Chemistry GCSE

Chemistry GCSE glossary

1283 key terms for Chemistry GCSE, A to Z, each linked to the topic pages that use it.

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A

Abbreviated LCA: A shortened assessment that studies only chosen stages or impacts rather than the full life cycle.AQA
Absence of oxygen: There was little or no oxygen gas in the early atmosphere, and the atmospheres of Mars and Venus today contain almost no oxygen.AQA
Absorption and re-emission of radiation: Greenhouse gas molecules absorb infrared radiation because their bonds can vibrate at frequencies matching that energy, and the absorbed energy is re-emitted…AQA
Absorption of infrared radiation: Greenhouse gases take in some of the outgoing longer wavelength infrared radiation that Earth re-emits after its surface warms.AQA
Abundance: How much of each isotope of an element is present, which the weighted average reflects.AQA
Acceptance of scientific models: A scientific model becomes accepted when its predictions match what experiments show, and agreement makes the model more widely accepted at the time.AQA
Accuracy: How close a measurement is to the true or accepted value.OCR
Accurate: Describes a method that measures a physical property precisely and compares it with reference data.AQA
Accurate measurement of volume: Using accurate procedural techniques, such as rinsing the burette with the acid and reading the bottom of the meniscus at eye level, to ensure precise volume…AQA
Acid rain formation: Sulfur dioxide dissolves in atmospheric water to form acid rain, which corrodes limestone buildings and acidifies lakes, damaging aquatic life.AQA
Acid reactions: Reactions in which acids react with some metals to produce salts and hydrogen.AQA
Acid reactions with carbonates: Carboxylic acids fizz with carbonates, producing a salt, water and carbon dioxide, for example sodium ethanoate from ethanoic acid and sodium carbonate.AQA
Acid reactions with insoluble bases: The solid may be a metal, a metal oxide, a metal hydroxide or a metal carbonate, and the acid dissolves the solid so a salt forms in solution.AQA
Acid-base reactions: Acids are neutralised by alkalis, which are soluble bases, and by bases, which include insoluble metal hydroxides and metal oxides, to produce salts and water.AQA
Acid–carbonate reactions: Reactions in which a carbonate reacts with a dilute acid to produce a salt, water and carbon dioxide.AQA
Acidic gas pollution: Sulfur dioxide and oxides of nitrogen are acidic gases released when fuels burn, and they cause respiratory problems and acid rain.AQA
Acidic solution: An aqueous solution of an acid has a pH value of less than 7, and the lower the number, the more acidic the solution.AQA
Acids: In aqueous solution, these produce hydrogen ions, H⁺.AQA
Activation: Removing a seal, pulling a tab or pressing a button so that reactants mix or air enters.AQA
Activation energy: The minimum amount of energy that particles must have to react, shown on a reaction profile as the vertical gap from the reactant energy level up to the peak…AQA
Activation energy as an energy barrier: The energy needed for a reaction to occur, labelled as the vertical gap from the reactant level to the peak.AQA
Actual yield: The amount of product that is obtained in practice, which is often lower than the theoretical yield.AQA
addition polymer: A long-chain saturated molecule made of repeating units, recognised from diagrams showing a saturated carbon chain.AQA
Addition polymerisation: A reaction in which no other molecule is formed, so the repeating unit of the polymer has the same atoms as the monomer.AQA
Addition reaction: A reaction in which bromine adds across the carbon–carbon double bond of an alkene, so the orange colour is decolourised and the mixture turns colourless.AQA
Addition reactions: Reactions in which the C=C double bond opens and two new single bonds form, so each of the two carbons forms a new single bond.AQA
Addition reactions of alkenes: The alkene adds bromine across its double bond in an addition reaction, decolourising the bromine water.AQA
Aerobic biological treatment: Treatment of effluent where microorganisms with oxygen break down remaining organic matter.AQA
Age of the Earth: The Earth is about 4.6 billion years old, an approximate value supported by evidence from meteorites and rocks.AQA
Agricultural production: Products that are grown or farmed, such as crops and livestock.AQA
Agricultural productivity: Chemistry improves agricultural processes by producing fertilisers that replace nutrients removed from soil by crops, so more food can be grown on the same…AQA
Agriculture: A human-managed supplement to naturally occurring resources, providing food, timber, clothing and fuels.AQA
Alcohols: A homologous series of organic compounds containing the –OH functional group, whose names end in -ol.AQA
Algebraic rearrangement and unit consistency: Changing the subject of a mathematical equation using inverse operations and keeping units consistent, converting between dm³ and cm³ where needed.AQA
alkali metals: The elements in Group 1 of the periodic table, which have characteristic properties because of the single electron in their outer shell.AQA
Alkaline batteries: A common example of non-rechargeable batteries, which are discarded once flat.AQA
Alkaline properties and pH: The presence of OH⁻ ions is linked to alkaline properties such as high pH, and the concentration of OH⁻ ions determines the pH value, with more OH⁻ giving a…AQA
Alkaline solution: An aqueous solution of an alkali has a pH value greater than 7, and the higher the number, the more alkaline the solution.AQA
Alkalis: In aqueous solution, these produce hydroxide ions, OH⁻.AQA
Alkane: A compound such as ethane that has no double bond, so no addition occurs and the orange colour remains, distinguishing it from an alkene.AQA
Alkanes: Hydrocarbons that have only single carbon–carbon bonds and are saturated, and are useful as fuels.AQA
Alkenes: Hydrocarbons that contain at least one carbon–carbon double bond and are unsaturated, and are reactive starting materials for making polymers.AQA
Allotrope: Different structural forms of the same element in the same physical state.OCR
Allotropes of carbon: Forms such as diamond and graphite that contain only carbon atoms but have different structural arrangements.AQA
Alloy: A mixture of a metal with other elements, such as iron with chromium, nickel and carbon.AQA
Alloy as a mixture containing different-sized atoms: A mixture of a metal with other elements, often other metals, whose atoms are a different size from the atoms of the main metal.AQA
Alloys: Mixtures of metals with small amounts of other elements added to improve strength or resistance to corrosion.AQA
Alloys in everyday use: Gold used as jewellery is usually an alloy with silver, copper and zinc, an everyday example of how alloying changes the properties of a metal.AQA
Alloys of copper: Bronze and brass are both alloys based on copper, but they contain different second metals.AQA
Alpha particle scattering: Alpha particles were fired at thin gold foil; most passed straight through, some were deflected through small angles, and a very small number bounced back.AQA
Alpha-particle scattering evidence: Most alpha particles passed straight through thin gold foil, but a few were deflected strongly, and some bounced back.AQA
Alternative metal extraction: Methods that can obtain metals from low-grade ores or waste, but which may be slower or need different conditions.AQA
Alternative pathway: A different route for the reaction that a catalyst provides, having a lower activation energy.AQA
Aluminium alloy: A mixture of aluminium with other elements such as magnesium, manganese, silicon or copper.AQA
Amide links: The link formed when the amine group of one amino acid reacts with the carboxylic acid group of another, releasing water.AQA
Amine groups: A functional group, –NH₂, contained in an amino acid molecule.AQA
Ammonia: The product of the Haber process, which can be used to produce nitrogen-based fertilisers.AQA
Ammonia as a feedstock: A versatile starting material because it is a base and a source of nitrogen, used to manufacture ammonium salts and nitric acid.AQA
Amounts of substance: The amount in moles, from which the number of particles is found by multiplying by 6.02 × 10²³.AQA
Anaerobic conditions: Conditions without oxygen, which favour fermentation rather than aerobic respiration.AQA
Anaerobic digestion: Treatment of sewage sludge in which microorganisms break it down without oxygen, reducing its volume and producing biogas that can be used as fuel.AQA
Anions: Negatively charged ions, which move to the positive electrode, the anode.AQA
Anode: The positive electrode, towards which negatively charged ions move because opposite charges attract.AQA
Applications of endothermic reactions: Sports injury packs use an endothermic reaction to cool an injury and reduce swelling and pain.AQA
Applications of nanoparticles: The effect depends on the application, such as catalysis, sun cream or medicine, where a smaller quantity may be effective.AQA
Applying knowledge to unfamiliar products: Suggesting why a component is included or how changing its amount would alter the product, using the evidence given rather than assumptions.AQA
Appropriate technology and sustainability: A suitable treatment is chosen based on the source, such as filtration and chlorination for fresh water or desalination for sea water or water with high salt…AQA
Aqueous: Dissolved in water, shown by (aq), used for acids and solutions of ionic compounds rather than (l).AQA
Atmospheric carbon dioxide decrease: The reduction in the percentage of carbon dioxide in the atmosphere caused by algae and plants carrying out photosynthesis.AQA
Atmospheric evolution: The change in the composition of the atmosphere over billions of years as photosynthesis removed carbon dioxide and added oxygen.AQA
Atmospheric pollution: Released substances such as carbon monoxide, sulfur dioxide and oxides of nitrogen enter the atmosphere and cause problems such as acid rain, respiratory harm…AQA
Atom: The smallest part of an element that can exist.AQA
Atom economy: A measure of the amount of starting materials that end up as useful products, also called atom utilisation.AQA, OCR
Atom utilisation: Another name for atom economy, the measure of the amount of starting materials that end up as useful products.AQA
Atomic (proton) number: The number of protons in the nucleus, which equals the number of electrons in a neutral atom.AQA
Atomic dimensions in metres: The typical radius of an atom is about 0.1 nm, which is 1 × 10⁻¹⁰ m.AQA
Atomic emission: When a sample is vaporised in a flame, excited electrons fall back to lower energy levels and emit light of characteristic wavelengths.AQA
Atomic neutrality: Atoms are neutral overall when proton and electron numbers are equal, and ions form when electrons are gained or lost.AQA
Atomic number: The number of protons in an atom of an element, which identifies the element and fixes its position in the periodic table.AQA, OCR
Atomic number ordering: Elements are arranged in order of atomic number, and after a set number of elements a new period begins.AQA
Atomic structure and electron shells: Atoms lower in Group 1 have more electron shells and a larger atomic radius.AQA
Atomic structure and periodic table position: An element's position in the periodic table reflects its atomic structure, because elements are arranged in order of atomic number, so proton number…AQA
Atoms and molecules: Typical atoms are about 0.1 to 0.5 nm across and small molecules are often about 0.5 to 2 nm across.AQA
Attraction between shared electrons and nuclei: Covalent bonds are strong because the shared pair is attracted to both nuclei.AQA
Avogadro constant: The number of atoms, molecules or ions in a mole of a given substance, with a value of 6.02 × 10²³ per mole.AQA

B

Balanced chemical equation: A symbol equation in which coefficients are adjusted so that the number of atoms of each element is the same on both sides.AQA
Balanced chemical equations: A balanced symbol equation for a named carbonate and dilute acid, including state symbols where required.AQA
Balanced equation: A symbol equation in which the number of atoms of each element is equal on both sides, achieved by placing whole-number coefficients in front of formulae.AQA
Balanced equations: Used to identify the mole ratio between the two reactants, which is applied to find moles of the second solution.AQA
Balanced equations and coefficients: Each species' Mᵣ must be multiplied by its coefficient in the balanced equation, and the equation must be balanced before totals can be compared.AQA
Balanced equations and mole ratios: A balanced equation gives the mole ratio of gaseous reactants and products, and its coefficients can be used to set the volume ratio.AQA
Balanced equations as mole ratios: A balanced symbol equation shows the ratio in which particles react, and that same ratio applies to amounts in moles.AQA
Balanced half equation: An equation showing the ion, product and electrons with correct formulae and state symbols, balanced so atoms and total charge are equal on both sides.AQA
Balanced symbol equations: Symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.AQA
Balancing chemical equations: Checking that each element has the same number of atoms on both sides, for example counting atoms of each element on both sides of the equation.AQA
Balancing equations: A balanced symbol equation can show the oxidation, checking that every element is balanced on each side.AQA
Balancing symbol equations: Writing the balanced symbol equation using those whole numbers as balancing numbers.AQA
Barium chloride solution: The reagent added dropwise that supplies barium ions, which combine with sulfate ions.AQA
Barium chloride with dilute hydrochloric acid: A test used for sulfate ions, giving a white precipitate.AQA
Barrier protection: A coating that stops oxygen and water reaching the metal surface, so corrosion cannot start.AQA
Barriers to change: Reasons why actions may be limited, such as renewable sources being unreliable or costly to install, or governments and individuals lacking money or motivation.AQA
Battery: Consists of two or more cells connected together in series to provide a greater voltage.AQA
Benefits and risks: New applications can bring benefits such as greater efficiency, lower material use or new functions, but possible risks to health and the environment must…AQA
Benefits and risks of nanotechnology: The useful properties of nanoparticulate materials must be weighed against drawbacks such as possible toxicity, unknown long-term effects and the difficulty…AQA
Benefits versus drawbacks: Comparing benefits such as low mass or targeted delivery with drawbacks such as possible toxicity, uncertain long-term effects and environmental impact, then…AQA
Bias: Selecting or emphasising only part of the evidence, or presenting one side more strongly.AQA
Bioleaching: Bacteria obtain energy by breaking down metal sulfide ores, producing a solution containing metal ions that can be processed to obtain the metal.AQA
Biological importance of DNA: DNA carries coded information needed by living organisms, which is passed from parents to offspring.AQA
Blast furnace: Where iron is extracted from iron ore by reduction with carbon.AQA
Blast furnace chemistry: In the industrial extraction of iron, carbon is added but first reacts to form carbon monoxide, which acts as the main reducing agent.AQA
Bleaching: The removal of colour from damp litmus paper by chlorine, so that the paper turns white because the dye is destroyed.AQA
Blue glass: Placed between the eye and the flame to absorb yellow light so that a lilac colour becomes visible.AQA
Boiling: Heating a liquid so particles gain enough energy to overcome the forces between them and escape as a gas throughout the liquid.AQA
Boiling point: Increases as molecular size increases because stronger intermolecular forces require more energy to overcome.AQA
Boiling point and molecular size: Hydrocarbons with low boiling points are typically small molecules and travel further up the column before condensing, while those with high boiling points…AQA
Boiling point elevation: An impure substance boils above the known boiling point, and the change happens over a range of temperatures.AQA
Bond breaking: Energy must be supplied to break bonds in the reactants, because the atoms are pulled apart against the attraction holding them together.AQA
Bond breaking and bond forming energy balance: Overall energy is absorbed because bond breaking takes in more energy than bond forming releases.AQA
Bond breaking and bond making: Energy is absorbed when bonds break in the reactants, and energy is released when new bonds form in the products.AQA
Bond energies: Supplied values in kJ mol⁻¹ used to total the energy absorbed for bonds broken and the energy released for bonds made.AQA
Bond energy: The amount of energy required to break one mole of a particular covalent bond.AQA
Bond formation: Exothermic, because energy is released as new bonds form and the system becomes more stable.AQA
Bond making: Energy is released when bonds in the products are formed, as the atoms come together into a lower-energy arrangement.AQA
Bond strength and energy transfer: A large amount of energy is needed to break covalent bonds, so they remain intact during physical changes such as melting and boiling.AQA
Bonding and forces between particles: The temperature of a change of state is linked to the type and strength of bonding or forces between particles.AQA
Brackets: A grouping in a formula where a subscript after the bracket multiplies every element inside the bracket.AQA
Brittle: Describes a material that cannot absorb much energy by bending, so it tends to crack or shatter rather than deform when struck or overloaded.AQA
Bromine water: An orange solution used to test for a carbon–carbon double bond, added to the sample and shaken, with the colour recorded before and after.AQA
Bromine water test: Shaking a substance with bromine water, which changes from orange to colourless, to identify ethene.AQA
Buckminsterfullerene: A spherical fullerene, C₆₀, whose surface is made from hexagons and pentagons.AQA
Build-up of nitrogen in the atmosphere: Nitrogen gradually built up in the atmosphere because it is relatively unreactive and was not removed by reactions or dissolving in the oceans.AQA
Bulk properties: The familiar properties of the material that coarse particles usually show because most of their atoms are inside the particle rather than at its surface.AQA
Burette: A piece of apparatus filled with the acid, from which the acid is added gradually while the flask is swirled.AQA

