Key definitions

AQA 8462 · 8462 mark-scheme terms

The definitions AQA marks to the word across the 8462 spec — bonding, quantitative chemistry, rates, acids and electrolysis. Say them exactly, not roughly.

Reading mode — read across, know each one cold.

Term	Mark-scheme definition
Element	A substance made of only one type of atom.
Compound	A substance made of two or more elements chemically combined (in fixed proportions).
Mixture	Two or more substances (elements or compounds) not chemically combined together.
Pure substance (chemistry)	A single element or compound, not mixed with any other substance.A pure substance melts and boils at one specific temperature.
Isotopes	Atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass (Ar)	The average mass of the atoms of an element, compared to 1/12th the mass of a carbon-12 atom, taking the abundance of each isotope into account.
Relative formula mass (Mr)	The sum of the relative atomic masses of all the atoms shown in the formula.
Ion	A charged particle formed when an atom (or group of atoms) loses or gains electrons.
Ionic bond	The strong electrostatic force of attraction between oppositely charged ions.
Covalent bond	A shared pair of electrons between two atoms.
Metallic bond	The strong electrostatic attraction between positive metal ions and a sea of delocalised electrons.
Giant covalent structure	A huge lattice of atoms held together by strong covalent bonds (e.g. diamond, graphite, silicon dioxide).
Mole	The amount of a substance that contains 6.02 x 10²³ particles (the Avogadro constant).
Concentration	The amount of solute dissolved in a given volume of solution, in g/dm³ or mol/dm³.
Empirical formula	The simplest whole-number ratio of atoms of each element in a compound.
Percentage yield	(Mass of product actually made / maximum theoretical mass) x 100.
Atom economy	(Relative formula mass of the desired product / sum of relative formula masses of all reactants) x 100.
Exothermic reaction	A reaction that transfers energy to the surroundings, so the temperature of the surroundings increases.
Endothermic reaction	A reaction that takes in energy from the surroundings, so the temperature of the surroundings decreases.
Activation energy	The minimum amount of energy that colliding particles must have to react.
Rate of reaction	The change in the amount of a reactant or product per unit time.
Catalyst	A substance that speeds up the rate of a reaction without being used up, by providing a different pathway with a lower activation energy.
Reversible reaction	A reaction in which the products can react to re-form the reactants.
Dynamic equilibrium	When the forward and reverse reactions occur at the same rate in a closed system, so the concentrations of reactants and products stay constant.
Oxidation	The gain of oxygen, or the loss of electrons.OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Reduction	The loss of oxygen, or the gain of electrons.
Acid	A substance that produces H+ ions in aqueous solution.
Base	A substance that neutralises an acid (e.g. a metal oxide or hydroxide).
Alkali	A soluble base that produces OH- ions in aqueous solution.
Strong acid	An acid that is completely ionised in aqueous solution (e.g. hydrochloric, sulfuric, nitric).
Weak acid	An acid that is only partially ionised in aqueous solution (e.g. ethanoic, citric, carbonic).
Neutralisation	The reaction of an acid with a base to make a salt and water.
Electrolysis	The breaking down of an ionic substance, molten or in solution, by passing an electric current through it.
Polymer	A large molecule made from many small molecules (monomers) joined together.

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