipitate and decolourising the bromine water.OCR
Reaction quotient: Compared with Kc to decide the direction of shift; increasing the concentration of a reactant makes it smaller than Kc, so the forward reaction is favoured…OCR
reaction with cold water: Sodium reacts vigorously, melting into a ball that skates over the surface and fizzes, while magnesium reacts only very slowly.AQA
Reaction with HCN: With HCN, supplied as KCN acidified in situ, the product is a hydroxynitrile; acid hydrolysis of that nitrile then gives a hydroxycarboxylic acid.OCR
reaction with steam: Heated magnesium burns in steam with a bright white flame to give the oxide, leaving a white solid.AQA
Reaction-dependent roles of species: The definition is about behaviour in a reaction, not a permanent label, so the same species can act as an acid in one reaction and as a base in another.OCR
Reactions with carbonates and metals: Carboxylic acids react with carbonates to release carbon dioxide and with reactive metals to release hydrogen.OCR
Reactions with water, oxygen and acids: The metals react with oxygen to form oxides, with water to form hydroxides and hydrogen, and with dilute acids to form salts and hydrogen.OCR
reactive intermediates: A species formed during a mechanism, such as a carbocation, whose correct structure must be drawn where the mechanism forms one.AQA
reactivity of alkenes: The relatively low bond enthalpy of the π-bond explains why alkenes are more reactive than comparable alkanes, whose σ-bonds are stronger.OCR
Reactivity of phenol compared with benzene: The electron-rich ring makes phenol more reactive towards electrophiles than benzene, so aqueous bromine at room temperature gives 2,4,6-tribromophenol…OCR
Reactivity series: A series used to decide whether a named metal reacts with a dilute acid, since metals below hydrogen do not react in this way.OCR
Reactivity trends down Group 2: Reactivity increases as ionisation energies fall, so the metal loses its two electrons more readily and reacts more vigorously.OCR
Reading the direction of an ordering question: Read whether the question asks for the order from lowest to highest or the reverse, and check whether the substance and conditions are the same for each state.OCR
Reagent and condition recognition: Checking that the reagents match the route, including the haloalkane and the nitrogen nucleophile, and confirming the stated solvent and excess condition…OCR
Reagent and condition selection: Every stage is named with a reagent and a condition that can deliver the required functional-group change.OCR
Reagent choice and safe delivery of HCN: Because HCN is a weak acid and hazardous as the pure gas or liquid, the reagent is supplied in practice as sodium cyanide solution followed by dilute acid.OCR
Reagent versus catalyst: The acid is a reagent here, not a regenerated catalyst, because it is consumed as the nitrile nitrogen is released as an ammonium ion, NH₄⁺.OCR
Reagent-to-product relationships: The named reagent decides the nucleophile and therefore the product, such as OH⁻ giving an alcohol or NH₃ giving an amine.OCR
Reagent–observation pairs: Each anion test learned as the reagent added together with the observation it gives, rather than as a single fact.OCR
reagent, condition, observation: Each test-tube reaction needs the right reagent, the right condition and a stated observation, written as three separate items.AQA
Reagents and conditions: Esterification requires an alcohol and an acid catalyst, and the reaction is reversible, so the equilibrium and catalyst should be shown.OCR
Realistic improvement: A change that addresses the named limitation and reduces the error it causes, rather than a vague fault.OCR
Rearranging equations: Rearranging gives n = pV ÷ RT, p = nRT ÷ V, V = nRT ÷ p and T = pV ÷ nR.OCR
Rearranging equilibrium expressions for related quantities: Used to find an unknown equilibrium concentration or partial pressure from a known K value, checking the direction of any rearrangement.OCR
rearranging the gas equation: The gas equation is rearranged for the required quantity, giving V = nRT/p for a volume.AQA
Rearranging the rate equation for k: The rate constant k is found by rearranging the rate equation to k = rate ÷ ([A]ᵐ[B]ⁿ).OCR
Recalculating the mean: Recomputing the mean from the remaining values after the anomalous value has been excluded.OCR
rechargeable cells: Cells in which both electrode reactions are driven backwards when an external voltage larger than the EMF is applied.AQA
Recognising end-points from colour change: Each colour change is linked to the species responsible, either excess MnO₄⁻ or loss of the starch–iodine complex.OCR
Recording and comparing a melting range: The learner records the temperature at which melting begins and ends, reports the range and compares it with a data book value.OCR
Recording and repeating readings: Record all readings with units and correct significant figures, and repeat titrations to obtain concordant results.OCR
Recording observations precisely: Observations such as colour changes, gas evolution or precipitate formation are recorded as brief, specific descriptions rather than vague words.OCR
Recording time and plotting graphs: Recording the measured quantity at regular intervals using a stopwatch, so that the rate is found from the gradient or from the change in quantity per unit…OCR
Recording values with units: Stating the unit alongside the numerical value rather than leaving the measurement unitless, so that no numerical result is left without a unit.OCR
Recrystallisation: Dissolve the crude solid in the minimum volume of hot solvent, filter hot to take out insoluble impurities, then cool so the product crystallises while the…AQA, OCR
Recrystallisation of organic solids: The impure solid is dissolved in the minimum volume of hot solvent, the hot solution is filtered, and the filtrate is cooled so the purified solid…OCR
recycling: Conserves crude oil and uses less energy than making new polymer, but the waste has to be collected, sorted by type and cleaned, and each reprocessing…AQA
Recycling and energy recovery: Common responses are to recycle polymers by sorting and reprocessing, and to recover energy by controlled combustion, in which toxic gases must be prevented…OCR
recycling unreacted gases: Recovering much of the yield lost to the compromise, which raises the overall conversion.AQA
red copper(I) oxide: The red precipitate that appears when copper(II) is reduced to copper(I) by an aldehyde.AQA
Redistillation: Distilling a liquid product a second time to raise its purity.OCR
Redox and ionic equations: Writing the ionic equation for the displacement and identifying the oxidation and reduction changes.OCR
Redox and oxidation states: Group 2 metals act as reducing agents and are oxidised from oxidation state 0 to 2⁺ by losing two electrons, while another species is reduced.OCR
Redox and reactivity trends: The metal is oxidised and the hydrogen ions in the acid are reduced, and reactivity increases down the group.OCR
Redox at electrodes: The fuel is oxidised at the negative electrode (anode) and oxygen is reduced at the positive electrode (cathode).OCR
Redox half-cells: A half-cell containing two ions of the same element in different oxidation states in contact with a platinum electrode.OCR
Redox interconversion: Converting Cr³⁺ to Cr₂O₇²⁻ is oxidation needing an oxidising agent, and the reverse is reduction needing a reducing agent such as zinc with acid.OCR
Redox pairing of oxidation and reduction: While the Group 2 metal is oxidised, another species gains electrons and is reduced, as the partner half of the redox reaction.OCR
Redox reactions with water, oxygen and acid: The trend is shown by the metals' redox reactions, for example with water, with oxygen and with dilute acid, where barium reacts most vigorously and magnesium…OCR
Redox roles of tin and hydrochloric acid: The tin is oxidised and supplies electrons, the acid provides protons, and the nitro group is reduced.OCR
Redox: reduction as electron gain: Gaining an electron is reduction, so the halogen atom that gains one electron to form a 1⁻ ion is itself reduced.OCR
reduced cost of development: Fewer compounds have to be made and tested, so cost and time fall.AQA
Reduced pressure: Pressure below the filter that is lower than atmospheric pressure, which draws liquid through the filter faster than gravity filtration.OCR
reducing ability of halide ions: Increases down the group because the larger ionic radius and greater shielding mean the outer electron is lost more easily.AQA
Reducing agent: The aldehyde acts as this because it is readily oxidised to the carboxylate, so the aldehyde is itself oxidised while Cu²⁺ or Ag⁺ is reduced.OCR, Pearson Edexcel
reducing agent strength: The species on the right of the most negative half-equations are the strongest reducing agents.AQA
reducing agents: Electron donors that supply the electrons, so the agent is itself oxidised.AQA
Reducing demand for crude-oil feedstock: Recovering monomers or chemical feedstock reduces demand for fresh crude-oil-derived raw materials.OCR
Reducing landfill: Processing routes keep waste polymers as a resource rather than a disposal problem, linking the resource or energy benefit to less landfill.OCR
Reduction: The process of electron gain, which lowers the oxidation state of the species reduced.AQA, OCR, Pearson Edexcel
Reduction and hydrolysis of nitriles: Reducing the nitrile, for example with LiAlH₄ in dry ether or H₂ with a nickel catalyst, gives a primary amine, and hydrolysing it with dilute acid gives a…OCR
reduction by zinc: Zinc supplies the electrons through Zn → Zn²⁺ + 2e⁻, so one mole of zinc reduces two moles of vanadium at each step.AQA
reduction half-equations: Written with the electrons on the left-hand side, and the tabulated E⦵ belongs to that direction.AQA
Reduction of carbonyl compounds: Reduction with NaBH₄ or LiAlH₄ in a suitable solvent converts aldehydes to primary alcohols and ketones to secondary alcohols.OCR
Reduction of Cr(VI) to Cr(III) and colour change: Chromium in dichromate(VI) is in oxidation state +6 and is reduced to Cr³⁺, giving the colour change from orange to green that signals oxidation.OCR
Reduction of Cu²⁺ to Cu⁺: Cu²⁺ is reduced to Cu⁺ by gain of one electron, with oxidation number falling from +2 to +1.OCR
Reduction of nitroarenes to aromatic amines: Nitroarenes such as nitrobenzene can be reduced to aromatic amines such as phenylamine using tin and concentrated hydrochloric acid.OCR
Reduction of silver ions: Silver ions are reduced to silver metal, giving a silver mirror, which happens only when the aldehyde is oxidised.OCR
Reduction of silver(I) to silver metal: The silver(I) complex is reduced to metallic silver, giving a silver mirror on the inside of the test tube or a grey-black precipitate if the tube is not clean.OCR
reduction to amines: Reducing the nitro group with tin and concentrated hydrochloric acid and then excess sodium hydroxide gives an aromatic amine such as phenylamine.AQA
Reduction to primary amines: Using H₂ with a nickel catalyst produces a primary amine, changing the functional group from –C≡N to –CH₂NH₂.OCR
Reduction with hydride reagents: With hydride from NaBH₄, aldehydes are reduced to primary alcohols and ketones to secondary alcohols.OCR
reduction with NaBH₄: The hydride ion H⁻ attacks the carbonyl carbon, the π bond breaks to give an alkoxide, and protonation gives a primary alcohol from an aldehyde or a secondary…AQA, OCR
Reference standard TMS: Tetramethylsilane, at 0 ppm, is the standard relative to which shifts are measured.OCR
Reference standards in NMR: A small amount of tetramethylsilane, TMS, is added to the sample so that every other peak is reported as a shift from the TMS line.OCR
Reflux: Heating a reaction mixture so that volatile vapours are returned to the reaction flask, preventing loss of reactants and solvent.AQA, OCR
Reflux apparatus: A round-bottomed flask carrying a vertical condenser only, in which condensed vapour is returned to the flask.OCR
reflux with hydrochloric acid: The protein is heated under reflux for several hours with about 6 mol dm⁻³ hydrochloric acid because the process is slow.AQA
Regeneration of monomers from condensation polymers: The polymer backbone is cleaved at every ester group, so the products are the monomers or their salts, not a single small molecule.OCR
regeneration of the catalyst: It usually reacts, forming an intermediate, and is then regenerated in a later step, which is why the amount is unchanged.AQA
Regeneration of the free amine by alkali: Adding aqueous sodium hydroxide to the salt reverses the reaction by removing the proton, regenerating the free amine.OCR
regions of electron density: When predicting shape, a double or triple bond counts as one region, not two or three, because both pairs lie between the same two nuclei.AQA
rejecting alternatives: Stating the formula that matches and rejecting the others with a reason, quoting the value that rules each one out.AQA
Relating absorption strength to concentration: The strength of infrared absorption is related to the concentration of the gas being monitored.OCR
Relating oxidation state to redox behaviour: A change in oxidation state is what makes these elements useful as oxidising or reducing agents.OCR
Relationship between [H⁺(aq)] and pH: pH is found by taking log₁₀ of the concentration in mol dm⁻³ and applying the negative sign, and reversed with [H⁺(aq)] = 10^(–pH).OCR
Relationship between [H⁺] and [OH⁻]: In pure water [H⁺] = [OH⁻] = 1.00 × 10⁻⁷ mol dm⁻³ at 298 K, and neutrality means [H⁺] = [OH⁻].OCR
Relationship between [H⁺], pH and pKa: Combining the approximations gives [H⁺] ≈ √(Ka × [HA]₀), so pH = −log₁₀[H⁺] and pKa = −log₁₀Ka.OCR
Relationship between [HA]/[A⁻] ratio and colour: The two forms have different colours, so the colour you see depends on the ratio [HA]/[A⁻].OCR
Relationship between aliphatic and alicyclic: Alicyclic sits inside aliphatic, so every alicyclic compound is also aliphatic, but not every aliphatic compound is alicyclic.OCR
Relationship between amount, concentration and volume: A solution of concentration c and volume V contains n = c × V moles of solute, provided V is converted to dm³.OCR
Relationship between configuration and group number: The configuration links to the group number, so Group 17 elements have one vacancy in the outer p subshell.OCR