C

Calibration: Comparing the sample spectrum with spectra from solutions of known concentration.AQA
Carbon cycle and carbon stores: Carbon dioxide is removed from the atmosphere when carbon becomes locked into solid or buried stores rather than remaining as a gas.AQA
Carbon dioxide: A greenhouse gas released by burning fossil fuels such as coal, oil and natural gas.AQA
Carbon dioxide and methane reduction: Cutting emissions by using renewable sources, improving energy efficiency, using public transport or electric vehicles, and capturing landfill gas or reducing…AQA
Carbon dioxide sources: Burning fossil fuels such as coal, oil and natural gas, and deforestation, which reduces photosynthesis so less carbon dioxide is removed from the atmosphere.AQA
Carbon footprint: The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.AQA
Carbon monoxide: Produced by partial oxidation of carbon in the fuel when oxygen is limited.AQA
Carbon nanotubes: Cylindrical fullerenes with variable lengths, forming extended structures rather than simple molecules.AQA
Carbonate reactions: Metal carbonates neutralise acids but produce carbon dioxide as well as salt and water.AQA
Carboxylic acid: A compound containing the –COOH functional group, which can be identified from a displayed, structural or molecular formula.AQA
Carboxylic acid groups: A functional group, –COOH, contained in an amino acid molecule.AQA
Carboxylic acids: Compounds that contain the –COOH group, from which alcohols are distinguished.AQA
Catalysis: Transition metals act as catalysts, for example manganese(IV) oxide catalyses the decomposition of hydrogen peroxide.AQA
Catalysis in industrial and laboratory reactions: Transition elements and their compounds are useful catalysts, for example iron catalyses the Haber process and nickel catalyses hydrogenation.AQA
Catalyst: A substance that increases the rate of reaction by providing an alternative pathway with a lower activation energy, and is regenerated at the end of it.AQA, OCR
Catalysts: Nanoparticles provide a large surface area for reactions, so a smaller quantity may be effective.AQA
Catalytic cracking: Hydrocarbon vapour is passed over a hot catalyst, commonly a zeolite, at a high temperature, and the catalyst allows reaction at a lower temperature than…AQA
Catalytic oxidation: Ammonia is oxidised industrially, typically over a heated catalyst, to nitrogen monoxide, which is further oxidised to nitrogen dioxide and dissolved in water…AQA
Catenation and structural variety in carbon compounds: Carbon atoms bond to one another, forming chains, branched chains and rings of varying length, which generates structural variety.AQA
Cathode: The negative electrode, towards which positively charged ions move because opposite charges attract.AQA
Cations: Positively charged ions, which move to the negative electrode, the cathode.AQA
Causal mechanisms: Warmer air holds more water vapour, ice melts and ocean water expands, altering weather patterns, sea level and habitats.AQA
Cell: A device that converts chemical energy into electrical energy, containing chemicals which react to produce electricity.AQA
Ceramics: Glass is a ceramic material made mainly from sand, which is silicon dioxide.AQA
Chain growth: Two functional groups per monomer allow chain growth in two directions, while a monomer with only one functional group would terminate chain growth.AQA
Changes of state: Energy is transferred during a change of state, but the temperature stays constant until the change is complete.AQA
Char or decompose: What a thermosetting polymer may do at very high temperatures instead of melting, because the covalent bonds of the network are too strong to be overcome.AQA
Characteristic colours of metal hydroxides: Copper(II) forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.AQA
Characteristic flame colours of metal cations: Lithium, sodium, potassium, calcium and copper compounds produce distinctive colours in flame tests, and the colour is characteristic of the metal cation…AQA
Characteristic line spectra: The instrument separates the emitted light into a line spectrum, and the pattern of lines identifies the metal.AQA
characteristic properties: Features explained by the single outer electron: soft, low density, shiny when freshly cut but tarnishing quickly, and reacting vigorously with water, oxygen…AQA
Charge and relative numbers of protons and electrons: Loss of electrons produces a positive ion because the number of protons now exceeds the number of electrons, and the size of the charge matches the number of…AQA
Charge balance: The overall compound is neutral because the total positive charge equals the total negative charge, which helps work out formulas.AQA
Charge balance in atoms: The positive and negative charges balance, so the atom has no overall charge.AQA
Charge carriers: Charged particles, such as ions or delocalised electrons, that are free to move and are required for conduction.AQA
Charge flow in circuits: The movement of ions through the electrolyte allows charge to flow and completes the circuit.AQA
Charge notation for ions: The ion symbol carries a superscript charge whose sign shows the direction of electron transfer, as in Mg²⁺ and O²⁻.AQA
Chemical amount: A quantity measured in moles, where the symbol for the unit mole is mol.AQA
Chemical behaviour and ion formation: Metals generally form positive ions when they react, and this behaviour applies to the majority of elements.AQA
Chemical bonding: There are three types of strong chemical bonds: ionic, covalent and metallic, which hold atoms or ions together in compounds and elements.AQA
Chemical bonding and bond breaking: The elements in a compound are held together by chemical bonds, so separating them requires breaking those bonds.AQA
Chemical change: A change in which one or more new substances form, with different properties from the reactants, because atoms are rearranged into different combinations.AQA
Chemical combination of elements: A compound contains two or more different elements joined by chemical bonds, not just mixed together.AQA
Chemical formula: Shows the number and type of atoms in a substance.AQA
Chemical formulae: A way of representing compounds, such as NaCl or H₂O.AQA
Chemical reactions: The process by which compounds are formed from elements, involving the formation of chemical bonds between atoms.AQA
Chemical stability: A stable arrangement of electrons, which a full outer shell provides.AQA
Chemical symbol: A representation of an atom of each element, usually one or two letters, where the first letter is always a capital and the second letter, if present, is…AQA
Chemical treatment of pollutants: Chemical methods such as neutralisation, precipitation or adsorption onto activated carbon, used with physical separation to remove substances from waste water.AQA
Chemical versus physical properties: Chemical properties are unchanged in a mixture, while physical properties such as melting point and boiling point may change because the components affect…AQA
Chemical versus physical separation: Because the metal is chemically bonded in the compound, chemical reactions are needed to extract it, as physical separation alone cannot break the…AQA
Chlorination: Sterilisation of water, often used to kill microbes.AQA
Chlorine: Added as a gas or in a solution, it kills microbes by disrupting their cell processes and provides some residual protection as water travels through pipes.AQA
Choosing fuels for particular uses: The choice of fuel depends on matching these properties to the intended use, for example a gas for a quick hot flame and a liquid for storage and transport.AQA
Chromatography: A separation technique in which a mixture is carried by a mobile phase through a stationary phase, and substances separate because they move at different rates.AQA
Classification of elements: Elements can be distinguished as metal or non-metal by their location in the periodic table rather than by memorising every element.AQA
Climate modelling: Computer models that predict climate change have limitations because they may simplify processes, rely on incomplete data, or struggle to represent clouds and…AQA
Climate versus weather: Weather is short term and local, whereas climate is long term and average.AQA
Closed System: A system in which no substances can enter or leave during a reaction.OCR
Closed systems: Apparatus which prevents the escape of reactants and products, so no reactants or products can escape.AQA
Closed systems and gas exchange: In a closed system the total mass before and after a reaction is unchanged, but an apparent mass change is usually because a gas has escaped or a gas from the…AQA
Cloudiness: Caused by calcium carbonate forming as a precipitate when carbon dioxide reacts with calcium hydroxide.AQA
Coarse particles: Larger particles, typically with diameters from about 1 × 10⁻⁵ m to 2.5 × 10⁻⁶ m, commonly called dust.AQA
Coefficient: A number in normal script before a formula that multiplies the whole formula unit, so every atom in that formula unit is multiplied.AQA
Coefficients: Whole-number multipliers placed in front of chemical formulae to balance an equation, never changing the formulae themselves.AQA
Collision frequency: The number of collisions per second between reactant particles, which rises when concentration, pressure or surface area is increased.AQA
Collision theory: For a reaction to happen, particles must collide with at least the activation energy, and the rate depends on the frequency and energy of collisions between…AQA
Collisions: Particles collide constantly, but most do nothing because the particles simply bounce apart; only those with energy at or above the activation energy can react.AQA
Colour changes: The change observed as the halogen formed appears, for example orange for bromine and brown for iodine.AQA
Coloured compounds: Transition metals form coloured compounds, for example hydrated copper(II) sulfate is blue and cobalt(II) chloride is pink in aqueous solution.AQA
Coloured transition element compounds: Compounds of transition elements are often coloured, such as blue copper(II) sulfate, yellow-brown iron(III) chloride and purple potassium manganate(VII).AQA
Colourless: Carbon monoxide has no colour, so it cannot be seen.AQA
Combustion: Burning a fuel in oxygen, releasing energy as bonds form in the products.AQA
Combustion and oxidation: Oxygen supports combustion more effectively than air, so the glowing tip bursts back into flame.AQA
Combustion products: The products of complete combustion are carbon dioxide and water, with carbon oxidised to carbon dioxide and hydrogen oxidised to water.AQA
Combustion reactions: Burning a fuel in oxygen, releasing energy, and in a hydrocarbon producing carbon dioxide and water vapour when complete.AQA
Comparative LCA: A simple comparison that sets the same stages side by side for two products, such as shopping bags made from plastic and paper.AQA
Comparing measured and known values: A measured melting point or boiling point is set against the known value for the pure substance at the same pressure.AQA
Comparing melting and boiling points: Comparing the type and strength of forces between the particles of substances and linking this to the energy needed for the change of state.AQA
Comparison of analytical methods: Stating advantages such as speed, sensitivity, accuracy and precision, and use with very small samples, with reference to the chemical tests in this…AQA
Comparison with known values: Checking a measured melting or boiling point against the known value for the expected pure substance, where a close match to the sharp known value indicates…AQA
Comparison with Mars and Venus: The Earth's early atmosphere may have been like the atmospheres of Mars and Venus today, which provide a useful model for the early Earth.AQA
Complementary base pairing: The two chains are held together by bonds between the bases of the nucleotides, and the pairing of bases means the chains are complementary.AQA
Complete and incomplete combustion: Complete combustion of a hydrocarbon gives carbon dioxide and water vapour, while a limited supply of oxygen causes incomplete combustion, which also produces…AQA
Complete ionisation in aqueous solution: A strong acid is completely ionised in aqueous solution, so essentially every acid molecule releases a hydrogen ion, H⁺, to a water molecule.AQA
Complex systems: Global climate is one of these, with many interacting parts including the atmosphere, oceans, ice, land and living things.AQA
composite: A material that combines two or more materials so that the useful properties of each are retained in the finished article.AQA
Composite materials: Materials made from two or more materials combined so that the product has properties better than either material alone.AQA
Composition: The specific amounts of carbon and other metals in a steel, which are adjusted for a purpose.AQA
Composition and carat: The proportion of gold in the alloy is measured in carats, with 24-carat gold being pure gold and 18-carat gold containing 18 parts gold out of 24 by mass.AQA
Composition and function: Identifying the function of each listed component and explaining how the combination of components gives the product its required properties.AQA
Composition and properties: Because alloys are mixtures, their composition can be varied to adjust properties.AQA
Composition of the early atmosphere: The Earth's atmosphere at the start of this period may have consisted of mainly carbon dioxide with little or no oxygen gas.AQA
Compound: A substance made of two or more different elements chemically combined in fixed proportions.AQA
Concentrated: Describes a solution with a large amount of substance in a given volume.AQA
Concentration: The number of moles of solute dissolved per cubic decimetre of solution, found by dividing the moles by the volume in dm³.AQA
Concentration and pH: The concentration of H⁺ ions determines the pH, so a higher concentration of H⁺ gives a lower pH value.AQA
Concentration and pressure effects: More reactant particles in the same volume of solution, or the same number of gas particles squeezed into a smaller volume, makes collisions more frequent.AQA
Concentration and volume: Moles of the known solution are calculated using amount = concentration × volume in dm³, and the unknown concentration is found by dividing its moles by its…AQA
Concentration calculations: The mass of solute is found from concentration multiplied by volume, and the relationship can be rearranged to find concentration or volume.AQA
Concentration changes: When the concentration of one reactant or product is changed, the concentrations of all the substances will change until equilibrium is reached again.AQA
Concentration versus strength: Strength is about degree of ionisation, whereas concentration is about the amount of substance per unit volume.AQA
Concordant Results: In a titration, results (titres) that are within 0.10 cm³ of each other.OCR
Concordant results and reliability: Repeating the titration until concordant titres agree, which improves reliability, and using a mean of concordant values.AQA
Condensation and the formation of oceans: As the planet cooled, water vapour condensed into liquid water, which fell as rain and collected in hollows to form the oceans.AQA
Condensation polymerisation: A type of polymerisation in which monomers with two functional groups react together, and a small molecule is eliminated for each new bond formed.AQA
Condensation reactions: Reactions in which monomers join together, usually losing small molecules such as water.AQA
Condensing: Cooling a gas so particles lose energy and move closer together, forming a liquid.AQA
Conditions for life: The natural greenhouse effect maintains temperatures on Earth high enough to support life.AQA
Conduction by free-moving ions: When an ionic compound melts or dissolves, the lattice breaks down and the ions become free to move, which allows the molten or dissolved compound to conduct…AQA
Conflict between ordering principle and chemical properties: Strict atomic-weight ordering sometimes placed elements in groups that did not match their chemical properties, as with tellurium and iodine.AQA
Consequences for people and ecosystems: Four potential effects are rising sea levels, more frequent and severe extreme weather, changes to ecosystems and habitats, and impacts on agriculture and…AQA
Conservation of chemical identity: The substances keep their original chemical identities because no chemical reactions occur during the separation.AQA
Conservation of chemical identity in mixtures: Each substance in a mixture keeps its own chemical properties because no chemical bonds form between the components, so the particles remain the same.AQA
Conservation of energy: The total energy of a closed system stays constant during any chemical change, so energy cannot be created or destroyed in a chemical reaction.AQA
Conservation of mass: The total mass of reactants equals the total mass of products for a fully balanced equation, as when 4 g plus 32 g gives 36 g.AQA
Conservation of mass in equations: Every element must have the same number of atoms on each side of the equation.AQA
Conservation of matter: Atoms are conserved, so fuel and oxygen atoms rearrange into the product gases and particles rather than being destroyed.AQA
Conservation of particles during changes of state: Ice, water and steam all contain H₂O particles, but the spacing and motion of the spheres differ.AQA
Conservation of protons in chemical change: The proton number does not change in chemical reactions; atoms gain, lose or share electrons instead.AQA
Contamination: A dirty wire gives a misleading colour, so the wire is cleaned with concentrated hydrochloric acid and heated until no colour is seen before each test.AQA
Context in scientific language: When a question uses the word pure, the context determines which meaning applies, such as a food label or a melting point investigation.AQA
Continuous electricity generation: The cell produces a potential difference while the fuel and oxygen supplies continue, unlike a battery that stores a fixed amount of chemical energy.AQA
Contrast between mixtures and compounds: In a compound the elements have reacted and the compound has different chemical properties from its constituent elements, whereas in a mixture each substance…AQA
Contrasting gas tests: A burning splint popping would indicate hydrogen instead, rather than a glowing splint relighting.AQA
Controlled release of heat: Design features control the rate of reaction so heat is released gradually and safely rather than all at once.AQA
Converting between mass and amount: Moles are found from mass using n = m ÷ Mr, and mass is found from moles using m = n × Mr.AQA
Corrosion: The destruction of materials by chemical reactions with substances in the environment.AQA
Corrosion resistance: Aluminium resists corrosion despite being more reactive than iron because the oxide layer stays intact and stops further corrosion.AQA
Cosmetics and sun creams: Nano-sized titanium dioxide or zinc oxide gives transparent protection by absorbing or scattering ultraviolet radiation.AQA
Cost and energy in water production: Desalination by distillation or reverse osmosis is expensive and energy-intensive, which explains why it is used in some locations but not others.AQA
Counting atoms: Applying subscripts within the formula first and then coefficients, so each element's atoms in the formula unit are multiplied correctly.AQA
Counting atoms and relative formula mass: The formula lets you count atoms, calculate relative formula mass and balance equations.AQA
Covalent Bond: A shared pair of electrons between two non-metal atoms.AQA
Covalent bonding: Bonds that hold atoms together within molecules, with shared electrons held between the atoms and not free to move through the substance.AQA
Covalent bonding and shared electron pairs: Two halogen atoms can each contribute one electron to a shared pair, forming a single covalent bond.AQA
Covalent bonds: Bonds inside each molecule that are strong compared with intermolecular forces and remain intact when a molecular substance changes state.AQA
Covalent bonds in the network: These must be overcome to melt or boil a giant covalent substance, and the energy needed depends on their number and strength per atom.AQA
Covalent bonds remain intact: The strong covalent bonds within each molecule are not broken during melting or boiling, so the molecules themselves stay intact.AQA
Covalent cross-links: Strong bonds that join polymer chains to one another, holding them firmly in place so they cannot slide past each other.AQA
Cracking: Breaking large hydrocarbon molecules down into smaller molecules that are more useful as fuels and as starting materials for making polymers and other…AQA
Cradle-to-grave environmental assessment: An assessment carried out to assess the environmental impact of products in each stage, from extracting and processing raw materials to disposal at the end of…AQA
Crude oil as the feedstock for most plastics: Most plastics are produced from crude oil, a fossil fuel, through fractional distillation and cracking to give monomers that are then polymerised.AQA
Crystallisation: The process in which a salt solution is warmed to evaporate some water, raising the concentration until saturated, then cooled so that crystals of the salt…AQA
crystallise: After evaporating some water, cooling allows the salt to crystallise before it is filtered, washed and dried gently.AQA
Cullet: Small pieces of crushed glass, produced when glass bottles are crushed before being melted in a furnace.AQA

D

Damp litmus paper: Litmus paper moistened with distilled water, which is essential because dry litmus does not respond to chlorine gas.AQA
Data evaluation: Analysing provided data, such as energy costs, environmental impacts and the chemical reactions involved, to weigh the pros and cons of a metal extraction…AQA
Data interpretation: Using given data, such as equilibrium concentrations or amounts, to predict how a change in concentration of a reactant or product affects a reversible…AQA
Decolourised: Describes the change in which an alkene turns bromine water from orange to colourless, because bromine adds across the C=C double bond.AQA
Decomposition reactions: Chemical reactions that break a compound into its elements or simpler compounds, for example passing electricity through molten sodium chloride or heating…AQA
Deducing order from evidence: Ranking metals by comparing the vigour of reactions or the outcomes of displacement experiments.AQA
degree of ionisation: The extent to which an acid ionises, which is complete for strong acids and partial for weak acids.AQA
Degree of ionisation and acid strength: Acid strength depends on how completely an acid ionises, not on how much acid is dissolved.AQA
Delocalised Electron: An electron that is not associated with a single atom or a covalent bond, and is free to move through a structure.OCR
Delocalised electrons: Electrons in a metal that are not attached to any particular ion and are free to move throughout the structure.AQA
Demand for small-molecule fuels: High demand for fuels with small molecules for heating, cooking and transport, which drives cracking.AQA
Density: Mass per unit volume, so a low-density alloy gives a strong but light component.AQA
Desalination: A treatment used when supplies of fresh water are limited, removing dissolved salts from salty water or sea water to produce potable water.AQA
Desalination by distillation: A desalination method that requires much more energy because water must be heated to boiling and condensed.AQA
Desalination by reverse osmosis: A desalination method that requires energy to force water at high pressure through a membrane.AQA
Designed mixtures: A formulation is a mixture designed to have specific useful properties, not a random combination of substances.AQA
Detection: Because carbon monoxide has no colour or smell, it is not easily detected without a detector such as a carbon monoxide alarm.AQA
Detergents: Help water to remove grease and dirt and are used in cleaning products.AQA
Development of scientific theories: Theories about what was in the Earth's early atmosphere and how the atmosphere was formed have changed and developed over time.AQA
Development of the atomic model: The atomic model changed over time as new experimental evidence emerged, moving from Dalton's solid spheres through Thomson's plum pudding model and…AQA
Development of the periodic table: Mendeleev's table was not merely a convenient list but a model with genuine predictive power, and its predictive success increased confidence in it.AQA
Diamond: A giant covalent structure of carbon atoms, each bonded to four others in a tetrahedral network, so it is very hard and has a very high melting point.AQA
Diatomic molecules: Group 7 halogens exist as diatomic molecules, so relative molecular mass is twice the relative atomic mass for each element.AQA
Dihaloalkane: The product formed when a halogen adds to an alkene, with two halogen atoms added, one to each carbon of the former double bond.AQA
dilute: Having a small amount of acid per unit volume, which is not the same as being weak.AQA
Dilute hydrochloric acid: Added to acidify the sample before the barium chloride solution, removing carbonate ions by reacting them to form carbon dioxide.AQA
Dilution: Adding more solvent increases the total volume, which decreases the concentration.AQA
Discharge: The process during which the reactants inside the cell are used up.AQA
Discharge at electrodes: The ions gain or lose electrons at the electrodes, producing new substances.AQA
Discovery of new elements: Chemists found new elements whose measured properties closely resembled the values Mendeleev had forecast from the surrounding groups.AQA
Discovery of the electron: Experiments on cathode rays showed that atoms contain small, negatively charged particles called electrons.AQA
Discovery of the neutron: James Chadwick conducted experiments that yielded evidence for a previously undiscovered subatomic particle, which had a relative mass of 1 but carried no…AQA
Displacement: Iron is more reactive than copper, so iron displaces copper from a solution of a copper compound.AQA
Displacement of hydrogen: Metals above hydrogen displace it from dilute acids, while metals below hydrogen, such as copper, do not.AQA
Displacement Reaction: A reaction where a more reactive element takes the place of a less reactive element in a compound.OCR
Displacement reactions: Displacement occurs when a more reactive metal, which forms positive ions more readily, takes the place of a less reactive metal in a compound.AQA
Dissolved salts: Impurities in water that can affect taste and, in extreme cases, health, and may be reduced by desalination.AQA
Dissolved salts and water purity: Both methods produce water with low levels of dissolved salts.AQA
Distribution between phases: Each substance spends a different proportion of time in each phase, and this difference is what causes separation.AQA
Distribution of results: The pattern formed when repeated measurements are plotted, for example as a dot plot or a small bar chart, showing how they cluster around the mean.AQA
Disturbance and re-equilibration: Changing the concentration of a reactant or product means the system is no longer at equilibrium, and concentrations of all substances change until…AQA
Dominant emission: The brightest colour that the eye sees, so that the weaker emission from another cation is not visible against it.AQA
dot and cross diagram: A diagram that represents the electron transfer during the formation of an ionic compound, using dots for one atom and crosses for the other.AQA
Dot and cross diagrams: A representation that shows the outer-shell electrons of each atom, with shared pairs in the overlap between atoms.AQA
Double helix: The two chains are arranged in the form of a double helix, like a twisted ladder.AQA
Dynamic equilibrium: Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate, so both reactions continue but the amounts of reactants and…AQA, OCR

