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    Chemistry GCSE

    Chemistry GCSE glossary

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

    A

    Accuracy
    How close a measurement is to the true or accepted value.OCR
    Acid
    A substance that produces hydrogen ions ($H^+$) when dissolved in aqueous solution.Pearson Edexcel
    Activation Energy
    The minimum amount of energy that particles must have to react when they collide.OCR, Pearson Edexcel
    Addition reaction
    A reaction in which two or more molecules combine to form a single larger molecule.Pearson Edexcel
    Alkali
    A soluble base that produces hydroxide ions ($OH^-$) when dissolved in aqueous solution.Pearson Edexcel
    Allotrope
    Different structural forms of the same element in the same physical state.OCR
    Atom Economy
    A measure of the amount of starting materials that end up as useful products, expressed as a percentage.OCR
    Atomic Number
    The number of protons in the nucleus of an atom.OCR

    B

    Balanced Equation
    A chemical equation in which the number of atoms of each element is the same on both the reactant and product sides.Pearson Edexcel
    Base
    A substance that reacts with an acid to neutralise it and produce a salt and water.Pearson Edexcel

    C

    Carbon Footprint
    The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.Pearson Edexcel
    Catalyst
    A substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without being used up.OCR, Pearson Edexcel
    Chemical Cell
    A system that contains chemicals which react to produce electricity until the reactants are used up.Pearson Edexcel
    Chemical Formula
    A representation of a substance using symbols for its constituent elements, showing the ratio of atoms of each element present.Pearson Edexcel
    Closed System
    A system in which no substances can enter or leave during a reaction.OCR, Pearson Edexcel
    Collision Theory
    The theory that chemical reactions only occur when particles collide with sufficient energy.Pearson Edexcel
    Concordant Results
    In a titration, results (titres) that are within 0.10 cm³ of each other.OCR
    Covalent bond
    A shared pair of electrons between two non-metal atoms.Pearson Edexcel

    D

    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
    Diatomic
    Molecules composed of two atoms covalently bonded together.Pearson Edexcel
    Displacement Reaction
    A reaction in which a more reactive element takes the place of a less reactive element in a compound.OCR, Pearson Edexcel
    Double bond
    A covalent bond formed by the sharing of two pairs of electrons between two atoms.Pearson Edexcel
    Dynamic Equilibrium
    The point in a reversible reaction in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations…OCR, Pearson Edexcel

    E

    Early Atmosphere
    The atmosphere present on Earth roughly 4.6 billion years ago, consisting mainly of carbon dioxide, water vapour, and nitrogen, with little or no oxygen.Pearson Edexcel
    Electrode
    A solid conductor through which electricity enters or leaves a cell.Pearson Edexcel
    Electrolysis
    The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity.OCR
    Electrolyte
    A liquid or gel that contains ions and can be decomposed by electrolysis; in a cell, it allows ions to move between electrodes.OCR, Pearson Edexcel
    Endothermic Reaction
    A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.Pearson Edexcel
    Exothermic Reaction
    A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.Pearson Edexcel

    F

    Formulation
    A mixture that has been designed as a useful product, formed by mixing together several different substances in carefully measured quantities.OCR
    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.Pearson Edexcel
    Functional group
    The part of a molecule that determines its chemical reactions (e.g., the C=C double bond in alkenes).Pearson Edexcel

    G

    Global Warming
    The long-term heating of Earth's climate system observed due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas…Pearson Edexcel
    Greenhouse Effect
    The retention of heat in the Earth's atmosphere caused by greenhouse gases absorbing outgoing long-wave infrared radiation.Pearson Edexcel

    H

    Halide
    A compound containing a Group 7 element and one other element, usually a metal or hydrogen.Pearson Edexcel
    Halogen
    A non-metal element in Group 7 of the periodic table.Pearson Edexcel
    Homologous series
    A family of organic compounds that have the same general formula, similar chemical properties, and show a gradual trend in physical properties.OCR, Pearson Edexcel
    Hydrocarbon
    A compound containing only carbon and hydrogen atoms.Pearson Edexcel