Relationship between group number and ionic charge: For main-group elements the charge of the common ion follows directly from the group number.OCR
Relationship between Ka and pKa: Because Ka can span many orders of magnitude, it is often expressed as pKa = −log₁₀Ka, so a smaller pKa corresponds to a stronger acid.OCR
Relationship between orbital count and electron capacity: The number of electrons a sub-shell can hold equals twice the number of its orbitals.OCR
Relationship between polymerisation and hydrolysis: Hydrolysis is the reverse of condensation polymerisation.OCR
Relationship to molecular formula: The empirical formula is often a reduced version of the molecular formula, as C₆H₁₂O₆ and CH₂O share the empirical formula CH₂O.OCR
Relationship with lattice and hydration enthalpies: The net result of breaking the ionic lattice into gaseous ions and then hydrating those ions, so it can be positive or negative depending on which term…OCR
Relative abundance from peak intensity: The height, or relative intensity, of each peak gives the relative abundance of that isotope.OCR
relative acid strength: The equilibrium always lies away from the stronger acid and towards the weaker one.AQA
relative amounts: The result of dividing each mass or percentage by the relative atomic mass.AQA
Relative atomic mass: The weighted mean mass of an atom of the element divided by one-twelfth the mass of an atom of carbon-12.AQA, OCR
Relative Atomic Mass (Ar): The average mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12.AQA
Relative atomic mass as a weighted mean: A comparison of the weighted mean mass of an element's atoms, allowing for isotope abundances, with the same ¹²C standard.OCR
relative charge: A comparison, not a measurement in coulombs, on which the proton has +1, the neutron has 0 and the electron has −1.AQA
Relative dissociation and equilibrium position: The extent of dissociation is linked to the position of equilibrium, which lies far to the right for a strong acid and to the left for a weak acid.OCR
relative formula mass: Applies to any formula unit, including ionic compounds such as NaCl, and is likewise the sum of the relative atomic masses of the atoms in the formula.AQA, OCR
relative isotopic mass: The mass of one atom of that isotope compared with one-twelfth the mass of an atom of carbon-12.AQA, OCR
Relative isotopic mass from peak position: For a sample of an element, each peak corresponds to an isotope, and its position gives the relative isotopic mass of that isotope.OCR
Relative isotopic mass of a single isotope: A comparison of a single isotope's atom with the ¹²C standard, giving a dimensionless number.OCR
relative mass: A comparison, not a measurement in kilograms, on which the proton and neutron each have 1 and the electron has 1/1840, usually quoted as 0.0005.AQA
relative molecular mass: The mean mass of one molecule of the substance divided by one-twelfth the mass of one atom of carbon-12.AQA, OCR
Relative numbers of equivalent protons: Peak area is proportional to the number of protons of that type, so the areas give a ratio rather than absolute counts.OCR
Relative positioning of atoms: The arrangement of atoms in a molecule, shown explicitly in a displayed formula by drawing every bond as a line.OCR
Relative reactivity of phenol and benzene: Phenol reacts more readily than benzene in electrophilic substitution, because its ring is more electron-rich and so attracts electrophiles more strongly.OCR
relative stability: Benzene is more stable than the theoretical cyclohexa-1,3,5-triene because delocalisation of p electrons lowers its energy.AQA
Release of H⁺ ions by acids in aqueous solution: In aqueous solution an acid releases H⁺ ions, so HCl gives H⁺ and Cl⁻, and H₂SO₄ gives 2H⁺ and SO₄²⁻.OCR
Release of OH⁻ ions by alkalis in aqueous solution: An alkali releases OH⁻ ions in aqueous solution, as NaOH gives Na⁺ and OH⁻ and KOH gives K⁺ and OH⁻.OCR
removal of added acid and alkali: Added H⁺ is removed by the conjugate base and added OH⁻ is removed by the weak acid, converting one member of the pair into the other.AQA
Removal of added H⁺ and OH⁻ ions: In an acidic buffer, the weak acid removes added OH⁻ ions and the conjugate base removes added H⁺ ions.OCR
Removal of hydrogen from carbon and from the hydroxyl group: The alcohol loses one hydrogen atom from the carbon bearing the –OH group and one hydrogen atom from the –OH group itself, so a C=O double bond forms.OCR
Removal of soluble and insoluble impurities: The hot solution is filtered to remove insoluble impurities, and cooling causes the purified compound to crystallise while soluble impurities remain in…OCR
removing interfering ions: Nitric acid removes carbonate, hydroxide and sulfite ions, which would give their own precipitates with silver ions and be misread as a halide.AQA
Removing spectator ions: Remove ions that stay unchanged on both sides so only reacting species remain, with spectator ions such as chloride omitted.OCR
renewable monomer: The monomer comes from a renewable plant source rather than crude oil.AQA
Repeat unit: The part of the chain that has the same atoms as the monomer but with the double bond replaced by two single bonds extending to neighbouring units.OCR
Repeat unit construction: Locate the C=C double bond and open it so each of those two carbons forms a single bond to the next repeat unit, with the two-carbon backbone section enclosed…OCR
Repeat unit drawing: The repeat unit is drawn showing the new linkage and continuation bonds on both sides to show the chain continues.OCR
Repeat unit structure: The smallest section of a polymer chain that, repeated many times, reproduces the whole structure, shown inside brackets with the continuation bonds crossing…OCR
repeating unit: The smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside.AQA, OCR
replication blocked: The double helix can no longer unwind and separate, so the enzymes that copy DNA cannot run along it and the cell cannot divide.AQA
Reporting derived results: A calculated result is given to the same number of significant figures as the least precise measurement used, so a final answer should not claim more figures…OCR
Representing ionic charge in dot-and-cross diagrams: Adding one electron for each negative charge or removing one for each positive charge before drawing, then enclosing the whole diagram in square brackets with…OCR
repulsion explanation: An explanation in terms of repulsion between electron pairs, where the lone pairs reduce the angles.AQA
Required practical 8: Building two half-cells and measuring the potential difference between them to measure the EMF of an electrochemical cell.AQA
Required practical 9: An investigation of how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base.AQA
Requirement for two different groups on each alkene carbon: Each carbon of the double bond must carry two different groups; if either carbon carries two identical groups, no E/Z isomerism exists.OCR
Residue: The insoluble solid retained on the filter medium when a mixture is filtered.OCR
Resistance of ketones to further oxidation: The ketone has no hydrogen on its carbonyl carbon, so it resists further oxidation and the reaction stops there.OCR
Resistance of tertiary alcohols to oxidation: In a tertiary alcohol the carbon bearing the –OH group carries no hydrogen atom, so this loss cannot happen and the alcohol resists oxidation by acidified…OCR
Resistance to addition reactions: Benzene does not readily decolourise bromine water without a catalyst, because delocalisation stabilises the ring.OCR
Resonance donation versus inductive withdrawal: Resonance donation of the oxygen lone pair raises electron density at the ortho and para positions, while inductively the electronegative oxygen withdraws…OCR
restoring delocalisation: The return of the pair of electrons from the C-H bond at the attacked carbon to the ring, releasing H⁺.AQA
restricted rotation: Rotation about the planar carbon–carbon double bond is restricted because rotating one end would break the sideways overlap of p orbitals.AQA
Restricted rotation about C=C: Rotation about the double bond is restricted because the π bond locks the two carbon atoms in one plane, so groups cannot swap sides by rotation.OCR
Restricted rotation about the C=C double bond: Rotation about the C=C double bond is restricted, so groups attached to each carbon stay on fixed sides of the plane.OCR
Restricted rotation and its consequences: The π-bond restricts rotation about the C=C bond because the π-electron cloud must stay continuous for overlap to persist, which is why alkene stereoisomerism…OCR
Retention of the aromatic ring: The aromatic ring is retained during reduction, so nitrobenzene gives phenylamine rather than an aliphatic amine.OCR
retention time: The time at which each component left the column, which on its own is not enough to identify a component because two different compounds can share it.AQA, OCR
Retrosynthetic planning: Working backwards from the target and checking each group at every stage to keep the plan consistent.OCR
Reversibility and yield improvement: The reaction is reversible, so yield can be improved by removing water or using an excess of one reactant.OCR
Reversibility in rechargeable storage cells: The electrode reactions are reversible, so an external supply drives the reverse reaction to recharge the cell.OCR
reversible reaction: The reaction reaches an equilibrium mixture, so the yield is always well short of the theoretical maximum.AQA
reversible reactions: Reactions in which the products can re-form the reactants, so the reaction never goes to completion and some reactant always remains.AQA
Reversible systems with observable changes: A reversible system chosen for investigation shows a visible or measurable change, such as the colour of an indicator or of a transition-metal complex ion.OCR
Reversible versus irreversible reaction: Acid hydrolysis is reversible, whereas alkali hydrolysis is effectively irreversible because the acid is neutralised as it forms.OCR
Reversing polymerisation to find the monomer: Restoring the carbon–carbon double bond between the two backbone carbon atoms of the repeating unit and removing the single bonds that link the unit to its…OCR
Rf calculation: Divide the distance the spot moved by the distance the solvent front moved, both from the pencil line.AQA
Rf calculation and identification: The retention factor is the distance travelled by the component divided by the distance travelled by the solvent front, and it is compared with known values…OCR
Rf value: The distance travelled by the spot divided by the distance travelled by the solvent front, both measured from the pencil baseline with the spot measured to…AQA
Rf value calculation: The distance that a component's spot has moved from the origin divided by the distance the solvent front has moved from the same origin, measured along the…OCR
Rinsing and avoiding contamination: The burette is rinsed with the solution it will contain so the added solution is not diluted.OCR
risk and benefit: The risks and benefits to society associated with using electrochemical cells, which must be weighed against each other in an evaluation.AQA
risks and benefits: Because cisplatin attacks any rapidly dividing cell it also damages hair follicles and bone marrow, so dose is a balance of benefit against side effects.AQA
Role of a halogen carrier as catalyst: A substance such as iron or aluminium chloride that acts as a catalyst by polarising the halogen and helping generate the halogen electrophile.OCR
Role of acid: Acid is present to generate the hydrogen halide and to protonate the alcohol so it becomes a good leaving group.OCR
Role of activation energy and the pre-exponential factor: A is the pre-exponential factor and Ea the activation energy in J mol⁻¹; a larger Ea makes the exponent more negative and k smaller at a given T.OCR
Role of an acid catalyst: A few drops of concentrated sulfuric acid are added as a catalyst, protonating the carboxylic acid so the alcohol can attack, and the catalyst is regenerated…OCR
Role of anhydrous salts as drying agents: An anhydrous salt such as MgSO₄ or CaCl₂ is added to the crude liquid in a small conical flask, where it hydrates by taking up water and clumps together.OCR
Role of aqueous alkali as a source of OH⁻: Aqueous sodium hydroxide or potassium hydroxide supplies the hydroxide ion, OH⁻, which is the nucleophile that attacks the carbon bonded to the halogen.OCR
Role of catalysts: Catalysts change rate by lowering activation energy without altering ∆G.OCR
Role of catalysts and carriers: A halogen carrier such as FeBr₃ or AlCl₃ is required in halogenation to generate the electrophile, while nitration uses concentrated HNO₃ and concentrated…OCR
Role of concentrated sulfuric acid as catalyst: It protonates nitric acid and is regenerated, not consumed, acting as catalyst and dehydrating agent rather than supplying the nitro group.OCR
Role of ethanol as co-solvent: Ethanol is added as a co-solvent because the haloalkane and the aqueous silver nitrate do not mix well, so it brings them into one phase so the substitution…OCR
Role of ethanol as solvent: Ethanol is the solvent that brings the ionic cyanide into contact with the organic haloalkane.OCR
role of solvent: Conditions shift the balance rather than switching one route off: warm aqueous potassium hydroxide favours substitution, while hot potassium hydroxide…AQA
role of temperature: The contribution from the increase in dispersal of energy and matter grows with temperature, so temperature determines how much weight that factor carries.AQA
Role of thionyl chloride as chlorinating reagent: SOCl₂ supplies the chlorine and removes the –OH as SO₂ and HCl, both gases that leave the mixture.OCR
Roman numerals: Transition metals vary, so their charge is stated by the Roman numeral in the name, as in iron(III).AQA
Roman numerals as oxidation numbers: A Roman numeral in brackets after the element name states the oxidation number of that element in that particular species.OCR
rotations cancel: Each enantiomer rotates the plane of polarised light by the same angle but in opposite directions, so the two rotations cancel exactly.AQA
Rounding at the end of a calculation: Extra figures are kept through intermediate steps before rounding at the end.OCR
rules in order: Oxidation states are assigned by a fixed set of rules applied in order.AQA