E

Earth's early atmosphere: One theory suggests it was mainly carbon dioxide with little or no oxygen, formed by volcanic activity, while another emphasises water vapour and nitrogen.AQA
Earth's energy balance: A continuous balance between incoming short wavelength radiation, outgoing long wavelength radiation, and absorption and re-emission by greenhouse gases.AQA
Earth's finite raw materials: Metals, glass, building materials, clay ceramics and most plastics are produced from limited raw materials, so continued production at current rates will…AQA
Earth’s first billion years: During the first billion years of the Earth’s existence there was intense volcanic activity that released gases and water vapour.AQA
Economic and environmental choices in metal extraction: Scrap iron is cheap and often waste material, which makes this an economical method, while electrolysis needs electrical energy.AQA
Economic factors in industrial chemistry: Raw materials cost money, and separating, purifying and disposing of unwanted by-products adds further cost and energy use, so a process that converts most…AQA
Economic, social and technological factors: Reasons affecting the take-up of actions, such as countries relying on fossil fuels for jobs and income, or lacking the grid connections and storage to use…AQA
Ecosystem damage: Habitats such as coral reefs and polar ecosystems may be damaged, so species may migrate, decline or become extinct.AQA
Effect of changing conditions: Changing the solvent can change the separation because the distribution of the substances changes.AQA
Effect of concentration on equilibrium: If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.AQA
Effect of concentration, pressure and temperature: The principle applies to changes in concentration, pressure and temperature, and predictions must state both the direction of shift and the resulting effect…AQA
Effect of impurities on physical properties: An impurity lowers the melting point and raises the boiling point, and it spreads melting and boiling over a range of temperatures rather than at one value.AQA
Effect of temperature on collision energy: Raising temperature increases the proportion of particles with energy greater than or equal to the activation energy, so more collisions are successful.AQA
Efficiency of materials: A larger exposed surface area allows the effect to be achieved with a smaller quantity, which can reduce cost, waste and the mass of material needed.AQA
Electric current: The movement of charge that occurs when delocalised electrons drift towards the positive terminal after a potential difference is applied.AQA
Electrical conductivity: Graphite conducts electricity because of delocalised electrons, whereas diamond does not because all outer electrons are localised in bonds.AQA
Electrical conductivity and mobile charge carriers: All outer electrons are used in covalent bonding, so there are no delocalised electrons or free ions, and diamond does not conduct electricity.AQA
Electrical conductivity in carbon nanotubes: The ability of carbon nanotubes to conduct electricity because of their delocalised electrons and extended structure, allowing them to act as tiny wires or…AQA
Electrical neutrality of atoms: The atom is neutral overall because the positive and negative charges balance.AQA
Electrochemical cells: A simple cell's voltage is not fixed but depends on the chemicals chosen, including the metals used as electrodes and the electrolyte.AQA
Electrochemical oxidation: The fuel is oxidised at an electrode surface, transferring electrons through the circuit rather than releasing energy only as heat.AQA
Electrode processes: At the negative cathode positive ions gain electrons, and at the positive anode negative ions lose electrons, forming neutral atoms.AQA
Electrode products: The metal is produced at the cathode and the non-metal is produced at the anode.AQA
Electrode reactions in molten electrolysis: At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻.AQA
Electrodes: The places where oxidation and reduction happen, with hydrogen oxidised at the negative electrode and oxygen reduced at the positive electrode.AQA
Electrodes and ion movement: Positive metal ions are attracted to the cathode and gain electrons, while negative non-metal ions are attracted to the anode and lose electrons.AQA
Electrolysis: A process in which ions in an electrolyte move to the electrode of opposite charge and are discharged, requiring a direct current supply.AQA, OCR
Electrolysis and direct current: Passing a direct current through an electrolyte causes the mobile ions to move towards the electrodes, and this process is electrolysis.AQA
Electrolysis of aqueous solutions: Process in which an aqueous solution is electrolysed using inert electrodes, with the ions discharged depending on the relative reactivity of the elements…AQA
Electrolysis of molten compounds: Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode.AQA
Electrolysis of solutions: A direct current is used to reduce copper ions at the negative electrode, producing copper metal.AQA
Electrolyte: A liquid or solution that contains mobile ions and can conduct electricity.AQA, OCR
Electron arrangement and reactivity: Chemical reactions depend on the arrangement of electrons, especially the outer shell, and hence on atomic number.AQA
Electron configuration: The arrangement of electrons in an atom, such as helium having two electrons in its only shell and neon having eight in its outer shell.AQA
Electron counting: The mole can also count electrons, such as in electrolysis or redox reactions, where one mole of electrons is 6.02 × 10²³ electrons.AQA
Electron gain: Halogens react by gaining one electron to form a 1⁻ ion in reactions with metals and in displacement reactions.AQA
Electron gain and loss to form ions: A Group 1 metal loses one electron, a Group 2 metal loses two electrons, a Group 6 non-metal gains two electrons and a Group 7 non-metal gains one electron.AQA
Electron loss and positive charge: Metal atoms lose one or more outer-shell electrons and become positively charged ions, because the loss of negatively charged electrons leaves more protons…AQA
Electron loss as the basis of reactivity: Metals that lose electrons readily form positive ions easily and are more reactive, while metals that form positive ions only with difficulty are less reactive.AQA
Electron sharing: The process in which non-metal atoms share pairs of electrons to achieve a full outer electron shell.AQA
electron shells: Fixed shells or energy levels around the nucleus that electrons occupy.AQA
Electron transfer: In ionic bonding, electrons are transferred from a metal atom to a non-metal atom, producing positively charged cations and negatively charged anions.AQA
Electronic structure and stability: Each Group 0 atom has a full outer electron shell, which is a stable arrangement, so its atoms neither gain nor lose electrons easily.AQA
Electronic structure and the outer shell: Group 1 atoms each have one electron in their outer shell, and that single outer electron is lost relatively easily when the metal reacts, forming a 1⁺ ion.AQA
Electronic structure and the periodic table: The electronic structure of the ion formed is that of the nearest noble gas, for example Cl⁻ is 2,8,8 like argon.AQA
Electronic structure of atoms and ions: The arrangement of electrons in an atom changes by the number lost or gained when it forms an ion, so the ion's structure differs from the atom's.AQA
Electronic structure of sodium: Written as 2,8,1, meaning two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.AQA
Electronics: Nanoparticles are used in components such as conductive films, displays and high-surface-area electrodes where a high surface area or small size is useful.AQA
Electroplating: A method that coats a metal with another metal using electrolysis, also improving appearance and conductivity.AQA
Electrostatic attraction: The strength of the bond comes from the attraction between oppositely charged ions, which attract each other electrostatically.AQA
Electrostatic attraction between oppositely charged ions: Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions, which attract each other because opposite…AQA
Electrostatic forces: The attractions between positive and negative charges, which fundamentally drive all chemical bonding.AQA
Electrostatic forces of attraction: Strong forces that act in all directions between oppositely charged ions in a giant ionic lattice.AQA
Element: A substance that contains only one type of atom.AQA
Element identity: What makes an atom a particular element, determined by its proton number.AQA
elements: There are about 100 different elements, and they are shown in the periodic table.AQA
Elimination of small molecules: A small molecule, commonly water, is lost at each link formed as a new covalent bond forms between monomer units.AQA
Embedded electrons: Negative electrons are embedded in a ball of positive charge.AQA
Emission control: Reducing these emissions involves using low-sulfur fuels, removing sulfur from flue gases, and using catalytic converters.AQA
Emission of light: What happens when the sample is introduced into a flame, where the metal ions emit light of characteristic wavelengths.AQA
Emission of light by excited ions: The colour comes from light emitted by excited metal ions as they return to lower energy levels.AQA
Empirical formula: The simplest whole-number ratio of cations to anions in the diagram, found by counting the ions.AQA
Empirical ratios from moles: Dividing all mole values by the smallest mole value to start simplifying the ratio.AQA
Empty space in atoms: Most alpha particles passed straight through the gold foil, showing that most of the atom is empty space.AQA
End point: The point at which a suitable indicator changes colour, showing that the acid and alkali have reacted completely.AQA
End users: The people and organisations who consume products, and whose choices drive demand for raw materials.AQA
Endothermic: A reaction that takes in energy and lowers the temperature, such as thermal decomposition of calcium carbonate.AQA
Endothermic and exothermic reactions: When temperature is increased, the system absorbs the extra heat by favouring the endothermic direction, while for an exothermic reaction the relative amount…AQA
Endothermic reaction: A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases.AQA
Endothermic reactions: Reactions that take in energy from their surroundings, so the surroundings cool.AQA
Energy and cost of water treatment: Desalination is expensive and uses a lot of energy, so it is not the first choice where fresh water is available.AQA
Energy and modern lifestyle: Many of the fuels on which we depend for our modern lifestyle are produced from crude oil.AQA
Energy barriers in reactions: The energy barrier to starting a reaction, represented in a model by the height of a hill a ball must be pushed over.AQA
Energy changes and bonding: A large amount of energy is needed to break the many strong bonds, which is why these compounds have high melting points and high boiling points.AQA
Energy changes during state changes: Only a small amount of energy is needed to separate the molecules from one another when a substance melts or boils.AQA
Energy demand in water treatment: The processes that make water potable can demand significant energy, especially when water is scarce or salty.AQA
Energy demands of desalination: Both methods require significant energy, which affects their cost and practicality.AQA
Energy input required for decomposition: Energy must be taken in continuously, so thermal decomposition is endothermic.AQA
Energy level diagrams: Simple reaction profiles showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to…AQA
Energy levels and shells: The regions around the nucleus that electrons occupy, with the lowest available energy level being the innermost shell, closest to the nucleus.AQA
Energy transfer: The difference between the energy absorbed in bond breaking and the energy released in bond making appears as energy transferred to or from the surroundings,…AQA
Energy transfer during state changes: Energy transferred during melting or boiling is used to overcome weak intermolecular forces rather than to break strong covalent bonds.AQA
Energy transfer from fuels: Burning hydrocarbon fuels transfers energy to the surroundings, which is why they are useful as fuels.AQA
Energy transfer from the surroundings: Energy is transferred from the surroundings to the reacting system during an endothermic change.AQA
Energy transfer in reactions: Drawing conclusions about energy transfer from recorded results, using them to identify patterns or explain how a chosen variable affected the temperature…AQA
Energy transfer to the surroundings: The excess energy is transferred to the surroundings, usually shown by a temperature rise.AQA
Energy transfers: Energy transferred by heating is used to overcome forces between particles during melting and boiling.AQA
Energy transfers during changes of state: Energy transferred by heating overcomes the forces holding particles together, and stronger forces need more energy to weaken or break.AQA
Environmental and economic trade-offs in extraction: Weighing advantages such as working on low-grade ores and less landscape disruption against disadvantages such as slow rates and the need for large areas of…AQA
Environmental cost of extracting raw materials: Obtaining raw materials from the Earth by quarrying and mining causes environmental impacts.AQA
Environmental damage: Acid rain can damage trees, acidify lakes and soils, and corrode buildings and statues made from limestone or marble.AQA
Environmental impact: Acid rain acidifies lakes and damages aquatic life, and carbon particulates can cause global dimming by reflecting sunlight.AQA
Environmental impact and future generations: A process is not sustainable if it uses finite resources faster than they can be replaced or causes lasting environmental harm for later users.AQA
Environmental impact of extraction: Digging, moving and disposing of large amounts of rock damages habitats and creates spoil heaps, while these methods reduce landscape disturbance and waste.AQA
Environmental standards: Requirements that treated water must meet before being released into rivers or the sea.AQA
Enzymes: Provided by yeast, these catalyse the reaction that converts glucose into ethanol and carbon dioxide.AQA
Equilibrium: In reversible reactions, changing temperature, pressure, or concentration can shift its position and alter yield.AQA
Equilibrium position: Changing temperature affects the position of equilibrium because it changes the rates of the forward and reverse reactions differently, depending on whether…AQA
Equilibrium shift: The direction in which an equilibrium moves, determined by using the balanced equation to oppose the change described.AQA
Equilibrium shifts: The movement of equilibrium to the right if a reactant is added or a product removed, or to the left if a product is added or a reactant removed.AQA
Error and uncertainty: Repeating each concentration and calculating a mean reduces the effect of random errors and improves the reliability of the conclusion.AQA
Ester links: The link formed when an alcohol group reacts with a carboxylic acid group, with water eliminated at each link formed.AQA
Esterification with alcohols: At higher tier, carboxylic acids react with alcohols to form esters, usually with an acid catalyst.AQA
Estimating uncertainty: A simple estimate given by half the range, often written as ± half the range about the mean.AQA
Evaluate: Weigh advantages such as strength, density, corrosion resistance and cost against disadvantages.AQA
Evaluating applications: Weighing benefits such as portability against risks such as burns, cost, reversibility and environmental impact using the information given.AQA
Evaluating applications against a specified purpose: Use the information given in a question to judge whether a use is worthwhile, reaching a justified judgement about whether a particular use is suitable for…AQA
Evaluating competing reaction pathways: Atom economy is only one factor; a company must also consider yield, rate, equilibrium position, energy demands, safety and the usefulness of any by-products…AQA
Evaluating evidence and reaching justified conclusions: Weighing advantages against disadvantages using the information given, with a judgement based on the evidence provided, not on a general preference.AQA
Evaluating evidence for a purpose: Using the information supplied in a question to judge how well nanoparticles suit the stated purpose, reaching a clear judgement supported by reasons.AQA
Evaluating hydrogen fuel cells: Weighing advantages, such as continuous electricity while hydrogen and oxygen are supplied and water as the only chemical product, against drawbacks, such as…AQA
Evaporation and condensation: Crude oil is heated so that hydrocarbons evaporate and rise as vapours, and a vapour condenses when it reaches a level where the temperature is below its…AQA
Everyday materials: Low density poly(ethene) is used for bags and film, and high density poly(ethene) is used for bottles and pipes.AQA
Everyday meaning of pure: Often means clean or natural, which differs from the chemical definition of pure.AQA
Everyday products: Formulations are common in everyday products including medicines, paints, cleaning agents, fuels, alloys, fertilisers and foods.AQA
Everyday versus chemical meaning: In everyday language pure means a substance has had nothing added to it, while in chemistry it means a substance contains only one substance.AQA
Evidence and conclusion: Using collected data to state whether the hypothesis is supported, identifying anomalies and basing the conclusion on the overall pattern.AQA
Evidence and regulation: Research informs regulation, and conclusions may change as new evidence is collected, so the field is developing rather than a fixed set of facts.AQA
Evidence and revision: As evidence accumulates, theories are modified or replaced, and new evidence leads to theories being modified or replaced.AQA
Evidence and scientific explanation: New experimental evidence can show that an existing model cannot explain observations, so the model is changed or replaced.AQA
Evidence for no new substance: No new substances are made during filtration, crystallisation, simple distillation, fractional distillation or chromatography.AQA
Evidence from characteristic reactions and tests: The particles of each substance remain unchanged, so their reactions and characteristic tests are the same as when the substance is pure, such as oxygen in…AQA
Evidence interpretation: Identifying patterns, trends, or relationships in the given data or information.AQA
Evidence-based acceptance of models: The periodic table became widely accepted because the agreement between prediction and discovery provided strong evidence that the underlying pattern was real.AQA
Evidence-based conclusions: The measured value and the known value are quoted, the direction of any difference is stated, and the sample is concluded to be pure or impure.AQA
Evidence-based decision making: The correct approach is to weigh evidence for the specific use rather than assume a general danger, using information supplied in the question to reach a…AQA
Evidence-based identification: Using the information supplied, such as an ingredient list or composition table, to decide whether the product is a formulation.AQA
Evidence-based model change: Science is a sequence of models revised when new evidence appears, such as cathode rays showing smaller, negatively charged particles inside atoms.AQA
Evolution of the atmosphere: The change from an early mixture rich in carbon dioxide with little or no oxygen to today's nitrogen-rich, oxygen-containing air.AQA
Examples of strong acids: Hydrochloric, nitric and sulfuric acids are examples of strong acids.AQA
excess: Adding the insoluble solid until it is in excess means the acid is fully neutralised and no unreacted acid remains.AQA
Excess reactant: The reactant that is not fully used up and some of which remains unreacted after the reaction.AQA
Excess reactant after reaction: The reactant in excess remains after the reaction and may need to be separated or recovered.AQA
Excess solid method: Because the solid is insoluble, it can be added in excess and then removed by filtration, and the reaction stops when no more solid dissolves.AQA
Excitation and emission: The heat energy from the flame excites the metal ions, which then emit light of characteristic wavelengths as they return to their lower energy states.AQA
Exothermic: A reaction that transfers energy to the surroundings and raises the temperature, such as combustion.AQA
Exothermic and endothermic changes: An exothermic change releases energy to the surroundings, while an endothermic change absorbs energy from the surroundings.AQA
Exothermic and endothermic classification: Using the sign or direction of the temperature change to show whether the reaction is exothermic or endothermic.AQA
Exothermic and endothermic processes: These are underpinned by the principle of conservation of energy, with energy transferred between the chemical store of reactants or products and the thermal…AQA
Exothermic and endothermic reactions: Reactions identified from reaction profiles by comparing the two levels: lower products mean energy is released, so the reaction is exothermic; higher…AQA
Exothermic reaction: A reaction in which the energy released from forming new bonds is greater than the energy needed to break existing bonds.AQA
Exothermic reactions: A reaction that transfers energy to the surroundings so the temperature of the surroundings increases.AQA
Experimental control of variables: A valid comparison requires the same mass or amount of reactant when surface area is changed, so that the effect is due to surface area alone.AQA
Experimental evidence: Observations such as colour change, solid deposit or temperature change that provide evidence of displacement.AQA
Experimental evidence for reactivity: Reactions with water or dilute acid, and displacement reactions, show which metals are more reactive.AQA
Experimental sequence: The sample must be put into the flame, the emitted light must be collected and passed through the spectroscope, and the resulting spectrum is then analysed.AQA
Experimental technique: A splint is lit, then blown out so that it still glows at the tip, and lowered into the tube so the gas surrounds the tip.AQA
Experimental testing of models: The model was later tested by the alpha particle scattering experiment, in which most alpha particles passed straight through gold foil but a few were…AQA
Explaining physical properties of simple molecules: Strong covalent bonds explain why molecules stay together, while weak intermolecular forces explain low melting and boiling points.AQA
Explaining reaction rate in particle terms: A reaction occurs only when reacting particles collide with each other and with sufficient energy; collisions with less energy do not lead to reaction.AQA
External current: A current supplied in the opposite direction, for example from a charger, which reverses the reactions so the products are converted back into the original…AQA
External reactants: The fuel and oxygen or air are fed into the cell from outside rather than being stored as the cell's only energy source.AQA
Extraction of metals from ores: Metals are extracted from ores, which are rocks containing enough of a metal compound to be worth mining.AQA
Extraction of metals with carbon: Metals below carbon can be extracted from their oxides by heating with carbon, whereas metals above carbon cannot be extracted this way.AQA
Extraction of reactive metals: Aluminium is extracted from its ore by electrolysis because it is more reactive than carbon, so carbon reduction is ineffective.AQA
Extreme weather: Warmer oceans and air can make some storms and rainfall events more intense, while other regions become drier and face drought.AQA

