GCSE Chemistry

Overview

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Topics

Chemistry key terms

Original definitions for core course vocabulary, informed by the references below. Not an official or exhaustive board-specific glossary.

40 of 40 terms

Atom

The smallest particle of an element that retains its chemical identity, consisting of a nucleus surrounded by electrons.

Atomic structure and bonding

Element

A substance whose atoms all have the same number of protons.

Atomic structure and bonding

Compound

A substance made from two or more different elements chemically bonded in fixed proportions.

Atomic structure and bonding

Mixture

Two or more substances present together without being chemically bonded to one another.

Atomic structure and bonding

Isotope

An atom of the same element as another atom but with a different number of neutrons.

Atomic structure and bonding

Ion

An atom or group of atoms with an overall electric charge because it has lost or gained electrons.

Atomic structure and bonding

Atomic number

The number of protons in the nucleus of an atom.

Atomic structure and bonding

Ionic bonding

The strong electrostatic attraction between oppositely charged ions.

Atomic structure and bonding

Covalent bonding

A bond formed when atoms share a pair of electrons.

Atomic structure and bonding

Metallic bonding

The electrostatic attraction between positive metal ions and delocalised electrons.

Atomic structure and bonding

Relative atomic mass

The weighted mean mass of an element's atoms compared with one-twelfth of the mass of a carbon-12 atom.

Quantities and chemical reactions

Mole

An amount of substance containing approximately 6.02 × 10²³ specified particles.

Quantities and chemical reactions

Molar mass

The mass of one mole of a substance, usually expressed in g/mol.

Quantities and chemical reactions

Concentration

The amount of solute per unit volume of solution, commonly expressed in g/dm³ or mol/dm³.

Quantities and chemical reactions

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound.

Quantities and chemical reactions

Balanced equation

A chemical equation with equal numbers of atoms of each element on both sides and equal total charge where relevant.

Quantities and chemical reactions

Limiting reactant

The reactant used up first, which determines the maximum amount of product that can form.

Quantities and chemical reactions

Percentage yield

The actual yield divided by the theoretical maximum yield, multiplied by 100.

Quantities and chemical reactions

Oxidation

The loss of electrons by a substance; in oxygen-transfer reactions, it is the gain of oxygen.

Quantities and chemical reactions

Reduction

The gain of electrons by a substance; in oxygen-transfer reactions, it is the loss of oxygen.

Quantities and chemical reactions

Exothermic reaction

A reaction that transfers energy to the surroundings, usually causing their temperature to rise.

Energetics, rates and equilibrium

Endothermic reaction

A reaction that takes in energy from the surroundings, usually causing their temperature to fall.

Energetics, rates and equilibrium

Activation energy

The minimum energy that colliding particles must have for a reaction to occur.

Energetics, rates and equilibrium

Catalyst

A substance that increases reaction rate by providing an alternative pathway with lower activation energy and is regenerated overall.

Energetics, rates and equilibrium

Rate of reaction

The change in the amount or concentration of a reactant or product per unit time.

Energetics, rates and equilibrium

Collision theory

The explanation that reactions occur when particles collide with sufficient energy and suitable orientation.

Energetics, rates and equilibrium

Reversible reaction

A reaction in which products can react to form the original reactants.

Energetics, rates and equilibrium

Dynamic equilibrium

The state in a closed system where forward and reverse reactions occur at equal rates, so concentrations remain constant.

Energetics, rates and equilibrium

Electrolysis

The decomposition of an ionic substance, molten or in solution, by an electric current.

Energetics, rates and equilibrium

Electrolyte

A molten or dissolved ionic substance that conducts electricity through moving ions and undergoes chemical change.

Energetics, rates and equilibrium

Acid

A substance that produces hydrogen ions in aqueous solution.

Acids, analysis and organic chemistry

Base

A substance that neutralises an acid; many bases are metal oxides or hydroxides.

Acids, analysis and organic chemistry

Alkali

A base that dissolves in water to produce hydroxide ions.

Acids, analysis and organic chemistry

pH

A numerical measure related to hydrogen ion concentration that indicates how acidic or alkaline an aqueous solution is.

Acids, analysis and organic chemistry

Neutralisation

A reaction in which an acid and a base form a salt and usually water.

Acids, analysis and organic chemistry

Chromatography

A separation method based on different distributions of substances between a mobile phase and a stationary phase.

Acids, analysis and organic chemistry

Distillation

Separation using vaporisation followed by condensation, exploiting differences in boiling behaviour.

Acids, analysis and organic chemistry

Hydrocarbon

A compound containing only carbon and hydrogen atoms.

Acids, analysis and organic chemistry

Homologous series

A family of organic compounds with the same functional group and general formula, in which successive members differ by CH₂.

Acids, analysis and organic chemistry

Polymerisation

A reaction in which many small monomer molecules join to form large polymer molecules.

Acids, analysis and organic chemistry

Exams

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Revision

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Practice

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Resources

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