S

s and p subshell order: The added electron fills the s subshell before the p subshell, with the 2p subshell filling from B to Ne and the 3p subshell filling from Al to Ar.OCR
s- and p-block scope up to Z = 36: The scope is s- and p-block elements up to Z = 36, so you handle ions such as Na⁺, Mg²⁺, Al³⁺, O²⁻, F⁻, S²⁻ and Cl⁻.OCR
s-block: Elements whose highest-energy electron enters an s subshell, including hydrogen and helium.OCR
s-orbital: Spherical in shape, with a 1s orbital as a single sphere and a 2s orbital a larger sphere with a spherical node inside it.OCR
s, p, d and f blocks: An element is classified into one of these according to its position in the Periodic Table, which is determined by its proton number.AQA
Safe assembly of apparatus: Joints must be greased and clamped securely, and condenser water enters at the lower end and leaves at the upper end.OCR
Safe laboratory practice in organic synthesis: Using anti-bumping granules and an open system to prevent violent boiling and pressure build-up.OCR
Safe small-scale technique: Using clean test tubes, small volumes, a water bath for warming where needed, and adding reagents carefully.OCR
Safe use of the separating funnel: The stopper is removed or the tap opened carefully to let air in and allow smooth flow, and shaking includes a venting step to relieve pressure from volatile…OCR
safety and storage: The anhydride can be stored and handled safely and reacts far less violently with water if there is a spill.AQA
Salt and hydrogen formation: A Group 2 metal reacts with a dilute acid to give a salt and hydrogen gas.OCR
salt bridge: Usually saturated KNO₃, it completes the circuit by letting ions move between the two solutions and keeps each electrically neutral; it does not carry…AQA
Salt bridge and complete circuit: A salt bridge, such as filter paper soaked in potassium nitrate solution, joins the two solutions so the circuit is complete.OCR
Salt bridge function: A salt bridge, often filter paper soaked in potassium nitrate, completes the circuit without mixing the solutions.OCR
salt bridge notation: Shown as a double vertical line in a cell diagram, marking the salt bridge.AQA
salt formation: With acids the reaction is complete and a salt forms, so ethylamine and hydrochloric acid give ethylammonium chloride, an ionic solid.AQA
Salt formation from cation and anion: The salt formula follows from the ions involved, using the charges on the cation and anion.OCR
Same element, different oxidation states: Both ions in the solution are of the same element but differ in oxidation state, such as iron(II) and iron(III) ions.OCR
Same element, same proton number: Atoms sharing the same atomic number and therefore the same number of protons and the same number of electrons in the neutral atom.OCR
Same formula, different spatial arrangement: Both isomers share the same molecular formula, so isomerism is a difference in spatial arrangement, not in composition.OCR
same functional group: Every member of the series contains it, so they react in the same way.AQA
Same functional group across a series: Members of a homologous series share the same functional group, so they react in similar ways.OCR
same polymer, same properties: The polymer is chemically identical to poly(ethene) from oil, so it is no more biodegradable.AQA
Sample-to-solvent ratio: The sample is dissolved in a large excess of solvent, so any ordinary protonated solvent would dominate the spectrum.OCR
saponification: Alkaline hydrolysis carried out on a triglyceride, giving soap and glycerol.AQA
saturated: Every carbon to carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can.AQA
saturated and unsaturated chains: In a fat the chains are saturated, while in an oil the chains carry Z (cis) C=C bonds that kink them.AQA
Saturated compound: Contains single carbon–carbon bonds only, so every carbon is bonded to four atoms by σ bonds.OCR
Saturated hydrocarbon groups: They are saturated, so they contain only single carbon–carbon bonds.OCR
Saturation and single bonding in alkanes: Alkanes are saturated hydrocarbons containing only single C–C and C–H bonds, with no C=C or C≡C.OCR
Scaling for formula stoichiometry: Terms are scaled correctly when the formula contains more than one of a given ion, for example doubling an ionisation or electron affinity term.OCR
Scaling fractional ratios: Multiplying all ratio values by the same small integer when a fractional ratio appears, so the empirical formula stays whole-numbered.OCR
scaling subscripts: Multiply every subscript in the empirical formula by the multiplier to deduce the molecular formula.AQA
scaling to molar quantities: Multiplying the mass of one atom in grams by the Avogadro constant to give the mass of one mole.AQA
scaling up fractions: Where a value such as 1.5 appears, the whole set is multiplied by 2 to reach whole numbers, since the ratio must be whole.AQA
scandium and zinc exceptions: Neither element is a transition metal because Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰, one empty and one full.AQA
secondary alcohol oxidation: A secondary alcohol has just one hydrogen on the carbon carrying the hydroxyl group, so it can be oxidised once and no further.AQA
Secondary alcohols: An alcohol in which the carbon bearing the –OH group is bonded to exactly two other carbon atoms.OCR
Selecting apparatus for a purpose: The choice depends on the precision needed and whether a single reading or a continuous record is required.OCR
Selecting the correct mathematical relationship: Choosing the relationship that matches the quantities given, such as n = c × V for a solution of known concentration and volume.OCR
Selecting the correct route: Identifying which named preparation route the stem describes before choosing an answer, then checking the reagents, the solvent and the excess condition.OCR
Selective reagents: Reagents chosen to act on the intended group without unwanted side reactions on the other groups in the molecule.OCR
Selectivity of reducing agents: NaBH₄ does not reduce C=C bonds, so it is a selective reducing agent for the carbonyl group.OCR
Self-indicating manganate(VII) titration of iron(II): Acidified potassium manganate(VII) is run from a burette into a measured volume of iron(II) solution until a permanent pink colour persists, as manganate(VII)…OCR
separating funnel: The organic liquid is shaken in it with sodium carbonate solution to remove acid, venting the carbon dioxide.AQA, OCR
separating funnel wash: The crude distillate is shaken in a separating funnel with sodium carbonate solution, which neutralises the unreacted acids and releases carbon dioxide, so…AQA
Separating raw and processed data: Raw readings are kept in separate columns, with means or processed values calculated in a further column.OCR
Separation by differing affinity: Each component travels a distance depending on its balance between the two phases, so components separate because they have different affinities for the…OCR
Separation by distillation: Separates a volatile liquid from a mixture by boiling it and condensing the vapour, collecting a distillate.OCR
sequence and direction: Order matters, because glycine joined to alanine is a different dipeptide from alanine joined to glycine as the peptide link points the other way.AQA
Sequence of drying and redistillation in synthesis: Drying removes water only, not other impurities, so it follows separation and precedes redistillation.OCR
setting ∆G to zero: This gives the temperature at which a reaction just becomes feasible, rearranging to T = ∆H/∆S.AQA
shape of the energy tail: The tail falls away steeply, so a small rightward shift moves a disproportionately large slice of molecules past Eₐ.AQA
Shapes and bond angles of simple molecules and ions: Two bonding pairs give a linear shape with a 180° bond angle; three give trigonal planar with 120°; four give tetrahedral with 109.5°.OCR
Shapes of molecules and ions: Counting all pairs, bonding and lone, fixes the total arrangement: two pairs give a linear base arrangement, three trigonal planar, four tetrahedral, five…OCR
Shapes of s, p and d orbitals: An s orbital is spherical, the three p orbitals are dumbbell-shaped and mutually perpendicular, and the five d orbitals are more complex.OCR
Shared electron pair: One pair of electrons shared between two nuclei, sitting between them so that each nucleus is attracted to the same pair.OCR
Shared electron pairs: Each shared pair contributes one electron from each bonded atom, so a double bond contains four shared electrons and a triple bond six.OCR
Shared electron pairs and the octet rule: Each atom contributes one electron to the shared pair, giving both atoms a full outer shell in most simple molecules.OCR
Shared outer-shell electron count: Elements in the same group share the number of outer-shell electrons, so they show similar chemical properties that change gradually down the group.OCR
shared pair: One pair of electrons shared between two atoms, sitting between them in a single covalent bond.AQA
Shell boundaries and large jumps: A large jump in energy marks the start of a new shell, occurring when an electron is taken from a shell closer to the nucleus.OCR
Shell capacity and the 2n² relationship: The nth shell contains n² orbitals and each orbital holds two electrons, so the maximum number of electrons is 2n².OCR
shells and energy levels: The electrons occupy shells around the nucleus, held by electrostatic attraction between their negative charge and the positive nucleus.AQA
Shielding: The reduction in nuclear attraction felt by an outer electron because of repulsion and screening by inner-shell electrons.AQA
shielding and deshielding: A nucleus never feels the whole applied field because the electrons around it circulate and shield it, while anything that pulls electron density away leaves…AQA
shielding and distance: The outer pair of electrons lies further from the nucleus and is shielded by more inner shells, and these effects outweigh the extra protons.AQA
short residence time: Deliberately around one second, because the products are more reactive than the feedstock and would otherwise react further.AQA
Side chains on rings: Groups that an alicyclic ring may or may not carry, attached to the ring.OCR
sideways p orbital overlap: The π bond is formed by sideways overlap of p orbitals above and below the axis.AQA
Sign convention: A convention fixed relative to the system, so the sign of ∆H follows the direction of energy leaving or entering the system rather than the size of the change.OCR
Sign convention for exothermic and endothermic reactions: An exothermic reaction gives a negative enthalpy change and an endothermic reaction gives a positive one.OCR
Sign conventions: Reversing the equation reverses the sign of ∆rH, since exothermic becomes endothermic and vice versa.OCR
Sign conventions and reversing arrows: Reversing an arrow reverses the sign, so the sign of any step traversed in the opposite sense is reversed.OCR
Sign conventions for enthalpy terms: Lattice enthalpy and electron affinity are treated as exothermic, while ionisation and atomisation are treated as endothermic, with the correct sign applied…OCR
Sign conventions in thermochemistry: Each combustion enthalpy keeps its negative sign, and the final ΔrH is reported with the correct sign and units, kJ mol⁻¹.OCR
sign of ∆G: The test for feasibility: a reaction is feasible when ∆G is zero or negative, and is not feasible when ∆G is positive.AQA
sign of ∆H: ΔH is positive for an endothermic reaction and negative for an exothermic reaction, linked to the net balance of bond breaking and bond making.AQA, OCR
Sign of combustion enthalpy values: Combustion of fuels releases energy, so ∆cH values are usually negative.OCR
Sign of standard cell potential: A positive value predicts the reaction is thermodynamically feasible under standard conditions, and a positive E⦵cell corresponds to a negative standard Gibbs…OCR
Sign-first notation and its distinction from ionic charge: Oxidation numbers are written sign-first, for example +6, never as a charge superscript.OCR
Significant figure counting rules: Leading zeros are never significant, while trailing zeros after a decimal point are significant.OCR
significant figures: Quote answers to three significant figures, because burette readings support that precision.AQA
Significant figures and standard form: Rounding the final answer to an appropriate number of significant figures consistent with the data, and using standard form and correct powers of ten for very…OCR
Significant figures and unit consistency: Evaluates k to an appropriate number of significant figures with correct arithmetic, keeping concentrations in mol dm⁻³ and rate in mol dm⁻³ s⁻¹.OCR
Significant figures and units in pH calculations: pH quoted to two decimal places corresponds to concentration to two significant figures, concentration is in mol dm⁻³ and pH is dimensionless.OCR