F

Factors affecting reaction rate: Increasing temperature, concentration, pressure or surface area, or adding a catalyst, increases the frequency or success of collisions and so increases the…AQA
Fair testing: Only the concentration of the chosen reactant is changed while temperature, volumes and other reactant amounts are kept constant for a fair comparison.AQA
Feedback mechanisms: These are loops through which the parts of the climate system affect one another, such as melting ice reducing reflectivity and causing more warming.AQA
Feedstock: A starting material for the petrochemical industry, rather than something necessarily burned for energy.AQA
Fermentation: The breakdown of sugar by yeast to produce ethanol and carbon dioxide.AQA
Fertiliser specification and quality control: Industry monitors product purity and nutrient content, while laboratory preparation is mainly for identification or testing rather than bulk supply.AQA
Fertilisers: Supply nitrogen compounds that plants need for making proteins and DNA, so crops grow larger and yields increase.AQA
fibreglass: A composite of glass fibres set in a polymer resin, where the fibres give tensile strength while the resin binds them and resists impact.AQA
Filling of energy levels: Electrons fill the lowest available energy levels first, with the first energy level holding a maximum of two electrons.AQA
Filter beds: Passing the water through these removes insoluble solids such as grit, soil and small particles, but not dissolved substances.AQA
Filtration: Separates an insoluble solid from a liquid using filter paper and a funnel, because the solid particles are too large to pass through the filter paper pores.AQA
Filtration and crystallisation: Filtering the mixture to remove the excess solid and keep the filtrate, then evaporating the filtrate until crystals begin to appear, before cooling,…AQA
Finite and renewable resources: Finite resources cannot be replaced as they are used, whereas renewable resources are replenished naturally at a rate that can match use.AQA
Finite resources: Resources that will eventually run out if extraction continues at current rates, because they form over geological timescales but are extracted and used much…AQA
Five-membered rings: Rings containing five carbon atoms that pull the sheet into a dome and help close the cage.AQA
Fixed composition and atom ratios: The proportions of the elements in a given compound are fixed, so the ratio of atoms is always the same for that compound.AQA
Fixed proportions: Formulations are made in fixed proportions so that every batch has the same properties.AQA
Flame test: A test that can be used to identify some metal ions (cations) from the colour they give to a flame.AQA
Flame tests: Tests used for metal cations such as lithium, sodium, potassium, calcium and copper, in which the flame colour is observed.AQA
Flammability: Decreases as molecular size increases because larger molecules are less volatile and harder to vaporise.AQA
Food and clothing: Categories supplied by natural resources and supplemented by agriculture, with food from fishing and farming and clothing using cotton, wool and synthetic…AQA
Forces between particles: The strength of these determines the amount of energy needed to change state from solid to liquid and from liquid to gas.AQA
Formation of fossil fuels: Buried carbon-rich remains subjected to heat and pressure over millions of years form deposits such as coal, crude oil and natural gas.AQA
Formation of halide ions and hydrogen halides: The halogen achieves a full outer shell either by gaining an electron to form a 1⁻ ion or by sharing an electron pair.AQA
Formation of positive ions: The more readily a metal loses electrons to form a positive ion, the more reactive it is.AQA
Formation of sedimentary rocks: Shells and skeletons of marine organisms contain calcium carbonate; when these settle and are compacted, sedimentary rock such as limestone forms and stores…AQA
Formation of sediments and sedimentary rocks: The precipitates settled on the ocean floor and became sediments, such as limestone.AQA
Formation of the early atmosphere: The first atmosphere is thought to have formed from volcanic gases released early in Earth's history, after the Earth formed.AQA
Formula: A way of showing which elements are present and, through subscripts, how many atoms of each are present.AQA
Formulae as symbolic representations: A chemical formula uses the chemical symbols of the elements and subscript numbers to show how many atoms of each element are in one unit.AQA
Formulation: A mixture that has been designed as a useful product, made to a chosen recipe so that it works well.AQA, OCR
Formulations: These include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods, appearing across many areas of chemistry and everyday life.AQA
Forward and reverse reaction rates: Increasing the concentration of a reactant makes the forward reaction faster than the reverse reaction, and the shift continues until the two rates become…AQA
Fossil fuels: Coal, oil and natural gas, which are limited resources that will eventually run out and whose burning releases carbon dioxide, a greenhouse gas.AQA
Fraction: A mixture of hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range, not a single pure compound.AQA
Fractional distillation: The process by which the many hydrocarbons in crude oil are separated into fractions, each containing molecules with a similar number of carbon atoms.AQA
Fractional distillation products: These fuels are fractions obtained by fractional distillation of crude oil.AQA
Fractions as mixtures with boiling-point ranges: Each collected fraction is a mixture of hydrocarbons with a similar boiling-point range rather than a pure substance.AQA
Freezing: Cooling a liquid so particles lose energy and move less, so forces pull them into a regular solid arrangement.AQA
Frequency of collisions: How often particles collide, increased by raising concentration or pressure because there are more particles in a given volume, and by increasing surface area…AQA
Fresh and clear: The state limewater must be in before use, because stale limewater may already be cloudy and give a false positive.AQA
Fuel cell: An electrical cell that is supplied with a fuel and oxygen, and uses energy from the reaction between them to produce electrical energy efficiently.AQA
Fuel cells: Devices supplied by an external source of fuel and oxygen or air, generating a potential difference continuously as long as they are supplied.AQA
Fuels: Fractions from crude oil provide these for cars, lorries, ships and aircraft, and for heating and electricity generation.AQA
Fuels and energy resources: The energy released makes hydrocarbons valuable as fuels for heating, cooking, electricity generation and transport.AQA
Fuels from crude oil: Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are all produced from crude oil.AQA
Full outer electron shells: A stable electron configuration that means atoms do not easily gain, lose or share electrons.AQA
full outer shell: Each ion has eight electrons in its outer shell, except for small ions such as Li⁺ which have two.AQA
Full outer shells: Electron transfer allows both atoms to achieve a full outer electron shell.AQA
Fullerenes: Molecular forms of carbon in which every carbon atom bonds to three others, producing curved cage-like or tubular molecules.AQA
Fully displayed formulae: Structural formulae in which every atom is shown and every bond is drawn as a line, including all C–H bonds and the new C–H or C–X bonds.AQA
Function of components: Each component in a formulation has a defined function, such as solvent, detergent, colourant, fragrance, preservative, fuel additive or active ingredient.AQA
Functional group: The group present that tells you which reactions to expect, such as the complete –COOH group that distinguishes carboxylic acids from alcohols and esters.AQA
Functional groups: The groups in the monomers, such as the alcohol groups and carboxylic acid groups, whose reaction allows the chain to continue growing.AQA
Functional unit: The same basis used to make a comparison fair, such as carrying the same shopping for one year.AQA

G

Galvanising: A zinc coating on iron, which still protects the exposed iron when scratched because the two metals are in contact.AQA
Gas identification: Oxygen is identified by its ability to relight a glowing splint inserted into a test tube of the gas.AQA
Gas identification tests: Hydrogen can be identified by a lighted splint giving a squeaky pop.AQA
Gas production and observation: Carbon dioxide is seen as effervescence, and it turns limewater milky.AQA
Genetic information storage: DNA is essential for life because it stores and transmits genetic information needed by living organisms.AQA
Geological timescales and atmospheric change: Both processes are slow and geological, so they reduced carbon dioxide gradually over long time periods.AQA
Giant covalent structure: A structure with no separate molecules, so the strong covalent bonds themselves must be overcome throughout the lattice to melt or boil it.AQA
Giant covalent structures: Graphite is one of these, in which each carbon atom forms three covalent bonds to three other carbon atoms, producing layers of hexagonal rings.AQA
Giant ionic lattice: A regular structure in which oppositely charged ions pack together in a three-dimensional arrangement that extends throughout the crystal.AQA
Giant ionic lattice structure: A structure in which huge numbers of positive and negative ions are packed together in a regular repeating pattern.AQA
Giant lattice: A structure in which the bonds act throughout, so a large amount of energy is needed to break them.AQA
Giant metallic lattice structure: A regular arrangement in pure metals in which identical atoms are arranged in layers.AQA
Giant structure: A metal is a giant structure of atoms rather than a simple molecule, with no fixed small number of atoms.AQA
Giant structures: Very large numbers of atoms are joined together in one continuous lattice rather than in small molecules.AQA
Global climate change: A change expected to result from the rise in the temperature of the Earth's atmosphere at the surface.AQA
Global dimming: The effect where particulates in the atmosphere reflect sunlight back into space, reducing the amount of light reaching the Earth's surface.AQA
Global temperature: A long-term average calculated from many land and sea measurements around the world.AQA
Graph interpretation: Plotting a graph such as volume of gas against time allows the gradient or the time taken to be interpreted as a measure of rate.AQA
Graphene: A single flat layer of hexagonal carbon rings one atom thick, with each carbon bonded to three others.AQA
Greenhouse effect: Greenhouse gases absorb infrared radiation emitted by the Earth, so increasing their concentration strengthens this effect.AQA
Greenhouse effect and global warming: Greenhouse gases such as carbon dioxide and methane absorb infrared radiation re-emitted by Earth, warming the atmosphere and oceans.AQA
Greenhouse effect mechanism: Short wavelength radiation from the Sun passes through the atmosphere and is absorbed by Earth's surface, which then emits longer wavelength infrared…AQA
Greenhouse gas emissions: Gases released across the life cycle, with carbon dioxide and methane the main gases counted and others included by converting them to a carbon dioxide…AQA
Greenhouse gases: Water vapour, carbon dioxide and methane are gases in the atmosphere that absorb and re-emit infrared radiation emitted by Earth's surface.AQA
Ground water: Water such as water from aquifers, which is often naturally filtered through rock and may need only screening, aeration and chlorination.AQA
Group: Determined by the number of outer-shell electrons in an atom.AQA
Group 0: The final column on the right of the periodic table, containing the noble gases.AQA
Group 0 noble gases: These sit on the far right of the periodic table and include helium, neon and argon.AQA
Group 1 and Group 2 metals: Group 1 metals such as lithium, sodium and potassium sit on the far left, and Group 2 metals such as magnesium and calcium sit next to them.AQA
Group 1 elements: Soft, low density and highly reactive metals that form 1⁺ ions.AQA
Group 1 metals: Lose one electron, giving ions with a 1⁺ charge, such as Na⁺.AQA
Group 1, Group 7 and Group 0 trends: Group 1 metals lose one electron readily, Group 7 non-metals gain one electron, and Group 0 elements have full outer shells and are unreactive.AQA
Group 2 metals: Lose two electrons, giving ions with a 2⁺ charge, such as Mg²⁺.AQA
Group 6 non-metals: Gain two electrons, giving ions with a 2⁻ charge, such as O²⁻.AQA
Group 7 halogens: These sit just to the left of Group 0 and include fluorine, chlorine and bromine.AQA
Group 7 non-metals: Gain one electron, giving ions with a 1⁻ charge, such as Cl⁻.AQA
Group 7 reactivity: The reactivity trend shown by this reaction, which can be used to place halogens in order of reactivity.AQA
Group 7 trends: Down the group, atomic radius and shielding increase, so the attraction for an incoming electron decreases and reactivity decreases.AQA
Group number: The number used to work out how many bonds an atom forms, for example carbon in group 4 forms four bonds.AQA
Group number and ion charge: For metals the group number tells you how many electrons are lost, and for non-metals the number gained is 8 minus the group number.AQA
Group properties: Elements in the same group have the same number of outer electrons, which gives them similar chemical properties.AQA
groups: Vertical columns in the periodic table that contain elements with similar properties.AQA

H

Haber process: Used to manufacture ammonia, reacting nitrogen from air with hydrogen.AQA
Habitat and landscape destruction: Large areas of land are cleared, destroying habitats and landscapes, and the pits and spoil heaps remain after extraction ends.AQA
Half equation: Shows electrons gained or lost by one species, for example Mg → Mg²⁺ + 2e⁻ for oxidation or Cl₂ + 2e⁻ → 2Cl⁻ for reduction.AQA
Half equations: Balanced equations that demonstrate the transfer of electrons, with electrons on the right side for oxidation and on the left side for reduction.AQA
Half-equations for hydrogen and oxygen: At the cathode 2H⁺ + 2e⁻ → H₂, and at the anode 4OH⁻ → O₂ + 2H₂O + 4e⁻.AQA
Half-equations for reduction: The cathode half-equation for hydrogen production is 2H⁺ + 2e⁻ → H₂, and for a metal less reactive than hydrogen such as copper it is Cu²⁺ + 2e⁻ → Cu.AQA
Halide identification: Halide ions in aqueous solution can be identified by adding dilute nitric acid followed by silver nitrate solution, and the precipitate colour identifies the…AQA
halide ions: Ions such as Cl⁻, Br⁻ or I⁻, which are discharged in preference to hydroxide ions, so the halogen forms instead.AQA
Halide preference at the anode: Halide ions are discharged at the anode in preference to hydroxide ions, giving a halogen; otherwise oxygen is produced.AQA
Halide salts: Salts whose aqueous solutions are used in displacement reactions, where the less reactive halide ion is oxidised to form a halogen molecule.AQA
Halogens: The elements in Group 7 of the periodic table, which have similar reactions because they all have seven electrons in their outer shell.AQA
Hand warmers: Devices that commonly use the exothermic oxidation of iron by oxygen from the air to release heat.AQA
Hardness and melting point: The rigid network makes diamond very hard, and many strong covalent bonds must be broken to melt it, so it has a very high melting point.AQA
Harmful chemicals: Toxic dissolved substances including heavy metal ions, acids, alkalis or toxic compounds that must be removed or reduced to safe levels.AQA
Harmful microbes: Bacteria and viruses that can cause disease, can contaminate drinking water and cause illness, and must be reduced before water is released.AQA
Hazard control and apparatus choice: Evaporating the filtrate using a water bath or electric heater rather than a direct flame, because gentle heating avoids spitting and decomposition.AQA
Heating and cooling in material manufacture: The three materials are heated together to a high temperature and then cooled quickly so the mixture solidifies without forming an ordered crystal lattice.AQA
Hexagonal rings: Six-membered rings of carbon atoms, exactly like those in graphite, that form the main framework of fullerenes.AQA
High carbon steel: An iron alloy containing a relatively large proportion of carbon, which makes it strong and hard but brittle.AQA
Historical development of atomic theory: The nucleus was an established scientific idea well before the neutron was identified.AQA
Homologous series: A family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit.AQA, OCR
Homologous series and general formulae: Families with the same general formula and functional group, whose members differ by one CH₂ unit, such as alkanes and alkenes.AQA
Human activities: Activities such as burning fossil fuels, deforestation, livestock farming and landfill increase the amounts of carbon dioxide and methane.AQA
Human health: Both gases irritate the airways and can worsen respiratory problems such as asthma and bronchitis.AQA
Human health effects: Carbon monoxide binds to haemoglobin and reduces oxygen transport, causing dizziness and death, while carbon particulates cause respiratory irritation.AQA
Human impact on climate: Some human activities increase the amounts of greenhouse gases in the atmosphere, and these gases trap heat.AQA
Human impacts: Agriculture may suffer where rainfall changes or pests spread, affecting food supply, and human health may be affected by heatwaves and changing disease…AQA
Human needs: Warmth, shelter, food and transport are provided by using the Earth's resources.AQA
Hydrocarbons: Molecules that contain only carbon and hydrogen, as in petrol, diesel and natural gas.AQA
Hydrogen: A non-metal even though it appears at the top left, which is a useful exception to remember.AQA
Hydrogen as a fuel: A common fuel used in fuel cells, supplied from a cylinder or store, and oxidised at one electrode.AQA
Hydrogen fuel cell: Produces a voltage continuously while hydrogen and oxygen are supplied, because the reaction between them releases energy that is transferred electrically.AQA
Hydrogen ion concentration and pH: pH is a measure of hydrogen ion concentration, so a higher concentration of H⁺ ions gives a lower pH value.AQA
Hydrogen oxidation: In a hydrogen fuel cell, hydrogen is oxidised at one electrode while oxygen is reduced at the other.AQA
Hydrogen production: Hydrogen gas is produced when hydrogen ions in the acid gain electrons, seen as effervescence as bubbles form.AQA
Hydrogen source: Hydrogen is commonly obtained from natural gas, which is mainly methane, by reacting methane with steam.AQA
hydroxide ions: Ions, OH⁻, present from water, which are discharged to produce oxygen gas.AQA
Hypothesis: A testable prediction linked to scientific reasoning that identifies the independent variable, the dependent variable and the expected direction of change.AQA
Hypothesis development and testing: Developing a testable hypothesis linking the ions in solution to the products at each electrode, then testing it by electrolysing solutions and recording…AQA