    I

    Instrumental Analysis
    The use of scientific instruments to analyse chemical substances, offering greater speed, accuracy, and sensitivity.OCR
    Intermolecular forces
    The weak forces of attraction between separate molecules.Pearson Edexcel
    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
    Ionic Equation
    A chemical equation that shows only the ions that participate in the reaction, omitting spectator ions.Pearson Edexcel
    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

    L

    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
    Le Chatelier's Principle
    If a change is made to the conditions of a system at equilibrium, the system will shift to oppose that change.Pearson Edexcel
    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
    Lone pair
    A pair of outer shell electrons that is not involved in chemical bonding.Pearson Edexcel

    M

    Mass Number
    The total number of protons and neutrons in the nucleus of an atom.OCR
    Mixture
    Two or more elements or compounds that are mixed together but not chemically combined.Pearson Edexcel
    Mobile Phase
    The phase that moves during chromatography (e.g., the solvent).Pearson Edexcel
    Mole
    The unit for amount of substance.OCR
    Molecule
    A group of two or more atoms held together by covalent bonds.Pearson Edexcel

    N

    Neutralisation
    The reaction between an acid and a base to form a salt and water.Pearson Edexcel

    O

    Order of magnitude
    A class in a system of classification determined by size, typically in powers of 10.Pearson Edexcel
    Oxidation State
    A number assigned to an element in chemical combination that represents the number of electrons lost or gained.OCR

    P

    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
    Photosynthesis
    The biological process by which plants and algae use sunlight to convert carbon dioxide and water into glucose and oxygen.Pearson Edexcel
    Physical Change
    A reversible change where no new substances are formed, such as a change of state or dissolving.OCR
    Phytoextraction
    The use of plants to absorb metal compounds from the soil.OCR
    Precipitate
    An insoluble solid that emerges from a liquid solution during a chemical reaction.OCR
    Precision
    How close repeated measurements are to each other, regardless of whether they are accurate.OCR
    Pure Substance
    A substance consisting of only one type of element or one type of compound, which melts and boils at specific, sharp temperatures.OCR, Pearson Edexcel

    R

    Rate of Reaction
    The change in concentration (or amount) of a reactant or product per unit time.Pearson Edexcel
    Redox Reaction
    A reaction in which both reduction and oxidation take place simultaneously.Pearson Edexcel
    Repeatability
    The precision obtained when the same person repeats the experiment using the same equipment and method.OCR
    Reproducibility
    The precision obtained when a different person performs the experiment, or when a different method or equipment is used.OCR
    Resolution
    The smallest change in the quantity being measured of a measuring instrument that gives a perceptible change in the reading.OCR
    Reversible Reaction
    A chemical reaction where the products can react together to reform the original reactants.Pearson Edexcel

    S

    Saturated
    A molecule containing only single covalent bonds between carbon atoms.Pearson Edexcel
    Saturated Hydrocarbon
    A compound containing only carbon and hydrogen atoms, with only single bonds between the carbon atoms (e.g., alkanes).OCR
    Sedimentary Rock
    Rock formed by the accumulation and compression of mineral and organic particles, such as limestone formed from carbonate precipitates in the oceans.Pearson Edexcel
    Shielding Effect
    The repulsion between electrons in inner shells and outer shells, which reduces the net attractive force from the positive nucleus on the outer electrons.OCR, Pearson Edexcel
    Solute
    The substance that dissolves in a solvent to form a solution.Pearson Edexcel
    Solvent
    The liquid in which a solute dissolves to form a solution.Pearson Edexcel
    Spectator Ion
    An ion that exists as a reactant and a product in a chemical equation but does not undergo any chemical change.Pearson Edexcel
    State Symbol
    Letters used in equations to show the physical state of a substance: (s), (l), (g), or (aq).Pearson Edexcel
    Stationary Phase
    The phase that does not move during chromatography (e.g., the chromatography paper).Pearson Edexcel
    Strong Acid
    An acid that is completely ionised in aqueous solution (e.g., hydrochloric, nitric, sulfuric acid).OCR, Pearson Edexcel

    U

    Unsaturated
    A molecule containing at least one carbon-carbon double bond.Pearson Edexcel

    W

    Weak Acid
    An acid that only partially dissociates into its ions in aqueous solution.Pearson Edexcel

    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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