Significant figures in particle calculations: The value is kept to three significant figures as given, so 6.02 × 10²³ is used rather than rounding the constant to 6 × 10²³.OCR
silver halide colours: Chloride gives a white precipitate, bromide a cream one and iodide a yellow one, while fluoride gives no precipitate because silver fluoride is soluble.AQA, OCR
Silver halide precipitate colours: The colour of the precipitate identifies the halogen: silver chloride is white, silver bromide is cream and silver iodide is pale yellow.OCR
Silver halide precipitates: Adding dilute nitric acid then silver nitrate gives a white precipitate for chloride, a cream precipitate for bromide and a pale yellow precipitate for iodide.OCR
silver mirror: The deposit of silver on the tube wall when the silver ion is reduced by an aldehyde.AQA
Silver nitrate as a halide test: Silver ions, Ag⁺, react with the released halide ion to form a silver halide precipitate, whose appearance and colour identify which halogen was present.OCR
Similar electronegativity of carbon and hydrogen: Carbon and hydrogen have similar electronegativities, which makes the σ-bonds in alkanes essentially non-polar.OCR
Simple molecular lattice: A regular arrangement of discrete covalently bonded molecules attracted by intermolecular forces.OCR
simple molecular structure: The halogens are simple molecular, so boiling only separates molecules and the covalent bond inside each molecule is untouched.AQA
simplest ratio: The relative amounts divided by the smallest value, multiplied up if a half or a third remains.AQA
Simplest whole-number ratio: The result of dividing every mole value by the smallest mole value, giving the empirical formula.OCR
Simplest whole-number ratios in formulae: Reducing the ratio of atoms to its simplest whole numbers, such as dividing Fe₄O₆ through to Fe₂O₃.OCR
Single covalent bonding in alkanes: Each carbon atom in an alkane forms four single covalent bonds, so it is surrounded by four bonding pairs of electrons that are equivalent.OCR
single crossing point: The hotter curve crosses the original curve exactly once before running above it at all energies past that crossing point.AQA
site of nucleophilic attack: The electron-deficient carbon is attacked because a nucleophile is attracted to positive charge.AQA
Site of reaction: The group that reacts with a given reagent, chosen and justified from the structure.OCR
six co-ordinate complexes: Complexes with a co-ordination number of 6, usually allowed by small ligands such as H₂O, NH₃ and CN⁻.AQA
Six-fold coordination: Six coordinate bonds from ligands to the central metal ion, giving coordination number 6.OCR
size of the d orbital gap: The energy difference between the ground state and the excited state of the d electrons, calculated using h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.AQA
skeletal formula: A plain zigzag with carbons at the angles and ends and hydrogens on carbon left out.AQA
Skeletal formulae: A simplified organic formula shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups.OCR
skeletal formulae in mechanisms: Skeletal formulae are acceptable in mechanisms, but where a C–H bond breaks both that hydrogen and its bond must be drawn.AQA
slight pressure and high temperature: The conditions of catalytic cracking, at a slight pressure and a high temperature of about 720 K.AQA
Slow cooling and crystallisation: Cooling slowly allows purified crystals to form while impurities remain in solution, giving larger, purer crystals.OCR
Small-scale test-tube qualitative analysis: Adding reagents to a sample of the unknown compound in a test-tube and observing the resulting change to identify ions.OCR
soap as a salt: The sodium salt of a long-chain carboxylic acid, RCOO⁻Na⁺, produced along with glycerol.AQA
Sodium carbonate and carboxylic acids: Carboxylic acids liberate carbon dioxide from sodium carbonate, giving effervescence.OCR
sodium chlorate(I): The compound NaClO, formed with sodium chloride and water when chlorine reacts with cold, dilute, aqueous sodium hydroxide.AQA
sodium chloride lattice: A giant ionic structure with 6:6 co-ordination.AQA
sodium hydroxide tests: Adding sodium hydroxide solution dropwise and then in excess, giving precipitates such as blue for copper(II) and brown for iron(III).AQA
solids, liquids and gases: Entropy rises sharply with freedom of movement in this order, with gases much higher again.AQA
Solubility: Depends on the balance between the energy needed to separate the particles and the energy released when new solute–solvent intermolecular forces form.OCR
Solubility and choice of solvent: Choosing a solvent in which the solid is very soluble when hot but only slightly soluble when cold is essential.OCR
Solubility and formation of sparingly soluble hydroxides: The precipitate forms because the hydroxide ion concentration exceeds the solubility product of the metal hydroxide.OCR
Solubility in excess NaOH and excess NH₃: Most of the precipitates are insoluble in both excesses, but Cr(OH)₃ dissolves in excess NaOH to a green solution of [Cr(OH)₆]³⁻ and dissolves slightly in…OCR
Solubility in water and chain length: Hydrogen bonding to water molecules makes short-chain alcohols fully miscible with water, while longer chains become less soluble as the hydrocarbon part grows.OCR
Solubility of silver halides in ammonia: Dilute ammonia dissolves silver chloride but not silver bromide or silver iodide, and concentrated ammonia dissolves silver bromide while silver iodide…OCR
Solubility through ion–dipole interactions: Ion–dipole interactions between polar water molecules and ions help to separate the lattice, so many ionic solids dissolve in polar solvents such as water.OCR
solubility trend: Solubility in ammonia falls down the group, matching the increasing insolubility of the silver halide itself.AQA
Solubility trends of hydroxides and sulfates: Solubility of hydroxides increases down the group while solubility of sulfates decreases.OCR
Soluble complex ions: Complex ions formed when some transition metal hydroxide precipitates dissolve on addition of excess aqueous sodium hydroxide or excess aqueous ammonia.OCR
Solute–solvent interactions: Hydrogen bonds form between the acid and water molecules, releasing energy that offsets the energy needed to separate the acid molecules and the water…OCR
solvent below the spots: The plate stands in a tank with the solvent below the baseline, never above it, or the spots dissolve into the solvent instead of climbing.AQA
Solvent selection by temperature-dependent solubility: The solvent is chosen so the compound is very soluble when hot but only slightly soluble when cold.OCR
solvent-free processes: Working without a solvent removes volatile organic compounds from the air, removes the energy cost of recovering the solvent, and removes an entire separation…AQA
Source of H⁺ and OH⁻ ions: The hydrogen ion comes from the acid and the hydroxide ion comes from the base.OCR
sources of error: Rinsing the conical flask with acid adds extra moles and raises the titre, and drops falling from the funnel decrease the titre.AQA
Sources of error in enthalpy measurement: Heat loss, incomplete combustion and non-standard conditions are accounted for as sources of error.OCR
Sparingly soluble: Dissolving only to a small extent in a solvent.AQA
specific heat capacity: The energy needed to raise one gram by one kelvin, 4.18 J g⁻¹ K⁻¹ for water.AQA
Specifying the entity being counted: The entities must be stated, because a mole of chlorine atoms differs from a mole of chlorine molecules.OCR
Spectator ions: Ions that appear unchanged on both sides of an ionic equation and are cancelled so that only the species that change appear.OCR
Spin–spin splitting: The splitting of a proton's signal in high-resolution ¹H NMR caused by the magnetic states of non-equivalent protons on adjacent carbon atoms.OCR
Splitting and the n + 1 rule: Splitting follows the n + 1 rule, where n is the number of non-equivalent protons on adjacent carbon atoms, so a singlet means no neighbouring protons, a…OCR
spontaneous endothermic processes: An endothermic process can still occur spontaneously, for example ammonium nitrate dissolving in water or a thermal decomposition.AQA
spread of molecular energies: At any temperature the particles have a wide spread of kinetic energies, with only the small area under the tail beyond Eₐ representing particles energetic…AQA
Square planar geometry: The four donors lie in one plane at the corners of a square, giving bond angles of 90°.OCR
Square-planar geometry of Pt(II) complexes: The four ligands occupy positions in one plane around the Pt²⁺ centre, so the 90° and 180° positions are meaningful.OCR
Stability of the arenium intermediate: An unstable arenium intermediate forms when the ring attacks the electrophile, and its stability at each position explains why one position is favoured over…OCR
Standard cell potential equation: E⦵cell = E⦵(positive electrode) − E⦵(negative electrode), applied with the signs of both values retained.OCR
standard conditions: The agreed reference conditions of 100 kPa and a stated temperature, conventionally 298 K (25 °C), used so that enthalpy values can be compared between…AQA, OCR
Standard conditions and cell e.m.f.: Standard conditions of 298 K, 1 mol dm⁻³ solutions and 100 kPa gases give E⦵cell.OCR
Standard electrode (redox) potential: Measures the tendency of a half-cell to gain electrons relative to the standard hydrogen electrode, which is assigned E⦵ = 0.00 V.OCR
Standard electrode potentials: Values compared for two half-cells to interpret or predict whether a reaction occurs, with the more positive half-equation proceeding as a reduction.OCR
Standard electrode potentials and spontaneity: The half-cell with the more positive E° is reduced and the other is oxidised, giving the spontaneous direction.OCR
Standard enthalpies of formation and combustion: Typical cases of the calculation use standard enthalpies of formation from elements or standard enthalpies of combustion to the same oxides.OCR
Standard enthalpy of combustion: A measurable enthalpy change that can be used in an enthalpy cycle to find an enthalpy change that is hard to measure directly.OCR
Standard enthalpy of formation: A measurable enthalpy change that can be used in an enthalpy cycle to find an enthalpy change that is hard to measure directly.OCR
Standard entropy values and their units: Tabulated entropies of reactants and products, usually quoted in J K⁻¹ mol⁻¹, used to calculate the entropy change of a system.OCR
standard hydrogen electrode: The agreed reference to which electrode potentials are measured, assigned a potential of exactly 0.00 V.AQA, OCR
standard solution: Made by weighing the solid, dissolving it, transferring the solution and all the washings to a volumetric flask, making up to the graduation mark with…AQA
standard solutions: Solutions of known concentration made up by accurate dilution of a stock solution, whose absorbance is measured to plot the calibration curve.AQA
Standard state: The physical state of an element or compound under standard conditions, such as liquid water or gaseous oxygen at 298 K and 100 kPa.OCR
Standard states and zero values: Elements in their standard states, such as O₂(g), have zero enthalpy change of formation.OCR
Starch-indicated iodine/thiosulfate titration: Iodine is titrated with sodium thiosulfate using starch indicator, added near the end point, until the blue-black colour disappears.OCR
starting pH: About 1 for a strong acid and about 3 for a weak acid of the same concentration.AQA
State symbols: The symbols (s), (l), (g) and (aq), which must match the physical state of each species under the stated conditions.OCR
Stated equation: The equation as written, whose stoichiometric coefficients and direction fix the magnitude and sign of ∆rH.OCR
stationary and mobile phases: Every chromatographic method has a stationary phase that stays put and a mobile phase that moves through or over it.AQA
stationary phase: Silica or alumina, on which amino acids are retained by adsorption.AQA
steep section: The steep part of the curve, spanning roughly pH 3 to pH 11 for a strong acid with a strong base.AQA
Steep section of the titration curve: The steep vertical section of the titration curve contains the equivalence point and is compared with the indicator's pH range.OCR
stereoisomer: A molecule with the same structural formula as another but a different arrangement of atoms in space.AQA
stereoisomerism: A form in which molecules have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in…AQA, OCR
Stereoisomerism in complexes: Complexes have the same molecular formula and the same bonds but a different spatial arrangement of atoms.OCR
stereoisomers: Molecules with the same structural formula, the same atoms joined to the same atoms, but a different arrangement in space.AQA, OCR
stereospecificity: Because the amino acids that form the site are themselves chiral, the site is chiral, so it fits only one enantiomer of a substrate.AQA