I

Identification by comparison with reference data: The sample's chart or table is compared with a reference set in the same form, matching line positions or wavelengths rather than brightness alone.AQA
Identification of electrode products: The products can be identified by testing gases, for example a lighted splint for hydrogen and damp litmus for chlorine.AQA
Identification of elements: Using the line spectrum produced through a spectroscope, since each metal ion gives its own pattern of lines, to identify which metal ions are present.AQA
Identification of metal ions: Using the positions of the lines in the spectrum to identify which metal ions are present in the solution.AQA
Identification of substances: Comparing Rf values with known values to help identify substances, where a pure substance usually gives a single spot.AQA
Identifying transition metal ions: These colours are distinctive and allow the ions to be identified in a simple test.AQA
Impurities: Unwanted substances in a material that can change its properties, such as copper in steel making it crack during rolling.AQA
Incomplete combustion: Combustion in which some carbon in a fuel is not fully burned, increasing the release of solid particles and unburned hydrocarbons.AQA
Incompleteness of scientific evidence: Early tables contained gaps because many elements had not yet been discovered, so they were incomplete in the sense of missing elements.AQA
Indivisible particles: Tiny spheres that could not be divided, which is how atoms were thought of before the discovery of the electron.AQA
Industrial conditions: The conditions used during polymerisation, such as temperature, pressure and the presence of a catalyst, which affect how the chains form and pack together.AQA
Industrial costs and maintenance: The anode must be continually replaced because it is consumed by reaction with oxygen, adding to the cost of the process.AQA
Industrial efficiency: Chemistry improves industrial processes by developing catalysts and new materials that increase efficiency and reduce waste.AQA
Industrial energy and cost considerations: Cryolite dissolves the oxide and lowers the melting point, reducing energy cost, while the carbon anode is gradually used up and must be replaced.AQA
Industrial feedstocks: Alkenes act as starting materials for making many other chemicals, including ethanol and ethanoic acid.AQA
Industrial processes such as the Haber process: In the Haber process, increasing pressure shifts equilibrium to the right because there are fewer gas molecules on the product side, partially reducing the…AQA
Industrial processing of minerals: Phosphate rock must be treated with an acid to convert the insoluble calcium phosphate into soluble phosphate salts.AQA
Industrial versus laboratory processes: Both routes rely on the same reaction between an acid and the basic phosphate rock, but scale, equipment, safety controls and product specification differ.AQA
Inert electrodes: Electrodes such as graphite are used so they do not react with the products or electrolyte.AQA
Inert electrodes and control variables: Choosing inert graphite electrodes and controlling variables such as solution concentration, voltage and electrode material.AQA
Instrument resolution: Instruments have limited resolution, and the resolution of an instrument can be used to estimate the uncertainty in a single reading.AQA
Instrumental analysis: A method in which an instrument is used to produce a result, here limited to flame emission spectroscopy, that is interpreted using appropriate data in chart…AQA, OCR
Instrumental methods: Methods that use machines to detect and identify elements and compounds from a measured signal.AQA
Integrated industrial chemistry: The nitric acid produced can then be reacted with more ammonia to make ammonium nitrate, so the two products are linked within an integrated fertiliser process.AQA
Interference between overlapping test results: Sodium yellow is intense and easily masks other colours, so careful cleaning and viewing through a blue glass can help when potassium is suspected.AQA
Intermolecular forces: Forces acting between one polymer molecule and its neighbours, which are relatively strong compared with those between small molecules but weaker than the…AQA
Intermolecular forces and physical properties: Very weak forces between the atoms give very low melting and boiling points, which rise slowly down the group as atoms get larger and forces between them…AQA
Intermolecular forces and physical states: Weak intermolecular forces hold the small molecules together in the element, which explains why fluorine and chlorine are gases, bromine is a liquid and…AQA
Interpret: Read the composition and identify the elements and their relative proportions.AQA
Interpreting graphical models of reactions: Using reaction profiles to identify reactions as exothermic or endothermic and to explain that the activation energy is the energy needed for a reaction to…AQA
Inverse proportionality: The ratio is inversely proportional to side length, so dividing the side length by 10 multiplies the ratio by 10.AQA
Ion: An atom or group of atoms that has an electrical charge, either positive or negative, due to the loss or gain of electrons.OCR
Ion competition in aqueous solution: The compound supplies a positive ion and a negative ion, while water supplies H⁺ and OH⁻, and the appropriate ions are discharged at each electrode.AQA
ion discharge: The process linked to the relative ease of oxidation of the ions present, giving oxygen or a halogen at the anode.AQA
Ion formation: Metal atoms lose electrons to form positive ions, and non-metal atoms gain electrons to form negative ions.AQA
Ion formation and equations: Hydrochloric acid gives HCl(aq) → H⁺(aq) + Cl⁻(aq), nitric acid ionises to H⁺ and NO₃⁻, and sulfuric acid releases two H⁺ ions per molecule, forming SO₄²⁻.AQA
Ion formation by electron gain: Non-metal atoms gain electrons into their outermost shell until that shell is full, producing a negative ion.AQA
Ion formation by electron loss: Metal atoms lose electrons to become positively charged ions, losing the electrons in their outermost shell first because these are the least strongly…AQA
Ion formation by metals: When metals react with other substances the metal atoms form positive ions.AQA
Ion migration in electrolytes: Positively charged ions are attracted to the negative electrode and negatively charged ions are attracted to the positive electrode.AQA
Ionic Bond: The strong electrostatic attraction between oppositely charged ions.AQA
Ionic bonding: Occurs in compounds formed from metals combined with non-metals, where oppositely charged ions are held together by strong electrostatic attraction in a giant…AQA
Ionic conduction: The movement of mobile ions, either molten or in aqueous solution, towards the electrode of opposite charge.AQA
Ionic equation: Shows only the ions that change, omitting spectator ions, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s) for a precipitation reaction.AQA
Ionic equations: The ionic equation for chloride is Ag⁺(aq) + Cl⁻(aq) → AgCl(s), and equivalent equations are written for bromide and iodide.AQA
Ionic equations and state symbols: Balanced ionic equations show H⁺(aq) as a reactant where appropriate, balancing both atoms and charge and including state symbols where they are given or…AQA
Ionic formulae and charge balance: The correct positive and negative ions are combined in the correct ratio so that the charges balance, with subscripts showing the ratio needed for electrical…AQA
Ionic lattice: Represented by NaCl because it contains a metal and a non-metal.AQA
Ionic lattice recognition: Deducing that a compound is ionic from a diagram by identifying a giant lattice of oppositely charged ions.AQA
Ionic lattices: The oppositely charged ions attract each other electrostatically in a giant ionic lattice, arranging in a regular lattice of alternating charges rather than…AQA
Ionic mobility: When the compound is melted it dissociates into ions, which are free to move and carry charge.AQA
Ionic species and charges: Naming the species with its charge, such as Cu²⁺ or SO₄²⁻, rather than giving just the element name.AQA
Ionic versus covalent bonding: The nature of the compound depends on whether the other element is a metal or a non-metal, giving electron transfer or electron sharing.AQA
Ions in aqueous solution: When an acid dissolves in water, it ionises to release hydrogen ions, H⁺, and the subscript (aq) shows that the ions are dissolved in water.AQA
Ions with different charges: Transition metals form ions with different charges, such as iron in FeSO₄ being 2⁺ and iron in FeCl₃ being 3⁺.AQA
Isotope: Atoms of the same element with the same number of protons but a different number of neutrons (and therefore a different mass number).OCR
Isotope notation: The mass number is written as a superscript before the element symbol, such as ²³Na.AQA
Isotopes: Atoms of the same element with different numbers of neutrons and therefore different mass numbers.AQA

J

Justification using equations: Explaining a prediction by referring to Le Chatelier's Principle and the relevant feature of the equation, such as gas molecule numbers or…AQA
Justified judgement: A clear judgement that states whether the alloy is suitable and why, supported by at least one property.AQA

K

Key experiments in atomic history: Thomson's discovery of the electron and Rutherford's alpha-particle scattering experiment provided evidence that changed the atomic model.AQA

L

Landfill: The place glass would otherwise be sent, and the amount sent there is reduced by recycling.AQA
Large hydrocarbon molecules: Molecules in crude oil fractions that are less useful as fuels because they are viscous, hard to ignite and burn less cleanly.AQA
Lattice: A repeating three-dimensional arrangement of atoms that extends throughout the whole piece of metal.AQA
Law of Conservation of Mass: The principle that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.OCR
Layered lattice arrangement in pure metals: In pure metals, the positive ions are arranged in regular layers, like sheets stacked on top of one another.AQA
Layered structures: The bonded atoms arrange into flat layers of hexagonal rings, like a honeycomb, with no covalent bonds between the layers.AQA
Layers of atoms and sliding: Because the atoms are all the same size, the layers can slide over one another when a force is applied, so the metal is soft.AQA
Layers of iron atoms: The layers in the structure that can slide over one another more readily when a force is applied, because fewer carbon atoms disrupt them.AQA
Le Chatelier's principle: When a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change.AQA
Leachate: The solution produced by bacteria, containing copper compounds and other dissolved substances, not pure copper metal.AQA
Life cycle assessment: Tracks a product through raw material extraction, manufacture, use and disposal.AQA
Life Cycle Assessment (LCA): An evaluation of the environmental impact of a product throughout its entire life, from extraction of raw materials to disposal.OCR
Life-cycle thinking: Considering every stage from raw material extraction through manufacture, packaging, transport, use and repair to disposal, recycling or reuse.AQA
Limewater: An aqueous solution of calcium hydroxide used to test for carbon dioxide.AQA
Limitations of early classification systems: Early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.AQA
Limitations of early models: The model does not include a nucleus, protons or neutrons, and was inconsistent with large-angle alpha particle deflections.AQA
Limitations of flame tests: The method only works for some cations and cannot detect non-metal ions.AQA
Limitations of models: Solid inelastic spheres with no forces between them cannot explain energy changes during melting and boiling.AQA
Limited resources: Resources that will eventually run out, such as the fossil fuels supplying much of the energy for extracting raw materials and manufacturing products.AQA
Limiting reactant: The reactant that runs out first when reactants are not in the exact ratio of the balanced equation, fixing the maximum amount of product.AQA
Limiting rules to specified groups: The method applies only to the metals in Groups 1 and 2 and the non-metals in Groups 6 and 7.AQA
Line spectra: The output of flame emission spectroscopy, in which each metal ion produces a characteristic pattern of lines at particular wavelengths.AQA
Line-bond representations: Representations that use one line per single bond, for small molecules, polymer repeating units and parts of giant covalent structures.AQA
Link between ion formation and reactivity: The more readily a metal loses electrons, the more reactive it is, and this ion formation underlies the reactivity series.AQA
Link between ionisation and pH: Because ionisation is complete, the concentration of H⁺ ions equals the concentration of the acid for monoprotic acids, giving a low pH.AQA
Local and global water availability: The choice of treatment depends on the water source and local conditions, so treatment is not the same everywhere.AQA
Long-term changes in atmospheric composition: Volcanic gases shaped the atmosphere's long-term composition, and nitrogen is now the most abundant gas in it.AQA
Low carbon steel: An iron alloy containing only a small proportion of carbon, which is softer and more easily shaped.AQA
Low density: A small mass for a given volume, which makes aluminium alloys suitable for aircraft, bicycles, vehicle parts and drink cans.AQA
Low-grade ores: Ores that contain only small percentages of copper compounds, so large masses of rock must be processed to obtain useful copper.AQA
Lubricants: Reduce friction between moving surfaces, for example in engines and machinery.AQA

M

Macromolecules: DNA is a very large molecule, often described as a macromolecule, made of many repeating units.AQA
Malleability: The regular packing allows layers of atoms to slide over one another when a force is applied, so metals can be bent and shaped.AQA
Malleability and ductility: Malleable means the metal can be bent or hammered into shape, while ductile means it can be drawn into wires.AQA
Malleable: Able to be bent, pressed or rolled into shape without cracking, as low carbon steel is.AQA
Manufacture of glass, building materials and clay ceramics: Glass is made by melting sand with sodium carbonate and limestone, building materials are quarried directly, and clay ceramics are made by shaping and firing…AQA
Masked: Describes a flame colour that is hidden when a sample containing a mixture of ions is used, because one colour can hide another.AQA
Mass and volume relationships: For a fixed mass of solute, increasing the volume of the solution decreases the concentration, while for a fixed volume, increasing the mass of the solute…AQA
Mass number: A count of protons plus neutrons, which is not the same as the atomic number.AQA, OCR
Mass number calculation: The sum of the protons and neutrons in an atom is its mass number.AQA
Mass relationships in equations: Calculating the mass of each substance by multiplying its balancing number by its Mr, or by converting a given mass to moles and back to mass.AQA
Mass–amount relationship: For a pure substance, amount in moles n = mass m ÷ Mr, and rearranging the same relationship gives mass m = n × Mr.AQA
Mass–mole calculations: Converting between mass and chemical amount using the relationship amount in moles equals mass divided by molar mass.AQA
Mass–mole conversions: The numerical bridge that lets you convert between mass in grams and amount in moles using n = m ÷ Mr, rearranging to m = n × Mr when mass is required.AQA
Mass–volume relationships: For a fixed mass of solute, increasing the volume decreases the concentration, and the mass of solute and volume of solution are related to the concentration.AQA
Matching supply of fractions to demand: Cracking helps match the supply of hydrocarbon fractions to demand, since longer-chain fractions are less useful as fuels.AQA
Materials and their uses: Knowing the exact metals in each alloy allows you to compare their properties and suggest suitable uses.AQA
Materials from crude oil and plants: Plastic comes from crude oil, while paper comes from trees.AQA
Materials selection for function: Because the alloy is a mixture, its composition can be adjusted to produce the desired hardness, colour and cost.AQA
Materials selection for uses: Matching the demands of the job to the data rather than choosing the cheapest or most familiar option.AQA
matrix: The material, also called the binder, that surrounds and binds together the other material and helps transfer forces through the composite.AQA
Mean and range: The mean is the sum of the measurements divided by the number of measurements, and the range is the difference between the highest and lowest values.AQA
Measurement and uncertainty: Whenever a measurement is made there is always some uncertainty about the result obtained, which is the range of values within which the true value is likely…AQA
Measurement of concentration: Using the intensity of the lines, often by comparing the sample spectrum with spectra from solutions of known concentration.AQA
Medicine: Nanoparticles can carry drugs to specific cells or be used in imaging and sensors because of their small size and high surface area.AQA
Medicines: Combinations of an active ingredient with other components that affect dose, stability or delivery.AQA
Melting: Heating a solid so particles vibrate more until the forces holding the regular arrangement are overcome and the solid becomes a liquid.AQA
Melting and boiling: Changes that require energy to overcome the forces holding particles together.AQA
Melting and boiling as physical changes: Melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice, not breaking individual covalent…AQA
Melting and boiling of simple molecular substances: The molecules separate and move further apart or past one another, but each molecule keeps its covalent bonds.AQA
Melting and boiling point data: Measurements of the temperatures at which a substance melts and boils, which for a pure substance are single sharp temperatures at a stated pressure.AQA
Melting and boiling points: These are relatively low for small-molecule substances because the weak forces between molecules are overcome, not the covalent bonds within molecules.AQA
Melting point depression: An impure substance melts below the known melting point, and the change happens over a range of temperatures.AQA
Melting points: Very high for giant covalent substances because melting requires breaking many strong covalent bonds.AQA
Membrane separation and reverse osmosis: Salty water is forced at high pressure through a partially permeable membrane that allows water molecules through but retains most dissolved ions.AQA
Metal and acid reactions: Magnesium reacts rapidly with dilute acid, zinc and iron react with dilute acid with zinc faster, and copper reacts with neither water nor dilute acid.AQA
Metal and non-metal compounds: Compounds in which the metal atoms lose electrons to form positive ions and the non-metal atoms gain electrons to form negative ions.AQA
Metal and water reactions: Potassium, sodium and lithium react with cold water to form the metal hydroxide and hydrogen, while calcium reacts steadily and magnesium very slowly.AQA
Metal extraction: A process in which the metal oxide loses oxygen and is therefore reduced.AQA
Metal ions in solution: The type of sample the method requires, so the metal ions must be in solution for it to be applied.AQA
Metal reactions with water and acids: The reactions, if any, of the named metals with water or dilute acids, including the products and observations.AQA
Metallic Bond: The electrostatic attraction between positive metal ions and a sea of delocalised electrons.AQA
Metallic bonding: The electrostatic attraction between the positive ions and the shared delocalised electrons, acting throughout the structure so it is strong.AQA
Metallic bonding and physical properties: Melting points and boiling points decrease down the group because metallic bonding weakens as ions become larger and charge density falls.AQA
Metallic bonding and structure: Transition metals have stronger metallic bonding and more delocalised electrons than Group 1 metals, which is why the differences between the groups arise.AQA
Metallic bonds: Strong bonds that hold positive ions to a sea of delocalised electrons in a metal.AQA
Methane sources: Livestock farming, especially cattle and sheep, releases methane through digestion and waste, and landfill sites release methane as organic waste decomposes…AQA
Microbes: Impurities such as bacteria that can cause disease and are reduced by chlorination to kill them.AQA
Mining of raw materials: Potassium chloride, potassium sulfate and phosphate rock are obtained by mining.AQA
Misleading: Presenting a limited LCA as a complete comparison, when the LCA itself may be valid for its limited scope.AQA
Mitigation actions: Actions to reduce emissions of carbon dioxide and methane, such as generating electricity from renewable sources or capturing gas from landfill sites.AQA
Mixture: Two or more elements or compounds that are not chemically combined together, so the substances keep their own identities.AQA
Mixtures: Components that are not chemically combined and can be separated by physical means.AQA
Mixtures and compounds: Separation only divides a mixture, whereas a reaction forms new products.AQA
Mixtures in natural products: Pure milk is a mixture of water, fats, proteins, sugars and minerals, so it is not pure in the chemical sense.AQA
Mixtures versus compounds: Physical methods separate mixtures, not compounds, because the elements in a compound are chemically combined.AQA
Mixtures versus pure substances: A mixture usually melts or boils over a range of temperatures because its different components change state at different temperatures, while a pure substance…AQA
Mobile and stationary phases: Components partition between these two phases as the solvent rises, which separates them.AQA
Mobile charge carriers: The delocalised electrons in a metal, which are free to move through the giant metallic lattice.AQA
Mobile ions: Ions that are free to move within the liquid or solution and carry charge through it, so a current flows.AQA
Mobile phase: The phase that moves, a solvent or gas that carries the substances through or over the stationary phase.AQA
Model limitations: Each model has limits: dot and cross diagrams show electrons but not shape, ball and stick models show three-dimensional shape but not electron pairs, and…AQA
Molar gas volume at room temperature and pressure: The value of 24 dm³ that applies at room temperature and pressure and is used to find gas volume from moles.AQA
Molar mass: The mass of one mole of a substance, found by adding the relative atomic masses in the formula and expressed in g/mol.AQA
Mole: The amount of a substance obtained by dividing its mass by its molar mass, giving moles of atoms or molecules.AQA, OCR
Mole as the amount unit: The amount unit linking mass to particles, where one mole of a substance has a mass in grams equal to its relative formula mass, Mr.AQA
Mole calculations: Multiplying the amount in moles by 6.02 × 10²³ gives the number of particles, and dividing the number of particles by 6.02 × 10²³ gives the amount in moles.AQA
Mole calculations from mass and relative formula mass: Finding moles by dividing mass by relative formula mass, then multiplying by 24 dm³ to find gas volume at room temperature and pressure.AQA
Mole calculations from masses: Converting the masses in grams to amounts in moles, using the correct Mr for each substance.AQA
Mole concept: The mole is the chemist's counting unit for particles, and one mole of a substance has a mass in grams numerically equal to its relative formula mass.AQA
Mole ratio: The ratio required by the balanced equation, used to convert the amount of limiting reactant into the amount of product in moles.AQA
Mole ratios: The balancing-number ratio from the equation, applied after converting a given mass to moles, to find the moles of the required substance.AQA
Molecular formulae: The formulae C₂H₄, C₃H₆, C₄H₈ and C₅H₁₀ for the first four alkenes, all fitting the general formula CₙH₂ₙ.AQA
Moles and concentration: When concentration is in mol dm⁻³, moles are first calculated using n = c × V, then converted to mass using m = n × Mᵣ.AQA
Molten compounds: Compounds that are molten so that their ions are free to move and conduct electricity.AQA
Molten state: State in which the ions of the compound are free to move, unlike in the solid state.AQA
monomer: The molecule whose atoms all become part of the polymer chain, with its double bond becoming a single bond in the repeating unit.AQA
Monomer structure: Each monomer has two of the same functional group, such as a diol with two alcohol groups or a dicarboxylic acid with two carboxylic acid groups.AQA
Monomer to polymer: Many small monomer molecules join together to form polymer molecules.AQA
Monomers: Small repeating units that covalently bond to form a polymer.AQA
Monomers and polymerisation: Ethene is the monomer used to make poly(ethene), and the same monomer can produce two different polymers depending on the conditions used.AQA
Movement of delocalised electrons: The delocalised electrons move with the ions and continue to hold the structure together, so the metallic bonding is not broken.AQA

