IGCSE Chemistry Definitions Sheet: Every Essential Term You Need

Chemistry at IGCSE level demands precise use of scientific vocabulary. Examiners routinely award — and withhold — marks based on whether you use the correct terminology in your answers.

This IGCSE Chemistry definitions sheet covers every key term from the Cambridge IGCSE Chemistry (0620/0971) syllabus, organised by topic for efficient revision.

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1. States of Matter

Term	Definition
Solid	A state of matter with a fixed shape and volume; particles vibrate in fixed positions.
Liquid	A state of matter with a fixed volume but no fixed shape; particles are close together but can move past each other.
Gas	A state of matter with no fixed shape or volume; particles move rapidly and randomly with large spaces between them.
Melting	The change of state from solid to liquid at a specific temperature.
Boiling	The change of state from liquid to gas throughout the liquid at a specific temperature.
Evaporation	The change of state from liquid to gas at the surface of a liquid, occurring at any temperature.
Condensation	The change of state from gas to liquid.
Sublimation	The change of state from solid directly to gas without passing through the liquid state.
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration.
Brownian motion	The random, erratic movement of microscopic particles suspended in a fluid, caused by collisions with fast-moving molecules.

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2. Atoms, Elements and Compounds

Term	Definition
Atom	The smallest particle of an element that can take part in a chemical reaction.
Element	A substance that contains only one type of atom and cannot be broken down into simpler substances by chemical means.
Compound	A substance formed from two or more elements chemically combined in a fixed ratio.
Mixture	Two or more substances that are not chemically combined and can be separated by physical methods.
Molecule	A group of two or more atoms covalently bonded together.
Proton	A subatomic particle found in the nucleus with a relative mass of 1 and a relative charge of +1.
Neutron	A subatomic particle found in the nucleus with a relative mass of 1 and no charge.
Electron	A subatomic particle found in shells around the nucleus with negligible mass and a relative charge of −1.
Atomic number (Z)	The number of protons in the nucleus of an atom.
Mass number (A)	The total number of protons and neutrons in the nucleus of an atom.
Isotopes	Atoms of the same element with the same number of protons but different numbers of neutrons.
Ion	An atom or group of atoms that has gained or lost electrons, giving it a positive or negative charge.
Relative atomic mass (Ar)	The weighted average mass of the isotopes of an element relative to ¹⁄₁₂ the mass of a carbon-12 atom.
Relative molecular mass (Mr)	The sum of the relative atomic masses of all atoms in a molecule.

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3. Bonding and Structure

Term	Definition
Ionic bonding	The electrostatic attraction between oppositely charged ions, formed by the transfer of electrons from a metal to a non-metal.
Covalent bonding	The sharing of one or more pairs of electrons between two atoms.
Metallic bonding	The electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons.
Giant ionic lattice	A regular three-dimensional arrangement of alternating positive and negative ions held together by strong electrostatic forces.
Simple molecular structure	A structure consisting of small molecules held together by weak intermolecular forces, with strong covalent bonds within the molecules.
Giant covalent structure	A structure in which very large numbers of atoms are held together by covalent bonds in a regular lattice (e.g. diamond, graphite, silicon dioxide).
Electrolysis	The decomposition of an ionic compound using an electric current when molten or dissolved in water.
Electrolyte	A substance that conducts electricity when molten or in aqueous solution and is decomposed during the process.
Electrode	A conductor through which electric current enters or leaves an electrolyte.
Cathode	The negative electrode where cations (positive ions) gain electrons — reduction occurs.
Anode	The positive electrode where anions (negative ions) lose electrons — oxidation occurs.

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4. Stoichiometry and the Mole

Term	Definition
Mole	The amount of substance that contains 6.02 × 10²³ particles (Avogadro’s number).
Molar mass	The mass of one mole of a substance, in grams per mole (g/mol), numerically equal to its Mr.
Empirical formula	The simplest whole-number ratio of atoms of each element present in a compound.
Molecular formula	The actual number of atoms of each element present in one molecule of a substance.
Concentration	The amount of solute dissolved in a given volume of solution, typically expressed in mol/dm³ or g/dm³.
Avogadro’s number	6.02 × 10²³ — the number of particles in one mole of a substance.

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5. Chemical Reactions

Term	Definition
Chemical reaction	A process in which one or more substances are changed into one or more new substances.
Exothermic reaction	A reaction that releases energy (heat) to the surroundings, causing the temperature to rise.
Endothermic reaction	A reaction that absorbs energy (heat) from the surroundings, causing the temperature to fall.
Activation energy	The minimum amount of energy that reacting particles must have in order to react.
Catalyst	A substance that increases the rate of a chemical reaction without being used up, by providing an alternative pathway with lower activation energy.
Rate of reaction	The speed at which reactants are converted into products, measured as change in concentration per unit time.
Collision theory	The theory that reactions occur when particles collide with sufficient energy and the correct orientation.