Stoichiometric calculation of amounts: Using the balanced equation and the known starting amounts to work out the amounts of the other species from the measured amount of one species.OCR
Stoichiometric change at equilibrium: The change in each species is in proportion to its coefficient in the equation, so each change is scaled by the coefficient in the balanced equation.OCR
Stoichiometric coefficients as exponents: Each concentration in the Kc expression is raised to its stoichiometric coefficient from the balanced equation.OCR
stoichiometric ratio: The mole ratio from the balanced equation, such as 2 : 1 for HCl with Ba(OH)₂.AQA
Stoichiometric scaling: Doubling the coefficients doubles the magnitude of ∆rH, so the value depends on the equation as written.OCR
Stoichiometric scaling and sign conventions: Each enthalpy change of formation is multiplied by its coefficient, and the order is products minus reactants, the reverse of the combustion route.OCR
Stoichiometric scaling of enthalpy steps: Multiplying a step multiplies its value, so each step is scaled by the number of moles involved before combining it with other steps.OCR
Stoichiometry from balanced equations: The mole ratio from the balanced equation converts moles of one substance into moles of another, so the equation must be balanced first.OCR
Stoichiometry in enthalpy calculations: Each combustion enthalpy is multiplied by its stoichiometric coefficient from the balanced equation when summing.OCR
stoichiometry is not the rate equation: The stoichiometric equation counts what is consumed overall, whereas the rate equation reflects only the species involved up to and including the slowest step.AQA
Straight chains: One way in which the carbon and hydrogen of an aliphatic compound are joined together.OCR
stratospheric ozone: Ozone formed naturally in the upper atmosphere, which is beneficial because it absorbs ultraviolet radiation.AQA
strength of adsorption: Adsorption must be of the right strength: too weak and too few molecules stick, too strong and the products never leave.AQA
strength of halide reducing agents: Among the halides the iodide ion is the strongest reducing agent, which is why it alone is able to reduce sulfur in concentrated sulfuric acid as far as…AQA
strength versus concentration: Strength is about the fraction of protons given up, while concentration is about how many moles are in a cubic decimetre.AQA
Strong acid: A strong monoprotic acid such as HCl is fully dissociated, so [H⁺] equals the acid concentration and pH follows directly.OCR
strong acid pH: For a strong monoprotic acid the dissociation is complete, so every mole of acid delivers a mole of H⁺ and [H⁺] is simply the acid concentration, with pH =…AQA
Strong acids dissociate almost completely: A strong acid dissociates almost completely in aqueous solution, so nearly every acid molecule releases its H⁺ ions.OCR
Strong alkali full ionisation: The strong alkali is fully ionised, so its OH⁻ ions are neutralised almost completely by the weak acid.OCR
Strong and weak acids and bases: The relative strengths of the acid and base determine the starting pH, the height of the jump and the salt formed.OCR
Strong base dissociation: For a strong base such as NaOH or KOH, the hydroxide ion concentration equals the concentration of the base because each formula unit dissociates fully.OCR
Strong monobasic acid: An acid that releases exactly one proton per molecule and ionises essentially completely in water, so the equilibrium lies far to the right.OCR
Strong versus concentrated: Strong means full ionisation, whereas concentrated means many moles per dm³, so a dilute strong acid still has a low pH.OCR
Structural formula: The minimal detail that shows the arrangement of atoms in a molecule, listing each carbon with the atoms or groups attached to it in order along the chain.OCR
Structural formulae: The repeat unit is shown with the correct number of hydrogen atoms and substituents, with bracket lines through the single bonds that link one repeat unit to…OCR
structural isomer: Compounds with the same molecular formula but a different structural formula, as their atoms are joined in a different order.AQA
Structural isomers: Compounds with the same molecular formula but different structural formulae, meaning the atoms are joined in a different order or arrangement.AQA, OCR
Structural representation: Drawing the structure and marking equivalent hydrogens with the same label before counting the environments.OCR
Structural requirement for oxidation of alcohols: Oxidation requires the carbon bearing the –OH group to lose a hydrogen atom as well as the hydrogen of the –OH group, so that a C=O double bond can form.OCR
Structure and bonding across a period: Across Period 2 and Period 3, the bonding and structure of the elements change from metallic to giant covalent to simple molecular.OCR
Structure and classification of secondary alcohols: A secondary alcohol has the –OH group on a carbon that is bonded to two other carbon atoms and to one hydrogen atom.OCR
structure and property: Identify the structure first, then name the force broken, then predict the property.AQA
structure determines properties: Controlling the structure of the chain, usually through the catalyst, controls the properties of the material.AQA
Structure interpretation: Reading a displayed structure and treating each group on its own merits rather than assuming one group dominates.OCR
structure of the active site: Mapped by software after the three-dimensional structure of the enzyme is determined, covering its shape, the positions of charged and polar groups and where…AQA
Structure reading: Tracing the bonds around each candidate group in the displayed structure before committing to a name, and rechecking the structure after listing groups to…OCR
Structure–property links: Because the electrons are delocalised, they can carry charge when a potential difference is applied, which explains why metals conduct electricity in the…OCR
Structure–property relationships: Explaining a property such as basicity or solubility by reference to structure, for example lone-pair availability or hydrogen bonding.OCR
Sub-shell capacities: s = 2, p = 6, d = 10: Each s sub-shell holds 2 electrons, each p sub-shell holds 6 and each d sub-shell holds 10.OCR
Sub-shell composition of each shell: Shell 1 has 1s; shell 2 has 2s and 2p; shell 3 has 3s, 3p and 3d; shell 4 has 4s, 4p, 4d and 4f.OCR
sub-shell filling order: Electrons fill sub-shells in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.AQA
Sub-shells: Each shell is divided into these, and which ones are available depends on n, so shell 1 has only 1s and shell 2 has 2s and 2p.OCR
Sub-shells within each shell: The first shell holds only 1s, the second holds 2s and 2p, and the third holds 3s, 3p and 3d.OCR
subjective endpoint: Judging the moment the cross vanishes is subjective.AQA
Subshell filling across Periods 2 and 3: Each successive element across a period gains one proton and one electron, and the added electron enters the same outer shell until that shell is full.OCR
Subshell filling order: Subshells are filled in order of increasing energy, and orbitals of equal energy fill singly before pairing, each orbital holding at most two electrons.OCR
Subshell notation and principal quantum numbers: Configurations are written in subshell notation using s, p and d labels with the correct principal quantum number, for example Mg as 1s² 2s² 2p⁶ 3s².OCR
Substituent position and structure recognition: Every substituent stays attached to the same backbone carbon and in the same position when converting the polymer section back to the monomer.OCR
substituted alkenes: These behave identically to alkenes in polymerisation because the substituent is a spectator.AQA
Substituting n: Replacing n with a value and evaluating 2n + 2 in full to find the hydrogen count, for example n = 4 giving C₄H₁₀.OCR
substitution: The chlorine replaces a hydrogen atom, and the hydrogen that leaves is carried away as hydrogen chloride.AQA
Substitution of haloalkanes: Aliphatic amines are made by substituting haloalkanes with excess ethanolic ammonia and amines, with the halogen leaving and the nitrogen of ammonia bonding…OCR
Substitution of n: Taking the general formula for a series and substituting the value of n for the member required, checking the arithmetic in the subscript.OCR
substitution over addition: A substitution replaces a hydrogen atom so the delocalised ring is regenerated and the stability is kept, whereas addition would break the ring and lose that…AQA
Substitution Reaction: A reaction in which an atom or group of atoms is replaced by a different atom or group of atoms.AQA
Substitution restoring aromaticity: In each case the aromatic ring is regenerated, so the reaction is substitution rather than addition.OCR
Substitution versus elimination controlled by solvent: Aqueous conditions favour substitution, whereas if the alkali is dissolved in ethanol the dominant pathway becomes elimination, giving an alkene.OCR
successful collision frequency: The frequency of successful collisions, and therefore the rate, increases greatly at the higher temperature.AQA
successful collisions: The same fixed proportion of collisions has energy greater than or equal to Eₐ, so these become more frequent and the rate rises.AQA, OCR
Successive ionisation energies: The second ionisation energy is always larger than the first because the electron is pulled from a positively charged ion that is smaller and holds its…AQA, OCR
Successive members differ by CH₂: Each successive member differs from the previous one by a CH₂ unit, adding one carbon and two hydrogens.OCR
Suffixes for principal functional groups: Naming a functional group using its suffix where it takes priority, for example -ol, -oic acid or -al.OCR
Suitable recrystallisation solvent: Dissolves the solid when hot but only sparingly when cold, so crystals form on cooling.OCR
Sulfate test with barium chloride: Acidifying with dilute hydrochloric acid, then adding barium chloride solution, gives a white precipitate of barium sulfate that confirms sulfate.OCR
sulfur impurities in fuel: Crude oil contains sulfur compounds, and any sulfur left in a fuel is oxidised when that fuel burns.AQA
sum equals the charge: The states in a species sum to its overall charge, so zero for a neutral compound and the ionic charge for an ion.AQA
Sum of oxidation numbers equals overall species charge: The sum of oxidation numbers is zero for a neutral compound and the ion charge for a polyatomic ion, so an unknown value can be calculated by subtracting…OCR
supply and demand: The economic reason for cracking is converting a fraction in low demand into fuels and alkenes in high demand.AQA
support medium: An inert material with a very large surface area over which a thin layer of metal is spread, contributing nothing chemically.AQA
Surface adsorption: In heterogeneous catalysis the reaction occurs at the catalyst surface, where reactant molecules are adsorbed, react and then desorb.OCR
Surface-catalysed addition: Hydrogen and the alkene are adsorbed onto the nickel surface, where the reaction occurs, so it is not an electrophilic addition and no carbocation…OCR
Suspected link to global warming: The greenhouse effect is the basis of the suspected link between rising concentrations of such gases and global warming.OCR
Sustainability: Supported by high atom economy because fewer raw materials are wasted per tonne of product and the process makes more efficient use of finite resources.OCR
Sustainability and CO₂ emissions: Much industrial heating relies on burning fossil fuels, so using less fuel lowers energy demand and reduces the CO₂ emissions associated with that combustion.OCR
Sustainability trade-offs: Energy recovery is one option alongside recycling and feedstock reuse, and it does not remove the need to reduce plastic waste at source.OCR
Symmetry test for optical activity: A complex with a plane of symmetry is not optically active, so symmetry must be checked before claiming optical activity.OCR
Systematic naming of esters: The ester is named by combining the alkyl group from the alcohol with the -oate stem from the acyl group.OCR
Systematic nomenclature: Naming each functional group using correct chemical terminology, using the full systematic name of the group rather than a vague description.OCR
Systematic nomenclature of esters: Naming follows the pattern alkyl alkanoate, with the alcohol supplying the first part of the name and the acid supplying the second.OCR
systematic reading of a spectrum: Reading a spectrum in a fixed order, looking first for a strong sharp absorption near 1700 cm⁻¹ and then for O–H.AQA
Systematic structural reading: Reading a displayed or condensed structure by locating the carbon skeleton and then checking for each functional group it contains, one at a time.OCR