N

n = m ÷ Mᵣ: The relationship used to convert between mass and moles when the question gives masses in grams.AQA
N:P:K ratio: Numbers where each represents the percentage by mass of that element in the fertiliser.AQA
Naming compounds from ions: The salt is named by giving the positive ion name first and the acid-derived anion name second.AQA
Nanometre range: One nanometre is 1 × 10⁻⁹ m, so nano dimensions are much smaller than everyday objects.AQA
Nanometres as a unit of length: A unit of length in which 1 nm = 1 × 10⁻⁹ m, used when discussing atomic dimensions.AQA
Nanoparticle: A particle with dimensions between 1 and 100 nanometres, possessing a very high surface area to volume ratio.AQA
Nanoparticle properties: Controlling particle size, shape and surface at the nanoscale can produce new or improved properties.AQA
Nanoparticles: Particles with at least one dimension in the range 1 to 100 nanometres, sitting between individual atoms or molecules and bulk material.AQA
Nanoscience: Refers to structures that are 1–100 nm in size, of the order of a few hundred atoms.AQA
Native metals: Unreactive metals such as gold are found in the Earth as the metal itself, described as native.AQA
Natural greenhouse effect: Trapped energy keeps the average surface temperature higher than it would otherwise be, warm enough for liquid water and life.AQA
Natural resources: Resources that occur naturally and, supplemented by agriculture, provide food, timber, clothing and fuels.AQA
Natural versus synthetic carbon compounds: Many compounds such as carbohydrates and proteins occur naturally, while others such as plastics and medicines are synthetic.AQA
Natural versus synthetic materials: Natural products are obtained from plants, animals or the Earth without chemical synthesis, while synthetic products are made by chemical processes.AQA
Naturally occurring polymers: DNA is a naturally occurring polymer rather than a synthetic one, found in living organisms.AQA
Neutral atom: An atom in which the number of electrons equals the atomic number.AQA
Neutral atoms: In an atom the number of electrons is equal to the number of protons in the nucleus, so the positive and negative charges balance exactly.AQA
Neutral molecules: Molecules that have no overall electric charge, even though individual atoms in a bond may carry small partial charges.AQA
Neutralisation: The process in which acids are neutralised by alkalis and bases to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide.AQA
Neutralisation and ionic equations: In neutralisation, H⁺(aq) from the acid reacts with OH⁻(aq) from the alkali to form water, H₂O(l), and this ionic equation applies to any acid reacting with…AQA
Neutralisation and salt formation: Adding the insoluble oxide or carbonate a little at a time until it is in excess and no more dissolves, so the acid is used up and a soluble salt forms.AQA
Neutralisation endpoint: The point at which no more solid reacts or dissolves, showing that all the acid has been used up and excess solid remains.AQA
Neutrality: Means hydrogen ion concentration equals hydroxide ion concentration, giving pH 7 for aqueous solutions at the usual laboratory temperature.AQA
neutron: A neutral particle in the nucleus, supported by Chadwick's evidence.AQA
Nitrogen source: Nitrogen is obtained from air, which is approximately 78% nitrogen by volume, and can be separated from air by fractional distillation of liquid air.AQA
Noble gases: The elements in Group 0 of the periodic table, which are unreactive non-metals.AQA
Nomenclature: The names use the stem for the number of carbon atoms, eth-, prop-, but- and pent-, with the ending -ene showing the C=C double bond.AQA
Non-electrolyte: A substance such as pure water or a solid ionic compound that lacks mobile ions and so is not an electrolyte in this sense.AQA
Non-enclosed systems: In these, matter can leave or enter as a gas, so the mass reading may change even though atoms are conserved.AQA
Non-metal character of the halogens: The halogens are non-metals, so they lack the typical properties of metals such as conduction of electricity and malleability.AQA
Non-metals: Found towards the right and top of the periodic table, they are poor conductors of electricity, often brittle as solids, and tend to form negative ions or…AQA
Non-metals in the series: Hydrogen and carbon are included among the metals in the series for prediction.AQA
Non-rechargeable cell: A cell that produces electricity from chemical reactions and cannot be recharged by passing current through it.AQA
NPK fertilisers: Fertilisers that supply nitrogen, phosphorus and potassium, with the potassium coming from mined soluble potassium salts.AQA
Nuclear charge: The charge of the nucleus, which changes when the proton number changes.AQA
nuclear model: Describes an atom as a tiny, dense, positively charged nucleus surrounded by electrons in shells, with most of the atom being empty space.AQA
Nuclear model of the atom: The model in which the mass and positive charge are concentrated in a tiny central nucleus, replacing the plum pudding model.AQA
Nucleotide monomers: The monomers of DNA are called nucleotides, and there are four different nucleotides in DNA.AQA
Nucleus as the mass centre: Almost all of the mass of an atom is in the nucleus, which contains protons and neutrons.AQA

O

Observation and description of chemical change: Describing observations such as fizzing, movement on the water surface and the metal becoming smaller.AQA
Observation and product identification: Recording observations at both electrodes, such as bubbles, colour or gas test results, and identifying products using suitable tests.AQA
Odourless: Carbon monoxide has no smell, so it cannot be smelled.AQA
Opinion: A personal or group viewpoint rather than a conclusion drawn from the full evidence base.AQA
Oppositely charged ions: The particles between which ionic bonding is the electrostatic attraction.AQA
Ordering by experimental observation: Metals are placed in order by comparing the vigour of their reactions rather than their physical appearance or density.AQA
Orders of magnitude: How many powers of ten separate two values, where an order of magnitude is a factor of 10.AQA
Ores and metal compounds: Most metals are found as compounds in ores, which are rocks containing a metal compound in sufficient amount to be extracted economically.AQA
Organic matter: Material in sewage and agricultural waste water that includes food waste, faeces and plant material, and is broken down by microorganisms in rivers, using up…AQA
Origin of crude oil: The remains of an ancient biomass consisting mainly of plankton that was buried in mud.AQA
Outer electrons: The electrons in the outer shell of an element, which are the electrons involved in bonding and reactions.AQA
Outer shell electrons: Group 7 atoms have seven electrons in their outer shell, so each needs to gain one electron to achieve a stable noble gas configuration.AQA
Overall cell reaction: The overall chemical change is hydrogen plus oxygen forming water, with the balanced equation 2H₂ + O₂ → 2H₂O.AQA
Overall energy change: Energy absorbed for bonds broken minus energy released for bonds made; a negative result means the reaction is exothermic overall and a positive result means…AQA
Overcoming intermolecular forces: When a substance melts or boils, energy is transferred and used to overcome the weak intermolecular forces between the molecules.AQA
Oxidation: The gain of oxygen in a chemical reaction, for example when carbon gains oxygen and is oxidised to carbon dioxide.AQA
Oxidation and reduction: Oxidation is loss of electrons and reduction is gain of electrons.AQA
Oxidation and reduction at electrodes: Hydrogen is oxidised at the anode and oxygen is reduced at the cathode, so the overall reaction produces water as the only chemical product.AQA
Oxidation at the anode: At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻.AQA
Oxidation State: A number assigned to an element in chemical combination that represents the number of electrons lost or gained.OCR
Oxidation states of iron: Iron(II) and iron(III) give different colours, so a green precipitate shows the original solution contained iron(II) ions, not iron(III) ions.AQA
Oxide layers: Aluminium reacts quickly with oxygen in air to form a thin, tough layer of aluminium oxide, Al₂O₃, on its surface.AQA
Oxides of nitrogen: At the high temperatures inside engines and furnaces, nitrogen and oxygen from the air react to form these.AQA
Ozone: A reactive gas that destroys microbes by oxidation.AQA

P

Paints: Formulations of pigment, binder, solvent and additives that together give colour, adhesion and durability.AQA
Particle diagram: A representation showing different types of particles mixed but not bonded together.AQA
Particle Model: A scientific model used to explain the properties of solids, liquids, and gases by describing the arrangement and movement of the particles they are made of.OCR
Particle model of matter: A model linking the arrangement, spacing and movement of particles, and the forces between them, to properties such as shape, volume or compressibility.AQA
Particle size and atomic dimensions: A typical atom is roughly 0.1–0.3 nm across, so a 1–100 nm structure contains of the order of a few hundred atoms.AQA
Particle size and properties: Reducing particle size increases surface area for the same mass, which can make the material more effective per gram.AQA
Particle types: The mole can be applied to any stated particle, including atoms, molecules, ions and electrons.AQA
Particulate pollution: Tiny solid particles, mainly soot and unburnt carbon, released by incomplete combustion of fuels and able to remain suspended in the air.AQA
Particulates: Tiny solid or liquid particles suspended in the air, which solid particles and unburned hydrocarbons can join together to form in the atmosphere.AQA
Patterns in organic chemistry: The generality that compounds sharing a functional group undergo the same type of reaction, even when their carbon chains differ in length.AQA
Patterns in properties: Chemical and physical properties repeat as you move through the elements in order of atomic number.AQA
Peer review: Other experts examine methods and conclusions before publication, which improves quality but does not prove a claim.AQA
Peer-reviewed evidence: Research that has been checked by other experts before publication.AQA
Percentage by mass: The contribution of one element to the total mass of a compound, found by dividing the element's total mass by Mᵣ and multiplying by 100.AQA
Percentage composition: The percentages of all elements in a compound, which should sum to 100% as a useful check.AQA
Percentage yield: A value that depends on how much product is actually collected, unlike atom economy which depends only on the reaction pathway chosen.AQA
Period: Determined by the number of occupied electron shells in an atom.AQA
periodic table: A table in which elements are arranged in order of increasing atomic number, grouping together elements with similar properties.AQA
Periodic table group patterns: The number of electrons gained or lost equals the distance from the nearest noble gas group, and the sign shows whether electrons are lost or gained.AQA
Periodic table layout: Metals are found to the left and towards the bottom of the periodic table, while elements towards the top right are not metallic.AQA
Periodic table ordering: The arrangement of the modern periodic table by increasing atomic number rather than by atomic mass.AQA
Periodic table structure: Metals occupy the large block on the left, centre and lower part of the table, while non-metals form a much smaller region towards the top right.AQA
Periodic trends: Going down Group 0, relative atomic mass, atomic radius and boiling point increase, while reactivity stays very low.AQA
Periodic trends and group properties: Transition metals have high melting points, high densities and are strong and hard, whereas Group 1 metals have low melting points, low densities and are soft…AQA
Periodic trends in reactivity: Going down Group 1, the reactivity of the elements increases, so rubidium and caesium would react even more vigorously than potassium.AQA
Periodicity: Similar properties occur at regular intervals, which is why the table is called a periodic table.AQA
Periods: Horizontal rows in the periodic table, across which the outer-shell electron count increases by one per element.AQA
Petrochemical industry: Produces many useful materials on which modern life depends, such as solvents, lubricants, polymers and detergents.AQA
Petrochemicals: Products of the petrochemical industry made from hydrocarbon feedstocks, including plastics, synthetic fibres, detergents, medicines, lubricants and solvents.AQA
pH probe: An electronic sensor connected to a meter that gives a numerical reading and is more precise and objective.AQA
pH scale: A continuous scale running from 0 to 14 that tells you how acidic or alkaline an aqueous solution is.AQA
Phosphate rock: A substance containing insoluble calcium phosphate, meaning plants cannot absorb it.AQA
Photosynthesis: A process in which carbon dioxide is taken in from the atmosphere and used to produce glucose, while oxygen is released.AQA
Photosynthesis and oxygen accumulation: Photosynthesis by algae and plants removed carbon dioxide and added oxygen, allowing oxygen levels to rise.AQA
Physical and chemical properties of metals and non-metals: Metals and non-metals differ in physical properties such as conductivity, malleability and melting point, and in chemical behaviour such as forming positive…AQA
Physical change: A change such as melting ice in which the same substance remains, so no new substance forms.AQA, OCR
Physical properties of fractions: Boiling point increases, viscosity increases and flammability decreases as molecular size increases.AQA
Physical properties of metals: Characteristic properties such as electrical conductivity, malleability and shiny appearance apply to the majority of elements.AQA
Physical state: The state of a substance under the stated conditions, such as metals usually solid, water liquid, and gases including H₂, O₂ and CO₂.AQA
Physical versus chemical change: Physical separation processes do not involve chemical reactions, whereas a chemical reaction does make a new substance.AQA
Physical versus chemical separation methods: Physical methods such as filtration, distillation, crystallisation and chromatography separate mixtures by differences in physical properties and do not break…AQA
Phytoextraction: The use of plants to absorb metal compounds from the soil.OCR
Phytomining: Plants absorb metal ions from the soil through their roots, are harvested and burned, and the ash contains the metal compound, which is then processed.AQA
plum pudding model: A model picturing the atom as a ball of positive charge with negative electrons embedded in it, so charge was spread evenly throughout.AQA
Pollution of water, air and soil: Water running through spoil heaps can become acidic or carry dissolved metals, polluting rivers and groundwater, and fossil fuels burned by machinery and…AQA
Poly(ethene): The polymer of ethene, named by adding 'poly' before the monomer name, whose repeating unit is –CH₂–CH₂–.AQA
Polyester: The polymer formed when a dicarboxylic acid and a diol react, with water eliminated.AQA
Polymer architecture: Using two different monomers with matching pairs of groups gives a regular alternating pattern in the polymer chain.AQA
Polymer notation: A representation in which the repeating unit is placed in brackets with n outside, where n is a large number.AQA
Polymer structure: Polymer molecules are long chains, so many intermolecular forces act along each chain, and heating weakens the forces between chains so the chains can slide…AQA
Polymers: Materials such as poly(ethene) and poly(propene) made by polymerising alkenes from cracking, used for bags and bottles.AQA
Positive charge of the nucleus: Alpha particles, which are positive, were repelled, showing that the nucleus is positively charged.AQA
Positive metal ions: Ions that sit in a regular lattice and are held to the electron sea by strong electrostatic attraction.AQA
Positive result: The limewater turning milky or cloudy when carbon dioxide is present.AQA
Potability: Being about safety rather than purity, depending on safety standards rather than absence of all impurities.AQA
Potable: A term used to distinguish drinking water from other water, such as sea water or waste water, which may be unsafe.AQA
Potable water: Water that is safe to drink, containing dissolved substances but with sufficiently low levels of microbes and contaminants.AQA
Potential difference: Produced by the flow of electrons through an external circuit, it can drive a current that can do useful work.AQA
Potential energy and temperature: Energy supplied during melting or boiling increases the potential energy of the particles rather than their average kinetic energy, so the temperature stays…AQA
Practical data handling: Recording initial and final temperatures with the same thermometer and the same units, and stating the temperature change clearly, including whether it is a…AQA
Practical measurement and accuracy: Measure the distance travelled by each spot from the baseline to the centre of the spot, and measure the solvent front distance from the same baseline.AQA
Pre-determined conclusions: Results that a selective LCA can be misused to reach, for example in support of claims for advertising purposes.AQA
Precipitate: An insoluble solid that emerges from a liquid solution during a chemical reaction.OCR
Precipitate colours: The characteristic colours are white for silver chloride, cream for silver bromide and yellow for silver iodide, and these colours allow the halide ion…AQA
Precipitation of carbonates: Some dissolved carbon dioxide reacted to form carbonate compounds, which precipitated as insoluble solids.AQA
Precipitation reactions: Silver ions combine with halide ions to form insoluble silver halides, which form a solid suspension.AQA
Precipitation tests: The colour is an observation of the solid formed when sodium hydroxide solution is added, not of the solution before reaction.AQA
Precision: How close repeated measurements are to each other, regardless of whether they are accurate.OCR
Precision of measurements: A smaller range usually indicates more precise measurements, so the spread of results can be compared to comment on precision.AQA
Predicting electrolysis products: Identifying the ions present, including H⁺ and OH⁻ from water, then applying the cathode and anode rules to name the product at each electrode.AQA
Predicting properties: Predictions should be based on trends in the group, such as a halogen lower down being less reactive and having a higher melting point.AQA
Predicting trends from the reactivity series: The tendency to form positive ions explains observed reactions with water, acid and oxygen, and can be used to predict and explain trends in reactivity.AQA
Prediction and confirmation in science: Mendeleev forecast the properties of missing elements, and later discoveries matched his predictions, showing a model tested and strengthened by new evidence.AQA
Prediction from patterns: The trend can be used to predict that rubidium and caesium react more vigorously than potassium, sometimes explosively.AQA
Prediction from structure: Identifying the functional group of an unfamiliar compound and applying the known reactions of that group to predict its behaviour.AQA
Predictive power of scientific models: The approach turned the periodic table into a predictive model that not only organised known elements but also made testable forecasts.AQA
Prefixes meth-, eth-, prop- and but-: Stems used in the names to show one, two, three or four carbon atoms, counting the carbon of the –COOH group.AQA
Preparing soluble salts: A soluble salt can be prepared by reacting an acid with an insoluble solid, which dissolves the solid and forms a salt in solution.AQA
Prioritising chemical properties over atomic weight: Where the next atomic weight would have placed an element in a group that did not match its chemistry, he swapped the order.AQA
Procedural techniques for precision: Accurate procedural techniques, such as rinsing the burette with the acid and reading the bottom of the meniscus at eye level, to ensure precise measurements.AQA
Processing resources: Crude oil must be fractionally distilled before petrol can be used for transport.AQA
Product yield: The final yield of product is greater than before the change, although some of the added reactant remains.AQA
Product, service and event impacts: Emissions added at each stage, since transport, manufacture, use and waste disposal can each add emissions.AQA
Products of combustion: The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen.AQA
Progression from nuclear model to modern atom: The atomic model develops as a sequence of evidence-led changes rather than a single event.AQA
Properties linked to uses: Soda-lime glass is used for windows, bottles and drinking glasses because it is transparent, rigid and relatively cheap.AQA
Properties of gold and its alloys: Pure gold is a soft, dense metal that is too soft to wear well in jewellery, so added metals make the gold harder and more durable while keeping its…AQA
Properties of solids, liquids and gases: A solid has a fixed shape and volume, a liquid has a fixed volume but a shape that flows to fit the container, and a gas changes both shape and volume to fill…AQA
Properties required of the final product: The qualities the final product must have, such as strength, flexibility, appearance or electrical conductivity, which set how pure the recycled material must…AQA
Proportions and properties: The properties of the formulation depend on which components are present and on their proportions.AQA
Proportions of gases: The fractions of different gases in the atmosphere, which have been much the same as they are today for 200 million years.AQA
Protective oxide layer: A thin, adherent oxide layer formed on the surface by chromium that seals the metal beneath from water and oxygen.AQA
proton: The name given to the positive particles found in the nucleus of an atom, carrying a relative charge of 1⁺ and a relative mass of 1.AQA
Proton number: The number of protons in an atom, which defines which element the atom is.AQA
Provisional nature of science: Scientific explanations are provisional and develop over time, so theories are not fixed and may be revised with new evidence.AQA
Provisional nature of scientific models: Such mismatches showed that atomic weight was not the fundamental property controlling chemical behaviour, so early tables were provisional and required…AQA
Pure substance: A single element or compound, not mixed with any other substance.AQA, OCR
Pure water: In a chemistry context, water that contains only H₂O molecules and no dissolved substances.AQA
purity: The excess solid guarantees the acid is used up and filtration removes the excess solid, giving a pure salt.AQA
Purity and impurities: A pure substance contains only one substance, while an impurity is something else present that lowers the melting point and raises the boiling point.AQA
Purity testing: Melting point and boiling point data can be used to test purity, as a substance that melts sharply at the expected temperature is likely to be pure.AQA
Purpose of excess: To ensure that all of the other reactant is used and the reaction is driven to completion.AQA

Q

Qualitative: Describes a test that shows the presence or absence of sulfate ions rather than their concentration.AQA
Qualitative chemical tests: Bromine water, an orange-brown solution, is used as a test for alkenes and turns colourless when shaken with one.AQA
Qualitative identification: A positive result is a distinctive flame colour that matches a known cation, so the test identifies rather than measures the amount.AQA
Qualitative identification by comparison with standards: The observed colour is used to identify an unknown cation by comparison with the known colours, and the test is qualitative and does not measure the amount of…AQA
Qualitative prediction: A description of the direction in which an equilibrium shifts and whether the yield of a product increases or decreases, without calculating numerical…AQA
Quantitative comparison of acids: For a given concentration, the stronger acid ionises more completely and so produces a higher concentration of H⁺ ions, giving a lower pH.AQA
Quantitative comparison of properties: Comparing materials numerically rather than by impression, reading the table, identifying the property being compared and quoting the figures.AQA
Quantitative measurement: Quantities that are objective because instruments give numbers in defined units.AQA
Quantitative reasoning: Multiplying each species' Mᵣ by its coefficient and summing for reactants and products supports reacting mass calculations and shows why coefficients matter…AQA