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6. Acids, Bases and Salts

Term	Definition
Acid	A substance that produces hydrogen ions (H⁺) when dissolved in water.
Base	A substance that reacts with an acid to form a salt and water only.
Alkali	A soluble base that produces hydroxide ions (OH⁻) when dissolved in water.
pH scale	A measure of the acidity or alkalinity of a solution, ranging from 0 (strongly acidic) to 14 (strongly alkaline), with 7 being neutral.
Indicator	A substance that changes colour depending on the pH of the solution.
Neutralisation	The reaction between an acid and a base to form a salt and water: H⁺ + OH⁻ → H₂O.
Salt	An ionic compound formed when the hydrogen of an acid is replaced by a metal ion or ammonium ion.
Titration	A technique used to find the exact volume of one solution needed to react completely with a known volume of another solution.

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7. The Periodic Table

Term	Definition
Period	A horizontal row in the Periodic Table — elements in the same period have the same number of electron shells.
Group	A vertical column in the Periodic Table — elements in the same group have the same number of outer-shell electrons and similar chemical properties.
Alkali metals (Group I)	Soft, reactive metals that form +1 ions and produce alkaline solutions with water.
Halogens (Group VII)	Reactive non-metals that exist as diatomic molecules and form −1 ions.
Noble gases (Group VIII/0)	Unreactive elements with full outer electron shells, existing as monatomic gases.
Transition metals	Metals in the central block of the Periodic Table that often form coloured compounds, act as catalysts and have variable oxidation states.

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8. Metals

Term	Definition
Ore	A naturally occurring rock that contains enough of a metal or metal compound to make extraction worthwhile.
Reduction	The gain of electrons or the loss of oxygen by a substance.
Oxidation	The loss of electrons or the gain of oxygen by a substance.
Redox reaction	A reaction in which both oxidation and reduction occur simultaneously.
Reactivity series	A list of metals arranged in order of decreasing reactivity.
Displacement reaction	A reaction in which a more reactive element takes the place of a less reactive element in a compound.
Alloy	A mixture of a metal with one or more other elements, usually to improve hardness or strength.
Rusting	The corrosion of iron in the presence of both oxygen and water, forming hydrated iron(III) oxide.

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9. Air, Water and the Environment

Term	Definition
Fossil fuel	A fuel formed from the remains of ancient organisms over millions of years (e.g. coal, oil, natural gas).
Greenhouse effect	The trapping of heat in the Earth’s atmosphere by greenhouse gases such as CO₂ and methane.
Acid rain	Rain with a pH below 5.6, caused by sulfur dioxide and nitrogen oxides dissolving in atmospheric water.
Carbon cycle	The continuous movement of carbon between the atmosphere, living organisms, oceans and rocks.

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10. Organic Chemistry

Term	Definition
Organic chemistry	The study of carbon-containing compounds (excluding CO₂, CO and carbonates).
Hydrocarbon	A compound containing only hydrogen and carbon atoms.
Homologous series	A family of compounds with the same general formula, similar chemical properties and a gradual trend in physical properties.
Alkane	A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂, containing only single C–C bonds.
Alkene	An unsaturated hydrocarbon with the general formula CₙH₂ₙ, containing at least one C=C double bond.
Isomers	Compounds with the same molecular formula but different structural arrangements of atoms.
Polymer	A very large molecule made by joining together many small molecules (monomers) in a repeating pattern.
Monomer	A small molecule that can be joined to many others to form a polymer.
Addition polymerisation	A reaction in which unsaturated monomers join together to form a polymer with no other product.
Fractional distillation	The separation of a mixture of liquids with different boiling points by heating and collecting fractions at different temperatures.
Cracking	The breaking down of long-chain hydrocarbons into shorter, more useful molecules by heating with a catalyst or steam.
Fermentation	The anaerobic breakdown of glucose by yeast to produce ethanol and carbon dioxide.
Functional group	An atom or group of atoms responsible for the characteristic reactions of a compound.

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11. Experimental Techniques

Term	Definition
Filtration	A separation technique used to separate an insoluble solid from a liquid.
Crystallisation	A separation technique used to obtain a dissolved solid from a solution by evaporating the solvent.
Distillation	A technique used to separate a liquid from a solution by boiling and then condensing the vapour.
Chromatography	A technique used to separate and identify the components of a mixture based on their different solubilities in a solvent.
Rf value	The ratio of the distance moved by a substance to the distance moved by the solvent front in chromatography.

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Exam Tips for IGCSE Chemistry Definitions

1. Learn the difference between oxidation and reduction — remember OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons).
2. Be precise with “catalyst” — always mention “alternative pathway” and “lower activation energy.”
3. Don’t confuse atom and molecule — an atom is the smallest particle of an element; a molecule is two or more atoms bonded together.
4. Know your bonding types — examiners love asking you to explain properties (melting point, conductivity) in terms of bonding and structure.
5. Use correct formula notation — subscripts, state symbols and charges all count.

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Last updated: March 2026. Based on the Cambridge IGCSE Chemistry (0620/0971) syllabus.

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