T

Tabulating data with headed columns: Numerical data belong in a table with clear column headings that name the quantity and give its unit.OCR
Tangent construction at a chosen time: Draw a tangent to the curve at the chosen time, then divide the change in the quantity by the change in time.OCR
Tangent method: Draw a tangent to the curve at the chosen time, then pick two points far apart on the tangent and calculate the gradient.OCR
tangents and gradients: The rate at any moment is the gradient of the tangent at that point on a concentration against time graph.AQA
tangents and initial rate: On a concentration against time graph, the rate at any moment is the gradient of the tangent, and the initial rate is the gradient at time zero.AQA
temperature against time graph: A plot of recorded temperature before, during and after mixing, used to show how the temperature changes.AQA
temperature and rate: Raising the temperature broadens the distribution and increases the number of molecules beyond the activation energy, so more successful collisions occur per…AQA, OCR
temperature and the endothermic direction: Raising the temperature shifts the equilibrium in the endothermic direction, absorbing energy to oppose the rise, while cooling shifts it the exothermic way.AQA
Temperature as a rate factor: Raising the temperature increases the average kinetic energy of the particles, so the rate increases; lowering it slows the reaction.OCR
temperature change: ∆T, which can be read straight off the thermometer because a kelvin and a degree Celsius are the same size.AQA
temperature control: Keeping the temperature at about 50 °C, because above this a second nitro group is substituted and dinitrobenzene forms.AQA, OCR
temperature dependence: The dissociation is endothermic, so heating raises Kw, lowers the pH of water, and still leaves it neutral.AQA
Temperature dependence and threshold temperature: Temperature scales the T∆S term, so a reaction with positive ∆H and positive ∆S becomes feasible only above a threshold temperature, while one with negative…OCR
Temperature dependence of equilibrium constants: The numerical value of an equilibrium constant changes with temperature and is unaffected by concentration or pressure changes at constant temperature.OCR
Temperature dependence of feasibility: When ∆H and ∆S have the same sign, one temperature range is feasible and the other is not, so changing temperature alters the sign of ∆G.OCR
Temperature dependence of Kc: Changing temperature alters the value of Kc itself: for an exothermic forward reaction, raising temperature lowers Kc and shifts equilibrium towards…OCR
Temperature dependence of Kw: Because the self-ionisation is endothermic, Kw increases as temperature rises, so neutral water at a higher temperature has a lower pH than 7.OCR
Temperature effects: Heating shifts equilibrium in the endothermic direction and cooling shifts it in the exothermic direction.OCR
temperature gradient: The column is hot at the bottom and cooler towards the top.AQA
Temperature in kelvin and reciprocal plots: Temperatures must be converted to kelvin before calculating 1/T, and ln k is plotted against 1/T rather than against T so the graph is linear.OCR
temperature weighting: Because the temperature multiplies ∆S, the entropy contribution grows as the system is heated, which is why heating can make an endothermic reaction go.AQA
template for replication: Pull the strands apart and each one is a template, which is exactly what makes copying possible.AQA
tensile strength: Kevlar's chains are aromatic and rigid, and they hydrogen bond into flat sheets that stack, giving a tensile strength greater than steel for the same mass.AQA
termination: A step that joins two radicals to give a non-radical, removing radicals from the system.AQA, OCR
Tertiary alcohol: An alcohol in which the –OH group is on a carbon bonded to three other carbon atoms, for example 2-methylpropan-2-ol, (CH₃)₃COH.OCR
tertiary alcohols: Alcohols in which the carbon carrying the hydroxyl group is bonded to three alkyl groups and no hydrogen.AQA, OCR
tertiary folding: The way the whole chain, helices and sheets together, folds into one three-dimensional shape, held by interactions between R groups.AQA
tertiary structure: The fold of the chain that creates a small cleft, the active site, whose shape and charge distribution are complementary to one particular substrate.AQA
Test for phenol using bromine water: The observation of decolourisation and a white precipitate is a test for phenol.OCR
test for sulfate ions: Barium chloride is the reagent, giving a dense white precipitate of barium sulfate, and the solution must be acidified first with dilute hydrochloric acid.AQA
test with bromine water: Shaking with bromine water at room temperature turns it from orange to colourless as bromine adds across the double bond, while an alkane leaves it orange.AQA
test-tube identification: Carrying out simple test-tube reactions to identify transition metal ions in aqueous solution.AQA
Test-tube scale qualitative analysis: Carrying out reactions in a test tube to identify ions in an unknown compound, recording observations such as precipitate colour and whether it dissolves in…OCR
tetrahedral arrangement: Four pairs move into three dimensions, tetrahedral at 109.5°.AQA
tetrahedral complexes: Complexes in which four ligands arrange themselves tetrahedrally around the metal ion, with bond angles of 109.5°.AQA
Tetrahedral geometry: The four donors sit at the corners of a tetrahedron, giving bond angles of about 109.5°.OCR
tetrahedral intermediate: The structure formed in the addition step, with a negative oxygen and a positive nitrogen, whose formal charges must be drawn.AQA
Tetrahedral shape about each carbon in alkanes: Each carbon atom in an alkane is bonded to four other atoms by four single covalent bonds, so the shape is described as tetrahedral about each carbon.OCR
tetramethylsilane reference: The standard added to the sample, assigned a shift of zero, sitting at the right-hand edge.AQA
The ¹²C atom: The standard for atomic masses, with one unit equal to one-twelfth of its mass.OCR
The ¹²C standard for atomic masses: The mass of one ¹²C atom, defined as exactly 12 units, so one unit is one-twelfth of that mass.OCR
The Avogadro constant and its unit: The number of particles per mole, 6.02 × 10²³ mol⁻¹, where the unit mol⁻¹ shows it is a count per mole.OCR
The carbonate ion test with dilute acid and limewater: Adding dilute acid and passing the gas through limewater, which turns milky.OCR
The equilibrium constant expression: For a homogeneous equilibrium aA + bB ⇌ cC + dD, it is written with product concentrations in the numerator and reactant concentrations in the denominator,…OCR
The hydroxyl group and hydrogen bonding: The −OH group is polar, so hydrogen bonding occurs between alcohol molecules, raising boiling points above those of comparable alkanes.OCR
The ideal gas equation and its variables: The equation pV = nRT relates the pressure p, volume V, amount n and temperature T of a gas through the gas constant R.OCR
The mole and the Avogadro constant: The mole is the unit of amount of substance, and one mole contains the Avogadro constant of entities, approximately 6.02 × 10²³ mol⁻¹.OCR
the mole as a count: A counted amount that contains as many particles as the Avogadro constant.AQA
The n + 1 rule: If n non-equivalent protons sit on the neighbouring carbon atom(s), the signal splits into n + 1 peaks.OCR
the n+1 rule: A signal is split into one more line than the number of non-equivalent protons on the neighbouring carbons, limited to doublet, triplet and quartet formation…AQA
Theoretical and actual yield: The theoretical maximum comes from the balanced equation, while the amount actually obtained is the actual yield.OCR
Theoretical yield: The maximum amount of product predicted from the balanced equation, calculated in moles from the limiting reactant.OCR
thermal cracking conditions: High pressure and high temperature, with temperatures around 1000 K and pressures up to about 7000 kPa.AQA
Thermodynamic feasibility versus kinetic rate: A negative ∆G shows a process is thermodynamically feasible, but it says nothing about how fast the process happens, which is a kinetic matter.OCR
thermometer at the side arm: The thermometer bulb sits level with the side arm, where the vapour leaves, so it reads the temperature of the vapour being condensed rather than of the liquid.AQA
thin-layer chromatography: A plate is coated with a solid and a solvent moves up the plate.AQA
Thin-layer, column and gas chromatography: TLC uses a thin layer of solid adsorbent such as silica on a plate, column chromatography packs the stationary phase in a tube, and gas chromatography uses a…OCR
three ionisation energy factors: Nuclear charge, distance of the outer electron from the nucleus, and shielding by inner shells, used to compare two elements.AQA
Three p-orbitals of a sub-shell: They lie at right angles along the x, y and z axes, so they are often drawn as a set of three mutually perpendicular dumbbells.OCR
tin and hydrochloric acid: The reducing system used with nitrobenzene, heated under reflux, to reduce the nitro group.AQA
Titration against a standard solution: Titrating the quenched portion against a standard solution and using the result to calculate the amount of one species.OCR
Titration curves and the equivalence point: Choosing an indicator means matching its range to the sharp vertical section of the titration curve at the equivalence point.OCR
Tollens' and Fehling's: An aldehyde gives a silver mirror with Tollens' reagent or a red precipitate with Fehling's solution, both warmed in a water bath.AQA
Tollens' and Fehling's reagents: Tollens' reagent gives a silver mirror with an aldehyde while a ketone does not, and Fehling's solution gives a brick-red precipitate of copper(I) oxide with…OCR
Tollens' and Fehling's tests for aldehydes: Aldehydes reduce Tollens' reagent to a silver mirror and reduce Fehling's solution from blue to a brick-red precipitate, whereas ketones do not.OCR
Tollens' reagent: Colourless ammoniacal silver nitrate, [Ag(NH₃)₂]⁺, which an aldehyde reduces to a silver mirror of metallic silver when warmed gently, whereas a ketone gives…AQA, OCR
Tollens’ reagent composition: Ammoniacal silver nitrate, made by treating silver nitrate solution with ammonia until the grey-brown silver oxide just redissolves, leaving the colourless…OCR
Tollens' reagent test: Tollens' reagent supplies silver ions in an alkaline ammonia solution; warmed with an aldehyde it gives a silver mirror, while a ketone gives no such change.OCR
Tollens' silver mirror: Ammoniacal silver nitrate gives a silver mirror with an aldehyde and no change with a ketone.AQA
Toxic chlorine gas hazards: The risks include the toxicity of chlorine gas to workers and the public if it escapes.OCR
toxic combustion products: Burning PVC releases hydrogen chloride, which must be removed by passing the flue gases through a base, and too low a combustion temperature produces toxic…AQA
Toxic substances: Substances used in a fume cupboard or well-ventilated space with gloves and no mouth pipetting.OCR
Toxic waste products and their removal: Harmful products such as HCl must be treated before release, for example by passing the gases through an alkaline scrubber where the HCl is neutralised.OCR
Toxicity of carbon monoxide: Carbon monoxide is colourless, odourless and toxic because it binds to haemoglobin in the blood, reducing oxygen transport.OCR
toxicity of chlorine: Chlorine is a toxic gas, used as a chemical weapon in the First World War, and chlorinating water produces traces of chlorinated organic compounds that some…AQA
toxicity of cyanide: Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases.AQA
trade names rejected: Common trade names are not accepted in place of the IUPAC form, so polythene, PVC and polystyrene should be converted before you write them.AQA
trade-offs in replacement: Hydrofluorocarbons are potent greenhouse gases and hydrocarbons are flammable, so the choice trades one environmental or safety cost against another.AQA
trans isomer: The inactive form in which the two chloride ligands are opposite each other rather than adjacent.OCR
Transition element: A d-block element that has an ion with an incomplete d-sub-shell.OCR
transition metal: A d-block element that forms at least one stable ion with a partially filled d sub-level.AQA
transition state: The arrangement partway through the reaction where old bonds are stretched and new ones are only partly formed and the potential energy is at a maximum.AQA, OCR
transmitted and reflected light: The remaining wavelengths of light that are not absorbed are transmitted through the solution or reflected from a solid.AQA
Trend in boiling points down Group 17: The boiling point rises from Cl₂ to Br₂ to I₂ because stronger induced dipole–dipole interactions mean more energy is needed to overcome them, while the…OCR
Trend in reactivity down Group 17: Reactivity decreases from chlorine to bromine to iodine down Group 17.OCR
Trend in reactivity down Group 2: Reactivity of the Group 2 metals increases down the group from magnesium to barium, shown by their redox reactions.OCR
Trends in ionisation energy: Trends across periods and down groups follow from nuclear charge, shielding and atomic radius.OCR
Tribasic acid: An acid such as H₃PO₄ that can release three H⁺, giving H₂PO₄⁻, HPO₄²⁻ and PO₄³⁻.OCR
triglycerides: Formed when one glycerol plus three acid molecules gives three ester linkages and three molecules of water.AQA
Trigonal planar geometry at each C=C carbon: Three regions of electron density around each carbon adopt a trigonal planar arrangement, as the double bond counts as one region at each carbon.OCR
triple bond: Contains three shared pairs of electrons, six electrons in all.AQA
two angles in a bipyramid: A trigonal bipyramid has two different angles, 120° equatorial and 90° axial.AQA
two stage oxidation: A primary alcohol has two hydrogen atoms on the carbon carrying the hydroxyl group, so it can be oxidised twice.AQA
two step mechanism: The attack in which a pair of electrons leaves the delocalised system to form a bond, then the C-H bond breaks heterolytically and the pair returns to the ring.AQA
Two-way conversion: Converting between pH and [H⁺(aq)] works in both directions, keeping concentrations in mol dm⁻³ and quoting pH to two decimal places.OCR