R

Rapid: Describes a method that produces a result in seconds or minutes, quickly compared with many chemical tests.AQA
Rate: The change in amount of reactant or product per unit time, meaning how quickly reactants are used or products formed.AQA
Rate measurement: In the gas-volume method, the initial gradient of a volume-time graph gives a measure of rate, while in the turbidity method a shorter time means a faster rate.AQA
Rate of movement: A substance spending more time in the mobile phase moves more and travels further, while one held back by the stationary phase travels less far.AQA
Rate of reaction: How fast the reaction happens, rather than how much product is formed.AQA
Rates of forward and reverse reactions: Immediately after a concentration change the rates of the forward and reverse reactions become unequal, until a new equilibrium is established.AQA
Rates of reaction: The rate is the change in the number of moles divided by the time taken, giving units of moles per second (mol/s).AQA
Ratio of ions: The proportion of cations to anions in a model, which gives the empirical formula of an ionic compound.AQA
Raw material extraction and processing: The first stage of an LCA, in which the raw materials needed to make the product are extracted and processed.AQA
Raw materials: Soda-lime glass is made from sand with sodium carbonate and calcium carbonate, while borosilicate glass is made from sand with boron trioxide.AQA
Raw materials and their functions: Sand supplies silicon dioxide, sodium carbonate lowers the melting point, and limestone stabilises the glass and helps it resist water.AQA
Raw materials and their sources: Nitrogen and hydrogen are the raw materials, with nitrogen obtained from air and hydrogen commonly obtained from natural gas or by electrolysis of water.AQA
Reactants: The substances sealed inside the cell that are used up as the cell discharges.AQA
Reaction profile: A curve on which activation energy is shown as the vertical gap from the reactant energy level up to the peak.AQA
Reaction profiles: These show the energy of reactants and products and the activation energy as the height from the reactants' energy level to the peak of the curve.AQA
Reactions in solution: Dissolved particles mix freely and can collide, so reactions can happen quickly.AQA
Reactions of alkali metals: The first three alkali metals react with oxygen to form metal oxides, with chlorine to form white metal chlorides, and with water to form a metal hydroxide…AQA
Reactions of halogens with metals: The halogen gains one electron from the metal atom, which loses electrons to form a positive ion, so the compound is an ionic halide.AQA
Reactions of halogens with non-metals: Electrons are shared rather than transferred, so the compound formed is covalent, as when hydrogen and chlorine form hydrogen chloride.AQA
Reactions of hydrocarbons: During combustion, the carbon and hydrogen in the fuels are oxidised, which is why complete combustion needs plenty of oxygen.AQA
reactivity: Being highly reactive because only one electron must be lost, with vigorous reactions with water, oxygen and chlorine.AQA
Reactivity and electron gain: The ease of gaining an electron depends on the attraction between the nucleus and the incoming electron, which is affected by atomic radius and shielding.AQA
Reactivity and ion formation: The reactivity of a metal is related to its tendency to form positive ions.AQA
Reactivity of metals: Aluminium is more reactive than iron yet corrodes less because of its protective oxide layer.AQA
Reactivity of the C=C double bond: The carbon–carbon double bond makes alkenes reactive and allows addition reactions that convert them into a wide range of products.AQA
Reactivity of transition metals: These metals react slowly with oxygen and water compared with Group 1 metals, for example iron rusts slowly.AQA
Reactivity series: A greater difference in reactivity between the two electrode metals gives a larger voltage, because the more reactive metal has a stronger tendency to lose…AQA
Reactivity series and cathode discharge: The less reactive positive ion is usually discharged at the cathode; if the metal is more reactive than hydrogen, hydrogen is produced instead.AQA
Reactivity series and extraction: Metals more reactive than carbon cannot be extracted with carbon because carbon is not reactive enough to displace them, so they require electrolysis instead.AQA
Reactivity series and extraction methods: The method chosen depends on the metal's position in the reactivity series, with carbon reduction for metals less reactive than carbon and electrolysis for…AQA
Reactivity series and reduction: If the metal is more reactive than hydrogen, its ions are less easily reduced, so H⁺ ions are reduced instead and hydrogen gas is produced.AQA
Reactivity series and selective discharge: Relative reactivity of the elements involved decides which ions are discharged, since at the cathode the more easily reduced positive ion is discharged.AQA
Reactivity trends: Elements in a group show similar properties but trends in reactivity, such as Group 1 becoming more reactive down the group.AQA
Reactivity with oxygen, water and halogens: Group 1 metals react vigorously with oxygen, water and halogens, while transition metals react slowly or require heating.AQA
Reagent order and control: Dilute nitric acid is added before silver nitrate solution, because the nitric acid removes carbonate ions that would otherwise form a white precipitate and…AQA
Rearrangement of atoms: During a chemical reaction, bonds break and new bonds form, but the atoms themselves are rearranged rather than created or destroyed.AQA
Rearranging equations and unit consistency: Rearranging the same relationship gives both directions, and units must always be included: g for mass and mol for amount, with Mr dimensionless whereas molar…AQA
Recall a use: Name one clear use for each alloy specified in the course.AQA
Recall of examples: You need to recall two human activities for carbon dioxide and two for methane, naming the activity and the gas it increases.AQA
Rechargeable cells: Cells that can be recharged because the chemical reactions are reversed when an external electrical current is supplied.AQA
Rechargeable cells and batteries: These store reactants internally and must be recharged, but they are familiar and widely available.AQA
Recharging: Storing electrical energy as chemical energy, which is later converted back to electrical energy, rather than creating energy.AQA
Recognising molecules from formulas: Recognising a small molecule from its formula is a key skill, using formulas such as H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄ and CO₂.AQA
Recovery from leachate: Copper can be recovered from leachate by displacement using scrap iron or by electrolysis.AQA
Recycling: Collecting, sorting and reprocessing waste materials into new products, for example melting used aluminium cans to make new ones.AQA
Recycling and resource conservation: Recycling metals conserves ore resources, reduces the need for mining and lowers the energy demand and environmental impact of extraction.AQA
Recycling of reactants: The remaining hydrogen and nitrogen are recycled back into the reactor, giving unreacted gases further chances to react.AQA
Recycling, reusing and reducing: Ways of reducing the use of limited resources, including recycling materials such as metals, glass, paper and plastics, reusing items, and reducing consumption.AQA
Redox and electron transfer: Iron atoms lose electrons and dissolve as iron ions, while copper ions gain electrons and form copper metal.AQA
Redox at electrodes: At the cathode hydrogen ions gain electrons to form hydrogen gas, and at the anode hydroxide ions lose electrons to form oxygen gas.AQA
Redox reaction: A reaction in which one substance is oxidised and another is reduced, the two processes happening together.AQA
Reducing the use of resources: Manufacturing fewer new products, which avoids the energy and raw materials needed to make them.AQA
Reducing use: Designing and buying less, such as lighter packaging, longer-lasting goods and products that need fewer materials.AQA
Reduction: The loss of oxygen in a chemical reaction, for example when iron(III) oxide loses oxygen and is reduced to iron.AQA
Reduction with carbon: Metals less reactive than carbon can be extracted from their oxides by heating them with carbon, which removes the oxygen.AQA
Regular arrangement of ions: Each ion is surrounded by ions of opposite charge, and the structure extends in three dimensions.AQA
Regular pattern: The atoms in a metal are arranged in a regular, repeating pattern, not a random arrangement.AQA
reinforcement: The material being bound by the matrix, which may be in the form of fibres or fragments and usually provides strength and stiffness.AQA
Relationship between bonding and physical properties: Strong electrostatic forces explain the high melting points and high boiling points of ionic compounds.AQA
Relationship between structure and physical properties: The regular layered arrangement is the key reason pure metals are malleable and ductile, as layers can slide over each other when a force is applied.AQA
Relationship between structure and properties: The giant arrangement explains the high melting points because many strong attractions must be overcome.AQA
Relative atomic mass: A weighted average that reflects the abundance of each isotope, relative to 1/12 of the mass of a carbon-12 atom.AQA
Relative charge and mass: Protons have relative charge 1⁺, neutrons have no charge and electrons carry relative charge 1⁻, with protons and neutrons together giving the nucleus almost…AQA
Relative formula mass: The sum of the relative atomic masses of all atoms in a substance's formula, found by multiplying each element's relative atomic mass by its subscript before…AQA
Relative formula mass from relative atomic masses: Found by adding relative atomic masses for every atom in the formula; for a monatomic element Ar is used, and for a compound or molecular element the Ar…AQA
Relative mass of subatomic particles: Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible.AQA
Removal of carbon dioxide from the atmosphere: Carbon dioxide dissolved in the water when the oceans formed and carbonates were precipitated, reducing the amount of carbon dioxide in the atmosphere.AQA
Renewable resources: Resources such as timber and fresh water, which are not being depleted faster than they are naturally replaced.AQA
Repeat measurements: Repeating measurements can reduce uncertainty, and the mean of repeated readings is usually more reliable.AQA
Repeatability: The precision obtained when the same person repeats the experiment using the same equipment and method.OCR
Repeating unit: The part of the polymer shown with bonds extending from the carbon atoms to indicate continuation of the chain, containing the same atoms as the monomer.AQA
Repeating units: The unit placed in brackets with n outside, showing the correct atoms and bonds, with bonds extending beyond the brackets to show the chain continues.AQA
Representation of ions using symbols and superscripts: The charge on the ion is written as a superscript after the symbol, and the superscript charge must match the number of electrons removed, with a positive sign.AQA
Representing combustion symbolically: Complete combustion is written as hydrocarbon plus oxygen gives carbon dioxide plus water.AQA
Representing ions with symbols and charges: The charge on the ion equals the number of electrons lost by the atom, written as a superscript after the symbol, such as Na⁺, Mg²⁺ and Al³⁺.AQA
Representing organic structures: Carboxylic acids can be shown as displayed formulae, structural formulae or molecular formulae such as CH₃COOH.AQA
Reproducibility: The precision obtained when a different person performs the experiment, or when a different method or equipment is used.OCR
Required practical investigation: An investigation of what happens when aqueous solutions are electrolysed using inert electrodes, involving developing a hypothesis.AQA
Required practical skills: Skills in measuring temperature, controlling variables, recording data and drawing conclusions about energy transfer, developed by investigating variables…AQA
Resolution: The smallest change in the quantity being measured of a measuring instrument that gives a perceptible change in the reading.OCR
Resource management: Renewable resources can be used repeatedly if managed carefully, with use not exceeding the rate of replacement.AQA
Resource reduction as a mitigation strategy: These impacts are reduced when less raw material is needed, which is why reducing, reusing and recycling matter.AQA
Reuse: Using an item again for the same or a new purpose, such as refilling a glass bottle or passing on clothing, so no new material is extracted.AQA
reversible arrow: The double arrow used to show that the ionisation of a weak acid is reversible and incomplete, unlike the single arrow used for a strong acid.AQA
Reversible reaction: A reaction that may not go to completion because it can reach equilibrium before all reactants convert, so some remain unreacted.AQA
Reversible reactions: Reactions that continue in both directions, so the forward and reverse reactions occur at exactly the same rate at equilibrium.AQA
Rf values: The distance moved by a substance compared with the distance moved by the solvent front, which can be compared with known values to help identify substances.AQA
Rf values and identification: The distance each spot travels relative to the solvent front gives its Rf value, and matching both spot colour and Rf value with known reference spots…AQA
Risk and uncertainty in nanotechnology: A risk is a possible harm, so uncertainty about long-term effects is itself a valid reason for caution, because behaviour in the body or environment is not…AQA
Risk, vulnerability and adaptation: Risk is the likelihood and seriousness of harm, which is greater where populations are exposed and less able to adapt.AQA
Role of algae and plants: These organisms decreased the percentage of carbon dioxide in the atmosphere by photosynthesis.AQA
Role of carbon dioxide: Carbon dioxide was the main gas in the Earth's early atmosphere, and the atmospheres of Mars and Venus today are dominated by carbon dioxide.AQA
Role of evidence in accepting models: Scientific models are accepted only when evidence supports them, and acceptance can take time.AQA
Role of gaps in classification: He left deliberate gaps rather than forcing every known element into a continuous sequence, and for each gap predicted the properties of the missing element…AQA
Role of volcanic activity: Intense volcanic activity released large volumes of gases, including water vapour, carbon dioxide and nitrogen.AQA
Rusting: An example of corrosion, being the corrosion of iron and steel that requires both oxygen and water.AQA

S

Sacrificial protection: A more reactive metal is used so it corrodes in place of iron, losing electrons which flow to the iron.AQA
Safety and purity: Potable water is not pure water in the chemical sense, because it still contains dissolved substances.AQA
Safety in gas tests: The splint is held at the open end so the gas ignites safely at the mouth.AQA
Salt formation: The salt produced when an acid reacts with a base, alkali or carbonate depends on the acid used and the positive ions in the base, alkali or carbonate.AQA
Salt formation from acids: Treating phosphate rock with nitric acid, sulfuric acid or phosphoric acid gives different soluble salts that supply phosphate and sometimes nitrate or…AQA
Same conditions of temperature and pressure: Equal moles of gases occupy the same volume under the same conditions, so gas volumes are compared only at the same temperature and pressure.AQA
Saturated: Describes alkanes, which have only single carbon–carbon bonds.AQA
Saturated and unsaturated hydrocarbons: The bromine water test distinguishes unsaturated hydrocarbons, which decolourise it, from saturated hydrocarbons, which do not.AQA
Saturated Hydrocarbon: A compound containing only carbon and hydrogen atoms, with only single bonds between the carbon atoms (e.g., alkanes).OCR
Scale and apparatus in chemistry: Industry uses large-scale continuous plants with concentrated acids and controlled temperatures, whereas laboratory preparation uses small quantities and…AQA
Scale and measurement: A nanometre is 1 × 10⁻⁹ m, so nanoscale objects are far smaller than everyday particles but larger than single atoms.AQA
Scale and size of atoms: Atoms are extremely small, with a radius of about 0.1 nm and diameters around 0.2 nm.AQA
Scale and timescale of environmental change: The effects are global and long-lasting, spanning decades and continents rather than one local area.AQA
Scarce: Describes copper ores as rich, high-grade ores are being used up, while low-grade ores remain.AQA
scattering experiment: An experiment in which positively charged alpha particles were fired at thin gold foil, with most passing straight through but some deflected through large…AQA
Science communication: Communicating results to the public, policymakers and other scientists allows scrutiny and informed decisions.AQA
Scientific reasoning: Reasoning with the provided information to draw justified conclusions about which theory is best supported.AQA
Scientific research and development: New applications for nanoparticulate materials are an important area of research, developed alongside testing, regulation and life-cycle assessment.AQA
Scrap steel: Steel that has already been used and collected, for example from old cars, buildings or machinery.AQA
Screening: A treatment stage that removes large objects from waste water.AQA
Screening and grit removal: The first stage, which takes out large objects, rags, plastics and dense grit that would damage pumps or block pipes.AQA
Sea-level rise: Melting ice sheets and glaciers, plus thermal expansion of seawater, raise sea level and increase flooding of low-lying land.AQA
Secondary cells: Another name for rechargeable cells, which can discharge and then be recharged.AQA
Sedimentation: A stage that lets finer suspended solids settle, producing sewage sludge at the bottom and effluent above.AQA
Selective discharge at the cathode: At the negative electrode, the ion that gains electrons more readily is discharged, so either hydrogen or the metal is produced.AQA
Selective LCA: An assessment devised to evaluate a product that covers only chosen stages, materials or impacts.AQA
Self-heating cans: Cans that contain reactants which react exothermically when activated, transferring energy to the food or drink.AQA
Sensitive: Describes a method that can detect very small amounts of a substance, sometimes parts per million or less.AQA
Sensitivity and accuracy: Being more sensitive means detecting very low concentrations that chemical tests would miss, and being more accurate and precise gives reliable measurements…AQA
Separation: The process of sorting a material from a mixture of different materials before it can be reused.AQA
Separation by cooling and condensation: On cooling, the ammonia liquefies and is removed.AQA
Separation by distillation: The water is heated to boiling, the vapour is cooled and condensed, leaving dissolved salts behind in the original container.AQA
Separation of mixtures: Because different substances have different distributions, they move at different rates and separate into spots or bands.AQA
Separation of mixtures by physical processes: The process separates the mixture because evaporation and condensation occur repeatedly at different temperatures.AQA
Separation techniques: Paper chromatography separates coloured substances by allowing a solvent to move up paper, carrying dissolved dyes at different rates.AQA
Series: The positive terminal of one cell connects to the negative terminal of the next, so the voltages add.AQA
Shared pair: Each atom contributes one electron to it, and it is attracted to both nuclei, holding the atoms together.AQA
Shielding and nuclear attraction: More inner shells shield the outer electron, and because it is further from the nucleus the attraction is weaker, so it is lost more easily.AQA
Shielding Effect: The decrease in electrostatic attraction between the nucleus and outer electrons due to the presence of inner electron shells.OCR
Significance of evidence: Judging whether a difference is significant by comparing orders of magnitude to see if it is large enough to matter.AQA
Silicon dioxide: The main compound in sand, with a giant covalent structure in which each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two…AQA
Silver halides: Silver chloride is white, silver bromide is cream and silver iodide is yellow, and these are the silver halides formed when silver nitrate solution is added…AQA
Silver nitrate with dilute nitric acid: A test used for halide ions, acidified first so carbonate ions do not produce a misleading precipitate.AQA
Simple distillation: Heating a solution to evaporate the solvent, then condensing the vapour to collect the pure solvent.AQA
Simple molecular substance: A substance where intermolecular forces between molecules are overcome and the covalent bonds inside each molecule stay intact, so melting points are low.AQA
Simple molecular substances: Substances that contain a limited number of atoms joined by covalent bonds, such as hydrogen, chlorine, oxygen, nitrogen, carbon dioxide and methane.AQA
Simplest ratio shown by a formula: The formula shows the simplest ratio of ions, not a molecule.AQA
Simplest whole-number ratio: The result of comparing mole values by dividing each by the smallest value, used as the coefficients in the balanced equation.AQA
Simplified model: A model that omits some greenhouse gases, feedbacks or natural factors, and can be useful if its limitations are stated and it fits the purpose.AQA
Single superphosphate: A mixture of calcium phosphate and calcium sulfate produced by treating phosphate rock with sulfuric acid.AQA
Single, double and triple covalent bonds: A single bond is one shared pair, a double bond is two shared pairs, and a triple bond is three shared pairs.AQA
Size comparisons: Comparisons should use the same unit, such as nanometres, and may use a simple ratio or scale factor.AQA
Size-dependent properties: The ratio explains why smaller particles have a greater proportion of their atoms at the surface, so properties change with particle size.AQA
Small molecules: Contain only a few non-metal atoms, for example H₂O, CO₂, NH₃, CH₄, HCl, O₂, N₂, H₂ and Cl₂.AQA
Small molecules and simple molecular substances: Covalently bonded substances may consist of small molecules, meaning a modest number of atoms held together by strong covalent bonds.AQA
Small proportions of other gases: The remaining part of the atmosphere contains various other gases, including carbon dioxide, water vapour and noble gases.AQA
Small-molecule alkanes: Alkanes such as methane, ethane, propane and butane that ignite easily, burn with a clean flame and are in high demand as fuels.AQA
Soda-lime glass: The common glass used for windows, bottles and drinking glasses, made by heating a mixture of sand, sodium carbonate and limestone.AQA
Sodium hydroxide solution: A test used for metal cations that form coloured or white precipitates.AQA
Solubility and nutrient availability: A fertiliser must release its nutrient ions into soil solution so plant roots can absorb them, and insoluble compounds cannot do this quickly enough.AQA
Solubility of bases and alkalis: An alkali is a base that is soluble in water and produces OH⁻(aq), and not all bases dissolve.AQA
Soluble salt solution: The filtrate that passes through the filter paper, giving a clean salt solution without leftover acid.AQA
Soluble salts: Products formed when phosphate rock is treated with acid, allowing phosphate ions to dissolve in soil water and be absorbed by plant roots.AQA
Solution: A mixture formed when a solute dissolves in a solvent.AQA
Solvents: Substances used to dissolve other substances, for example in paints and cleaning products.AQA
Spectator ions: Ions that appear unchanged on both sides of a full equation and are cancelled so that only the reacting species are written.AQA
Spectroscope: An instrument that separates the light given out into its component wavelengths.AQA
Speculation: A claim that goes beyond the available evidence, such as predicting a specific local effect from global data.AQA
Speed of analysis: These methods are faster than the chemical tests in this specification because no lengthy test-tube reactions are needed.AQA
Spherical fullerenes: Closed cages built from hexagonal rings together with five-membered rings.AQA
square brackets: Drawn around each ion in the final diagram, with the charge written outside the bracket.AQA
Stability and full outer shells: When atoms lose or gain electrons so that their outer shell becomes full, the resulting ion is stable.AQA
Stability of atoms: An atom is most stable when its electrons occupy the lowest available energy states.AQA
Stability over geological time: The atmosphere has had much the same proportions of gases for about 200 million years, meaning they have not changed significantly over that period.AQA
Stainless steel: An alloy of iron with chromium and nickel, and also containing carbon, that is hard and resistant to corrosion.AQA
Standard form and unit conversion: A way of writing very small numbers and converting between units, used to express atomic dimensions such as 1 × 10⁻¹⁰ m.AQA
State symbols: Symbols that show the physical state of each substance in an equation: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).AQA
States of matter: The three states are solid, liquid and gas, and the state depends on the arrangement, spacing and movement of the particles and the strength of the forces…AQA
States of matter and boiling point: Boiling point increases down the group because more energy is required to overcome stronger intermolecular forces.AQA
States of matter and changes of state: A substance is solid below its melting point, liquid between its melting and boiling points, and gas above its boiling point.AQA
States of matter and the particle model: Atoms are rearranged, not destroyed, but gas particles can move into or out of the container.AQA
States of matter at room temperature: Small-molecule substances usually exist as gases or liquids at room temperature, although some, such as water and ethanol, are liquids.AQA
Stationary and mobile phases: In paper chromatography, the solvent is the mobile phase and the water held in the paper is the stationary phase.AQA
Stationary phase: The phase that does not move, such as the chromatography paper or a thin layer of solid such as silica on a plate.AQA
Steam cracking: The hydrocarbon feedstock is mixed with steam at a very high temperature and no catalyst is used.AQA
Steel: An alloy based on iron that always contains iron as the main metal, together with a controlled, specific amount of carbon and often other metals added for…AQA
Stem: The part of a name, meth-, eth-, prop- or but-, that gives the number of carbon atoms.AQA
Sterilising: The final treatment step described for producing potable water, which kills microbes.AQA
Sterilising agents: Chlorine, ozone or ultraviolet light are used for potable water to kill microbes that could cause disease.AQA
Stoichiometric calculations: The masses of reactants and products can be calculated from balanced symbol equations, converting the known mass to moles, using the balancing numbers, then…AQA
Stoichiometry from balanced equations: Using a balanced equation to set a gas volume ratio and scaling it to a given volume of a gaseous reactant or product.AQA
Stoichiometry of gaseous reactions: The volumes of gaseous reactants and products can be calculated from the balanced equation by scaling the volume ratio to the given volume.AQA
Strength and hardness: The ability to resist forces without deforming, produced in high carbon steel because carbon atoms disrupt the layers of iron atoms so they cannot slide easily.AQA
Strength of electrostatic forces: The strength of the forces increases with the charge on the ions, so compounds such as magnesium oxide have even higher melting points than sodium chloride.AQA
Strength of ionic bonding: The attractions are strong, so a large amount of energy is needed to overcome them.AQA
Strength of metallic bonding: This varies between metals, so melting points vary, and the more electrons donated per atom, the higher the melting point tends to be.AQA
Strong Acid: An acid that is completely ionised in aqueous solution (e.g., hydrochloric, nitric, sulfuric acid).OCR
Strong covalent bonds within molecules: The bonds that hold each molecule together internally, which remain strong.AQA
Structure and properties: The strong covalent bonds within a polymer molecule make it difficult to break apart, and they are distinct from the weaker intermolecular forces between…AQA
Structure and properties of monatomic gases: Because the stable arrangement means they do not need to bond to gain a full outer shell, they exist as single atoms and are chemically unreactive.AQA
Structure and property relationships: Longer molecules become tangled and slide over one another less easily, so the liquid flows more slowly.AQA
Structure and reactivity: Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes, whose molecules have only single bonds.AQA
Structure of noble gases: They exist as single atoms, not as diatomic molecules.AQA
Structure of the atom: A tiny, dense, positively charged nucleus sits at the centre and most of the atom is empty space, with electrons outside the nucleus.AQA
Structure of the nucleus: The modern nuclear model establishes that the nucleus contains both protons and neutrons, accounting for the full atomic mass.AQA
Structure recognition: Looking for a long chain rather than a small discrete molecule, and finding the shortest section that repeats along the chain.AQA
Structure-property relationships: Chains that are long, straight and closely packed give a denser, stronger, more rigid polymer with a higher softening temperature, while shorter or more…AQA
Structure-property-application links: Linking a named property, such as high tensile strength or electrical conductivity, to a specific use rather than listing uses without justification.AQA
Structure-property-use relationships: Iron is strong and dense and used in construction, while copper is a good conductor used in electrical wiring.AQA
Structure–property relationships: The way the arrangement of atoms in a structure leads to different properties, such as hardness and electrical conductivity.AQA
Subatomic particles: Protons, each with relative charge 1⁺, and neutrons, which have no charge, are found in the nucleus, while electrons carry relative charge 1⁻ and occupy shells.AQA
Subscript: A small, low number written after a symbol that multiplies only the atom or bracketed group immediately before it within that formula unit.AQA
Substance: A material that can be an element, compound or mixture, and all substances are made of atoms.AQA
Successful collisions: Collisions in which the particles have energy equal to or greater than the activation energy, so a reaction takes place.AQA
Sufficiently low: The phrase showing that water need not be chemically pure, but must meet safety standards for consumption.AQA
Sulfur dioxide: Produced when sulfur impurities in a fuel burn and react with oxygen.AQA
Sulfur impurities: Sulfur present in many fuels as an impurity rather than as the main fuel component, as in coal and crude-oil fractions.AQA
Surface area: Increasing this for a solid reactant, for example by using a powder, increases the rate of reaction.AQA
Surface area and reaction rate: Exposing more particles of a solid to the other reactant allows more frequent collisions, so the rate increases.AQA
Surface area to volume ratio: Nanoparticles have a very large one of these because a high proportion of their atoms lie at the surface.AQA
Surface protection: The strongly bonded, unreactive layer seals the metal underneath from oxygen and water, and if scratched it reforms immediately in air.AQA
Surface temperature rise: An increase in the temperature of the Earth's atmosphere at the surface, expected because human activities increase greenhouse gas concentrations.AQA
Sustainability: An issue raised by dependence on crude oil, which is a finite resource, so alternatives and recycling are important.AQA
Sustainable development: Development that meets the needs of current generations without compromising the ability of future generations to meet their own needs.AQA
Sustainable use of limited resources: Reducing the use of limited resources by recycling materials, reusing items, and designing products that last longer or use less material.AQA
Symbol equation: A way of representing a reaction that uses chemical formulae instead of names and gives the same reaction information more precisely.AQA
Systematic balancing strategy: Balance carbon first using the coefficient of CO₂, then hydrogen using H₂O, and finally oxygen by adjusting the number of O₂ molecules.AQA

T

Temperature change: The difference found by subtracting the initial temperature from the final temperature, showing how large the change is.AQA
Temperature change measurement: Measuring the initial temperature of the solution before adding the second reactant, then measuring the maximum or final temperature after the reaction and…AQA
Temperature change of surroundings: The basis for distinguishing the two types of reaction, where a rise indicates exothermic and a fall indicates endothermic.AQA
Temperature changes: If the temperature of a system at equilibrium is increased, the relative amount of products at equilibrium increases for an endothermic reaction and decreases…AQA
Temperature decrease as evidence: A temperature drop measured with a thermometer or temperature sensor is evidence that a reaction is endothermic.AQA
Temperature measurement: Recording the initial temperature of the surroundings or solution before the reaction and the highest or lowest temperature reached after the reaction.AQA
Testing predictions against new evidence: He predicted the existence and properties of an element he called eka-silicon, which was later discovered as germanium.AQA
The mole and Avogadro number: The unit used to measure chemical amount, containing the Avogadro number of particles, 6.02 × 10²³, which may be atoms, molecules, ions or electrons.AQA
Theoretical maximum: The maximum mass of product that could possibly be created from the given reactants.AQA
Theoretical yield: The maximum calculated amount of product, assuming all reactants convert fully into the desired product.AQA
Theory and evidence: Agreement between theory and evidence supports a scientific model, because models are judged by how well they explain and predict evidence.AQA
Theory evaluation: Comparing how well each theory explains the evidence provided, considering strengths and weaknesses such as how well it explains the data.AQA
Thermal decomposition: Breaking a compound down by heating, for example heating calcium carbonate to form calcium oxide and carbon dioxide.AQA
Thermosetting polymers: Polymers whose chains are joined by strong covalent cross-links, so they do not melt when heated and may char or decompose at very high temperatures.AQA
Three-dimensional network: A rigid structure formed when polymer chains are joined to one another by strong covalent cross-links.AQA
Timber and fuels: Categories supplied by natural resources and supplemented by agriculture, with timber from managed forests and fuels including wood, biodiesel and fossil fuels.AQA
Time scale of scientific discovery: The neutron was identified roughly two decades after the nucleus became an accepted scientific idea.AQA
Titration: A practical method used to measure the volume of one solution that reacts exactly with a known volume of another solution.AQA
Titration calculations: Use concentrations in mol/dm³ and balanced equations to link reacting solutions, calculating the concentration of the other solution when the volumes of two…AQA
Toxic: Carbon monoxide is a toxic gas, meaning it is poisonous to humans and can cause serious harm or death.AQA
Toxic gas: A gas such as chlorine that must never be smelled directly or prepared in open laboratory conditions without suitable ventilation.AQA
Trace gases methane and ammonia: There may have been small proportions of methane and ammonia in the atmosphere.AQA
Trade-off between purity and cost: A high-purity product usually needs more separation stages, which uses more energy and costs more.AQA
Trade-offs between environmental benefit and cost: Weighing benefits against drawbacks, for example energy saved by recycling aluminium against energy and money used in collecting and sorting the cans.AQA
Transition elements: Metals located in the central block of the periodic table between Groups 2 and 3, showing similar properties to each other.AQA
Transition metal chemistry: Transition metals show general properties such as forming ions with different charges, forming coloured compounds and acting as catalysts, exemplified by…AQA
Transition metals: Chromium, manganese, iron, cobalt, nickel and copper are examples, having high melting points and high densities.AQA
Trend description: Saying whether values increase, decrease or remain constant and quoting data points to support it.AQA
Trends in Group 0: Going down the group, relative atomic mass increases and boiling point increases from helium to radon.AQA
Trends in metallic character: Moving down a group, metallic character becomes more pronounced, so elements lower in a group behave more typically as metals.AQA
Trends in physical properties: As the chain lengthens, the molecules have greater intermolecular forces, so boiling points rise and solubility in water decreases.AQA
Trends in properties: Boiling point and viscosity increase with chain length.AQA
Trends in reactivity: Reactivity increases from lithium to sodium to potassium, so potassium reacts most vigorously, often igniting the hydrogen produced.AQA
Trends in reactivity down a group: Reactivity increases down Group 1 because the outer electron is further from the nucleus and shielded by more inner shells, so the attraction holding it is…AQA
Trends in reactivity down Group 7: Reactivity decreases down Group 7 because atomic radius increases and shielding increases, so the nucleus attracts an incoming electron less strongly.AQA
Trends with molecular size: As the molecules in a hydrocarbon fraction get larger, boiling point, viscosity and flammability change in a regular way.AQA
Triple superphosphate: Calcium phosphate produced by reacting phosphoric acid with further phosphate rock.AQA

U

Ultraviolet light: Used by passing water through a chamber where UV radiation damages the genetic material of microbes so they cannot reproduce.AQA
Unadulterated substances: A substance that has had nothing added to it, so it is in its natural state, as with pure milk.AQA
Unburned hydrocarbons: Fuel molecules that escape combustion unchanged.AQA
Uncertainty: A range or limitation arising from measurement limits, natural variation or model simplification, so scientists give ranges rather than single values.AQA
Unit conversion: Volume in cm³ is converted to dm³ by dividing by 1000 before substituting, and consistent units are used throughout.AQA
Units and dimensional reasoning: The unit mol/s represents the amount of substance in moles reacting or forming each second.AQA
Units of concentration: Common units include g dm⁻³ or mol dm⁻³.AQA
universal indicator: A mixture of dyes that gives a colour for each pH, which you compare with a colour chart; it is quick but only gives an approximate value.AQA
Unsaturated: Describes alkenes, which contain a carbon–carbon double bond.AQA
Unsaturation: The presence of a carbon-carbon double bond, shown by bromine water being decolourised from orange-brown to colourless.AQA
Use and limitations of scientific models: The simple model represents particles as small solid spheres, but it ignores forces, particle size and the internal structure of atoms.AQA
Useful products: Formulations are designed as useful products, and examples include paints, medicines, alloys, fertilisers and foods.AQA
Uses of polymers: Polymers made from alkenes are used for bags, bottles, pipes and fibres.AQA
Uses of water: Different uses need different quality, so washing, irrigation and industrial cooling may use lower-quality water.AQA
Using data to predict properties: Identify the trend from the data given, then extend it to the next element, justifying the prediction using outer-shell electron loss and increased atomic size.AQA
Using multiple observations: Combining evidence from several tests, because one observation alone may fit more than one species.AQA
Using named examples to support comparisons: Naming the property, stating the trend for each group and supporting it with a named example makes the comparison precise.AQA
Using the series to make predictions: The series can be used to predict whether a displacement reaction occurs.AQA

V

Value judgements: Decisions about how harmful each pollutant is, which are choices rather than readings from an instrument.AQA
Variable composition: The components of a mixture can be present in any proportion, unlike a compound which has a fixed composition by mass.AQA
Variable exhaust composition: The exact mixture depends on the fuel, the oxygen supply and the temperature, which is why the word may is important.AQA
Variable oxidation states: Many transition elements form ions with different charges, for example iron forms Fe²⁺ and Fe³⁺ and copper forms Cu⁺ and Cu²⁺.AQA
Variables affecting cell performance: The voltage of a cell depends on the type of electrode and electrolyte, and temperature can also affect the voltage in practical cells.AQA
Variables and control: Changing only the independent variable and controlling other variables such as temperature, volumes and reactant amounts.AQA
Variables and fair testing: Varying one independent variable at a time, such as concentration, volume, particle size or identity of metal, while controlling other variables to ensure a…AQA
Viscosity: Increases with molecular size, meaning longer chain hydrocarbons flow less easily.AQA
Volcanic production of nitrogen: Volcanoes produced nitrogen, which then gradually built up in the atmosphere.AQA
Voltage: The energy transferred per unit charge, which drives a larger current through a circuit when greater.AQA
Voltage and energy transfer: Voltage is described as the energy transferred per unit charge, and is not the same as current, which is the rate of flow of charge.AQA
Volume ratios of gases: The coefficients in the balanced equation give the volume ratio for gases measured under the same conditions of temperature and pressure.AQA

W

Waste: Products that are discarded after one use, some of which is sent to landfill.AQA
Waste minimisation and resource efficiency: A process that converts most of its starting materials into the desired product uses resources more efficiently and produces less waste.AQA
Waste reduction: Less rock is dug, transported and discarded, so waste is reduced.AQA
Waste water: Water including sewage and industrial effluent, which contains organic matter, harmful chemicals and microorganisms and so needs multiple treatment stages.AQA
Water as a product: The only chemical product of the overall reaction is water, which is why hydrogen fuel cells are described as producing no carbon dioxide at the point of use.AQA
Water as a source of ions: Water molecules break down producing hydrogen ions and hydroxide ions, and water is a weak electrolyte because only a small fraction of its molecules…AQA
Water quality and health: Water of appropriate quality is essential for life, as it supports drinking, sanitation, agriculture and industry.AQA
Water scarcity and supply: Some regions have limited fresh water because of low rainfall, high demand or both, so an alternative source may be required.AQA
Water treatment methods: The methods used to produce potable water depend on available supplies of water and local conditions.AQA
weak acid: An acid that is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions.AQA
Weak acids, ionisation and pH: Carboxylic acids dissolve in water and ionise only slightly, releasing few H⁺ ions, so their pH is higher than that of a strong acid of the same concentration.AQA
Weak and strong: Describe the degree of ionisation: a strong acid ionises completely, a weak acid only partially.AQA
Weak forces between molecules: The attraction between separate molecules in a simple molecular substance, which is weak.AQA
Weak intermolecular forces: The weak forces between separate molecules that must be overcome when the substance changes state, and which are the key factor in low melting and boiling…AQA
Weighting pollutant effects: Assigning a number to the harm caused by each pollutant so that effects can be compared.AQA
White precipitate: The fine solid of barium sulfate that appears when sulfate ions are present, showing they are present.AQA
Whole-number coefficients and formula integrity: Use the lowest whole-number coefficients, doubling the equation if an odd oxygen count appears, while keeping every formula fixed and adjusting coefficients…AQA
Whole-number ratios: Multiplying through if necessary to obtain the smallest whole-number ratio, rather than leaving decimals or fractions.AQA
wide range indicator: An indicator that can be used to measure the approximate pH of a solution, added as a few drops or as a piece of indicator paper dipped into it.AQA
Word equation: A way of representing a reaction that names all reactants and products, for example hydrochloric acid + sodium hydroxide → sodium chloride + water.AQA
Word equations: Nitrogen + hydrogen ⇌ ammonia summarises the reaction and shows that the process is reversible.AQA

Y

Yield: The amount of a product obtained in a reaction, usually measured in grams.AQA

Other terms

-anoic acid: The ending in a name that signals the –COOH group and shows a carboxylic acid.AQA
-ol ending: Used in a name as evidence that the compound is an alcohol.AQA
-ol suffix: The ending used to name alcohols, with a number added where needed to show the position of the –OH group.AQA
–COOH: A carbon atom joined to an oxygen by a double bond and to an –OH group, sitting at the end of a chain.AQA
–COOH unit: The group looked for in a formula, which may be written as COOH, CO₂H or drawn with the carbon double-bonded to oxygen and single-bonded to –OH.AQA
–OH group: An oxygen atom covalently bonded to a hydrogen atom, with that oxygen also bonded to a carbon atom of the hydrocarbon chain.AQA
1⁺ ion: Formed when a Group 1 atom loses its single outer electron, giving a stable full outer shell.AQA

Where these terms come from

Each definition is one we publish on a Chemistry GCSE topic page or study guide. Open a term to see how each exam board words it and which topic it belongs to. For the full course, go to Chemistry revision or the glossary for every subject.

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