U

ultraviolet absorption: Ozone absorbs ultraviolet radiation, the energy breaking it back to O₂ and an oxygen atom, which then reforms ozone.AQA
ultraviolet light: Methane and chlorine do not react in the dark, but in this condition they react readily, and it is the essential condition for the reaction.AQA
Ultraviolet radiation: The energy source in the upper atmosphere that breaks the carbon–chlorine bond in a CFC by homolytic fission to form halogen radicals.OCR
unchanged co-ordination number: The co-ordination number stays the same when a bidentate or multidentate ligand replaces the monodentate ligands; only the number of separate particles changes.AQA
Unchanged energy distribution: The proportion of collisions with energy greater than or equal to the activation energy is fixed by temperature, so the energy distribution remains untouched…AQA
Unchanged enthalpy change of reaction: A catalyst does not change the overall enthalpy change of the reaction, so reactants and products occupy the same levels on the profile.OCR
unchanged temperature: Adding a catalyst does not change the temperature, so the distribution curve is not redrawn at all.AQA
Unequal sharing of bonding electrons: In a bond between two atoms of different electronegativity, the shared pair is pulled towards the more electronegative atom.OCR
unfamiliar molecules: For a molecule that is not familiar, work from the structure to judge the size of the induced dipole-dipole forces from the number of electrons and the shape.AQA
unique pattern: The pattern as a whole is unique to one compound, which is why it is called the fingerprint region.AQA
Unit consistency in calculations: Matching the computed value and stating g or mol, and checking significant figures in the chosen option.OCR
Unit consistency: kelvin, J mol⁻¹ and J K⁻¹ mol⁻¹: R is the gas constant 8.314 J K⁻¹ mol⁻¹, Ea is in J mol⁻¹ and T must be in kelvin, so convert from degrees Celsius by adding 273.OCR
unit conversion: Convert ∆H into joules or ∆S into kilojoules so the two are compatible, and this step alone carries a mark.AQA
unit conversion at the end: The final answer is converted into the unit demanded by the question, such as multiplying by 10⁶ to change cubic metres to cubic centimetres.AQA
Unit conversion between cm³ and dm³: Because 1 dm³ = 1000 cm³, a volume in cm³ is divided by 1000 before use, so 250 cm³ becomes 0.250 dm³.OCR
Unit conversion between J K⁻¹ mol⁻¹ and kJ K⁻¹ mol⁻¹: Dividing by 1000 to express a result in J K⁻¹ mol⁻¹ as kJ K⁻¹ mol⁻¹ when a related quantity needs that unit.OCR
Unit conversion for pressure, volume and temperature: Pressure is in pascals, volume is in cubic metres, and temperature must be in kelvin, so add 273 to the Celsius value.OCR
unit conversions: The wavelength must be in metres, so a wavelength quoted as 600 nm becomes 6.00 × 10⁻⁷ m before substituting into ΔE = hc/λ.AQA
Units and magnitude checks: The answer is reported in kJ mol⁻¹ with a sensible magnitude and sign consistent with the cycle, keeping units consistent throughout.OCR
Units derived from axes: Units come from the axes, for example mol dm⁻³ s⁻¹ for concentration against time, or cm³ s⁻¹ for gas volume against time.OCR
units of cm⁻¹: The unit in which characteristic wavenumbers are measured, and it should be written every time, including inside explanations.AQA
Units of dm³ mol⁻¹: The units in which molar gas volume is quoted.OCR
Units of entropy: Entropy is measured in J K⁻¹ mol⁻¹, so keep the kelvin and per-mole units when values are quoted.OCR
Units of equilibrium constants: The units of Kc and Kp follow from the powers used in the expression.OCR
Units of gradient: Derived from the units of the axes so the gradient carries units, for example mol dm⁻³ s⁻¹ for concentration against time.OCR
units of k: Fixed by the overall order: mol dm⁻³ s⁻¹ for zero order overall, s⁻¹ for first, mol⁻¹ dm³ s⁻¹ for second and mol⁻² dm⁶ s⁻¹ for third.AQA, OCR
units of molar mass: The value is quoted in g mol⁻¹ rather than left as a bare number.OCR
Units of rate: Calculating gradient as change in concentration divided by change in time gives units such as mol dm⁻³ s⁻¹.OCR
Units of the rate constant: For a first order reaction k carries units s⁻¹ because rate (mol dm⁻³ s⁻¹) is divided by concentration (mol dm⁻³).OCR
unreactive aryl halides: Compounds such as chlorobenzene, in which the carbon-halogen bond is strengthened by overlap with the ring, so ammonia will not displace the halogen.AQA
Unsaturated compound: Contains at least one multiple carbon–carbon bond, including C=C, C≡C and aromatic rings.OCR
unsaturation: Containing a carbon–carbon double bond, so the molecule can add extra atoms across that double bond without losing anything.AQA
Unsaturation and the C=C double bond: An alkene is a hydrocarbon that is unsaturated because it contains a C=C double bond, so it holds fewer hydrogen atoms than the corresponding alkane.OCR
unsuccessful collisions: Most collisions involve particles below the barrier, so the particles simply bounce apart unchanged.AQA
unsymmetrical alkenes: Alkenes in which H⁺ can attach to either double-bond carbon, so that two products are possible in addition reactions.AQA
upstream emissions: Most hydrogen is made from methane, so the carbon dioxide appears at the plant rather than the exhaust.AQA
Use of a base to remove hydrogen chloride: The HCl by-product is acidic, so a base such as pyridine is often added to remove it.OCR
Use of brackets for repeated polyatomic ions: A polyatomic ion is placed in brackets when more than one unit is required in the formula.OCR
Use of data tables: A data table of typical shifts is used by matching each observed shift to the closest entry in the table to assign the environment type.OCR
Use of melting point to identify derivatives: The precipitate can be recrystallised and its melting point compared with literature values to identify the specific carbonyl compound.OCR
Using atomic number and charge: Use the atomic number to count electrons and the charge to add or remove electrons, then write the configuration.OCR
Using diagrams to describe bonding: The diagram describes which electrons are shared and which are lone pairs, and shows the ion's overall charge.OCR
Using mol dm⁻³ for concentration: Expressing concentration in mol dm⁻³ and multiplying it by a volume in dm³ to give an amount in mol.OCR
Using overall charge in calculations: The sum of oxidation numbers equals the overall charge of the species.OCR
using the data sheet: Values are looked up on the data sheet rather than recalled, and any value inside the tabulated range is credited.AQA
Using the gas constant R: R = 8.314 J K⁻¹ mol⁻¹, and values are substituted with the correct gas constant to evaluate the answer with a sensible unit.OCR

V

vacant orbital: The species that receives the lone pair must have a vacant orbital.AQA
Valency and atom counting: Each candidate is checked by counting atoms and confirming the valency of carbon, hydrogen, oxygen and nitrogen.OCR
Valency of carbon as four bonds: Each carbon must have the correct hydrogen count so that every carbon forms four bonds.OCR
Validity of weak-acid approximations: The approximations work well only when the acid is weak enough that a small fraction dissociates, and become unsafe when a significant fraction of the acid…OCR
Value of 24 dm³ mol⁻¹ at room temperature and pressure: At room temperature and pressure the molar gas volume is taken as 24 dm³ mol⁻¹, so one mole of any gas occupies 24 dm³ under those conditions.OCR
van der Waals forces: Forces between molecules that take more energy to overcome as each step down the group adds electrons, making the electron cloud larger and more easily…AQA
Variable oxidation numbers of transition elements: Some transition elements form more than one compound or ion, so the name alone is ambiguous.OCR
Variable oxidation state: Arises because the 4s and 3d sub-levels lie close in energy, allowing different numbers of electrons to be lost.AQA
variable oxidation states: Transition elements show this behaviour, in which more than one oxidation state is accessible.AQA
Variable oxidation states of copper: Copper shows two common aqueous oxidation states, Cu²⁺ (blue) and Cu⁺ (colourless), with Cu⁺ unstable in aqueous solution.OCR
Variable oxidation states of transition metals: Transition metals are variable and are not predicted this way.OCR
Variable R side chain: The R group varies between amino acids, so glycine has R = H while other amino acids carry different side chains.OCR
variation of ∆G with temperature: Treat ∆G = ∆H − T∆S as a straight line of ∆G against T with intercept ∆H and gradient −∆S, so a positive ∆S makes ∆G fall as the temperature rises.AQA
vector addition: Bond dipoles are arranged in space and add as vectors.AQA
Vector addition of bond dipoles: The dipoles are vectors, so they add tip-to-tail, and identical opposed bond dipoles in a linear or symmetrical molecule cancel.OCR
Vibrational energy levels: A molecule can take up infrared radiation only when the energy matches a gap between these levels.OCR
Volatile impurities: Impurities separated from the product during redistillation when their boiling points differ enough from the product's.OCR
volatility: The tendency of ester molecules to enter the vapour phase readily, because they cannot hydrogen bond to one another and the forces holding them together are…AQA
Volatility compared with alkanes: Intermolecular hydrogen bonds must be overcome to separate molecules, so alcohols are less volatile than alkanes of similar Mr, whose only intermolecular…OCR
Voltage measurement and units: The two electrodes are connected to the voltmeter terminals and the voltage is read once it settles and recorded with the correct unit, the volt.OCR
volume increments: Added in 1.0 cm³ portions at first but in 0.1 or 0.2 cm³ portions near the equivalence point, where the pH changes fastest.AQA
Volumetric flask calibrated to a fixed volume: A flask calibrated to contain its stated volume at a specified temperature, so it is used to prepare a solution of known concentration.OCR
volumetric flask technique: Transferring the solution and the beaker washings into a volumetric flask, making up to the graduation mark with distilled water, then inverting or shaking…AQA
Volumetric measurement of solutions: Use a volumetric pipette for a fixed accurate volume and a burette for variable delivered volumes, reading the bottom of the meniscus at eye level.OCR

W

Washing and drying a crude solid: The solid is washed with a little cold solvent to remove soluble impurities and mother liquor, and air is drawn through it to help drying.OCR
water as only product: The only product at the point of use is water, so no carbon dioxide, nitrogen oxides or particulates are released in the street.AQA
water as the leaving group: Protonation turns the poor leaving group, OH⁻, into a good one, water.AQA
water bath equilibration: The solutions are brought to temperature separately in a water bath before mixing, and the actual temperature of the mixture is measured rather than assumed.AQA
water bath heating: Warming is done in a water bath, not over a flame, because the organic liquids are flammable.AQA
water of crystallisation: The water shown after the dot in a hydrated formula, whose mass must be included in the calculation.AQA, OCR
Water solubility of alcohols: Small alcohols dissolve readily in water because hydrogen bonding to water molecules lets the alcohol both donate and accept hydrogen bonds.OCR
water treatment: Chlorine is added to drinking water and to swimming pools to kill bacteria and other micro-organisms.AQA
Water treatment chemistry: Chlorine added to water undergoes a reversible reaction, which is why chlorine is used in treating drinking water and in swimming pools.OCR
water-borne disease: Untreated supplies spread cholera, typhoid and dysentery, which killed people in enormous numbers before treatment became routine and still do where water is…AQA
wavelength of maximum absorbance: The peak of the absorption spectrum curve, written as λmax, which is the wavelength most strongly absorbed.AQA
wavenumber and transmittance axes: An infrared spectrum plots percentage transmittance against wavenumber in cm⁻¹, with wavenumbers read from the horizontal axis.OCR
Wavenumber in cm⁻¹ as the horizontal axis: The spectrum is a plot of absorbance (or transmittance) against wavenumber in cm⁻¹, usually read from about 4000 cm⁻¹ down to 500 cm⁻¹.OCR
Wavenumber ranges quoted in cm⁻¹: The O–H absorption of an alcohol is quoted as roughly 3200–3600 cm⁻¹, and a C=O absorption near 1700 cm⁻¹ would indicate a carbonyl compound instead.OCR
Wavenumber regions for the C=O bond: The C=O bond of an aldehyde or a ketone gives a strong, sharp absorption in the region around 1680–1750 cm⁻¹.OCR
Weak acid and conjugate base pairing: The mixture supplies undissociated weak acid to remove added OH⁻ ions and conjugate base to remove added H⁺ ions, so the pH stays nearly constant.OCR
weak acid and its salt: The two components an acidic buffer contains, for example ethanoic acid with sodium or potassium ethanoate.AQA
Weak acid and weak base equilibria: When one partner is weak, the strong partner still reacts essentially completely because the weak acid or weak base equilibrium shifts to replace the species…OCR
Weak acid behaviour: Carboxylic acids only partially dissociate in aqueous solution, so they are weak acids.OCR
Weak acid behaviour of indicators: An indicator is itself a weak acid whose undissociated and conjugate forms differ in colour.OCR
Weak acid equilibrium of indicators: An acid–base indicator is itself a weak acid, written HA, that dissociates in water as HA ⇌ H⁺ + A⁻.OCR
Weak acid partial dissociation: The weak acid is only partially dissociated, so undissociated CH₃COOH survives and remains in excess.OCR
Weak acidity of phenol: Phenol is a weak acid because the aromatic ring stabilises the phenoxide ion, so the O–H bond releases a proton more readily than in ethanol but far less…OCR
Weak acidity of phenols: Phenol is only weakly acidic because the phenoxide ion formed when it loses a proton is stabilised by delocalisation, but less than a carboxylate ion.OCR
Weak acids dissociate partially: A weak acid dissociates only partially, so most molecules remain undissociated and only a small fraction release H⁺ ions.OCR
weak base and its salt: The mixture of a weak base with the salt of that weak base, which forms a basic buffer.AQA
Weak intermolecular forces between molecules: Forces between neighbouring molecules that are much weaker than the covalent bonds and are overcome first on melting or boiling.OCR
Weak-acid approximation for [H⁺]: It is assumed that [H⁺] ≈ [A⁻] because the acid is the main source of protons and autoionisation of water is negligible.OCR
Weak-acid approximation for [HA]: The undissociated concentration is treated as essentially the initial concentration, so [HA] ≈ [HA]₀.OCR
Weighted calculation of relative atomic mass: Each isotope's relative isotopic mass is multiplied by its relative abundance, the products are added and the total is divided by the total abundance.OCR
Weighted mean mass: A mean in which each isotope mass is multiplied by its relative abundance, the products are added, and the sum is divided by the total abundance.OCR
whole-number multiplier: The molecular formula is always a whole-number multiple of the empirical formula, so molecular = n × empirical.AQA
why racemates are inactive: The two enantiomers are produced in equal amounts, and their equal and opposite rotations then cancel.AQA
Writing and balancing ionic equations: Check that atoms and total charge balance on both sides of every ionic equation.OCR
Writing organic oxidation equations: The equations are ethanol to ethanal, CH₃CH₂OH + [O] → CH₃CHO + H₂O, and ethanal to ethanoic acid, CH₃CHO + [O] → CH₃COOH.OCR

Y

yeast enzymes: These act as the catalyst that converts glucose to ethanol, and they work too slowly below the temperature range and are denatured much above it.AQA
Yield above 100%: A signal of an error such as an impure or wet product, not a genuine result.OCR
yield against rate: The conflict in which the conditions giving the best yield give an unacceptably slow rate, so a compromise condition balances an acceptable yield against an…AQA

Z

zeolite catalyst: An aluminosilicate honeycomb with a huge surface area and acidic sites, used in catalytic cracking.AQA
zero order reactants: Changing the concentration of such a species leaves the rate unchanged, so its concentration has no effect on the rate.AQA
Zero order: constant gradient: The concentration falls at a constant rate, so the graph is a straight line with a constant negative gradient, and the half-life shortens as concentration…OCR
Zero value for elements: An element in its standard state has ∆fH = 0 by definition.OCR
zero, first and second order: Zero order means concentration has no effect at all, first order means doubling concentration doubles the rate, and second order means doubling it quadruples…AQA, OCR
zwitterion: The form present around the isoelectric point, with NH₃⁺ and COO⁻ and both charges shown and balancing.AQA

Other terms

[O] as shorthand oxidant: A way of writing the oxidant in organic equations that keeps the focus on the organic change.AQA
∆H of combustion: The enthalpy change when one mole of a substance burns completely in oxygen, with reactants and products in standard states.OCR
∆H of formation: The enthalpy change when one mole of a compound forms from its elements in their standard states, with all substances in standard conditions.OCR
∆H of neutralisation: The enthalpy change when an acid and alkali react to form one mole of water.OCR
∆solH in energy cycles: Obtained by combining the reverse of lattice enthalpy with enthalpies of hydration in a cycle.OCR
2-deoxyribose: A pentose: a five-membered ring of four carbons and one oxygen, with an OH on carbon one, an OH on carbon three and a CH₂OH side group on carbon four.AQA
2,4-dinitrophenylhydrazine: Brady's reagent gives a yellow-orange precipitate with both aldehydes and ketones, confirming a carbonyl group.OCR
2,4-dinitrophenylhydrazine test: A test in which 2,4-dinitrophenylhydrazine condenses with aldehydes and ketones to give an orange or yellow-orange crystalline precipitate of a…OCR
2,4-dinitrophenylhydrazone: Yellow-orange crystalline precipitate formed when 2,4-dinitrophenylhydrazine reacts with the carbonyl group of aldehydes and ketones.OCR
2,4-dinitrophenylhydrazone derivative: The yellow, orange or red-orange precipitate formed when 2,4-DNP reagent is added to a carbonyl compound dissolved in a minimum of ethanol.OCR
2,4-DNP: A reagent, also called Brady's reagent, used as a qualitative test for the carbonyl group in aldehydes and ketones.OCR
4p above 3d: In the sequence, 3d comes before 4p, so 4p is higher in energy than 3d.OCR
4s and 3d energies: These sub-levels lie close in energy, so once 4s electrons are removed, 3d electrons can be lost without a large jump in ionisation energy.AQA
4s before 3d: The 4s sub-shell lies just below 3d in energy so it fills first, but once the 3d sub-shell is occupied it drops below 4s.AQA
4s below 3d: 4s sits below 3d, so the 3d sub-shell is not filled until after 4s.OCR
4s lost first: When ions form the 4s electrons leave first, so Fe²⁺ is [Ar]3d⁶ and Fe³⁺ is [Ar]3d⁵.AQA
4s lower in energy than 3d: The 4s orbital fills before 3d because it lies at lower energy in these atoms.OCR
90° and 180° arrangements: The bond angle between two ligands in the cis arrangement is 90°, and between two ligands in the trans arrangement is 180°.AQA
α-helix: A secondary structure in which the chain coils and the hydrogen bonds run parallel to the axis.AQA
β-pleated sheet: A secondary structure in which lengths of chain lie side by side with hydrogen bonds bridging between them.AQA
Δ and Λ propeller forms: The two mirror-image senses in which the three chelate rings wind around the metal like a propeller and cannot be superimposed.OCR
π electron density and electrophilic attack: Lower π electron density in benzene means weaker attraction for a bromine molecule, so the ring is less able to induce a dipole in, and attract, bromine.OCR
π-bond bond enthalpy: The π-bond has a relatively low bond enthalpy compared with the σ-bond, so less energy is needed to break it.OCR
π-bond formation by sideways p-orbital overlap: The π-bond is formed by sideways overlap of adjacent p-orbitals, giving electron density above and below the bonding carbon atoms.OCR
σ-bond formation by direct orbital overlap: A σ-bond is formed when orbitals on the two bonding atoms overlap directly along the line joining the nuclei, giving a region of electron density concentrated…OCR
σ-bond formation by end-on orbital overlap: The σ-bond is formed by overlap of orbitals directly between the bonding carbon atoms, along the line between the two carbon nuclei.OCR

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