all chemistry definitions

A comprehensive glossary of terms used in GCSE Chemistry, covering key definitions and concepts.

Atomic Number

The number of protons in the nucleus of an atom.

Mass Number

The total number of protons and neutrons in the nucleus of an atom.

Isotopes

Atoms that have the same number of protons but different numbers of neutrons.

Cation

A positive ion.

Anion

A negative ion.

Compound

A substance formed when two or more elements are chemically combined.

Ionic Bond

The attraction between oppositely charged ions.

Covalent Bond

A bond formed by a shared pair of electrons.

Diatomic

a molecule that consists of two atoms.

Metallic Bonding

The attraction between delocalised electrons and positive ions in a regular lattice.

Malleable

Can be hammered into shape.

Ductile

Can be drawn out into wires.

Alloy

A mixture of two or more elements, at least one of which is a metal.

Nanometre

A unit of measure equal to 10^-9 meters.

Nanoparticle

A structure that is sized between 1 and 100 nanometers.

Allotropes

Different forms of the same element in the same physical state.

Element

A substance that consists of only one type of atom.

Atom

The simplest particle of an element that can exist independently.

Molecule

A particle consisting of two or more atoms chemically bonded together.

Period

A horizontal row in the Periodic Table.

Group

A vertical column in the Periodic Table.

Relative Atomic Mass

The mass of an atom compared to that of the carbon-12 isotope.

Percentage Yield

Calculated by (actual yield/theoretical yield) × 100.

Water of Crystallisation

Water that is chemically bonded into the crystal structure.

Hydrated

Describing solid crystals that contain water of crystallisation.

Dehydration

The removal of water of crystallisation.

Anhydrous

A substance that does not contain water of crystallisation.

Degree of Hydration

The number of moles of water of crystallisation in 1 mole of a compound.

Empirical Formula

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

Molecular Formula

The actual number of atoms of each element in a molecule.

Indicator

A chemical that gives a color change in acidic, alkaline, and neutral solutions.

Base

A substance that reacts with an acid to produce a salt and water.

Alkali

A soluble base.

Strong Acid

An acid that ionises completely in water.

Weak Acid

An acid that ionises partially in water.

Neutralisation

The reaction between an acid and an alkali producing a salt and water.

Salt

A compound formed when some or all hydrogen ions in an acid are replaced by metal ions or ammonium ions.

Pure Substance

A single element or compound not mixed with others.

Mixture

Two or more substances mixed together that are usually easy to separate.

Melting Point

The temperature at which a solid changes into a liquid.

Boiling Point

The temperature at which a liquid changes into a gas.

Formulation

A mixture designed as a useful product with carefully measured quantities.

Soluble Substance

A substance that dissolves in water.

Insoluble Substance

A substance that does not dissolve in water.

Solute

A soluble substance that dissolves in a solvent.

Solvent

The liquid in which the solute dissolves.

Solution

A mixture of solute dissolved in a solvent.

Miscible Liquids

Liquids that mix well.

Immiscible Liquids

Liquids that do not mix.

Filtrate

The filtered solution.

Residue

The solid that remains on the filter paper.

Saturated Solution

A solution in which no more solid will dissolve at a particular temperature.

Precipitate

A solid formed when mixing two solutions.

Solubility

The mass of solid that can saturate 100g of solvent at a certain temperature.

Displacement Reaction

A reaction where a more reactive metal replaces a less reactive metal in a compound.

Redox Reaction

A reaction where oxidation and reduction occur simultaneously.

Oxidation

Gain of oxygen, loss of hydrogen or loss of electrons.

Reduction

Loss of oxygen, gain of hydrogen or gain of electrons.

Rust

Hydrated iron(III) oxide.

Activation Energy

The minimum energy required for a reaction to occur.

Catalyst

A substance that increases the rate of a chemical reaction without being used up.

Reversible Reaction

A reaction where reactants can convert to products and vice versa.

Dynamic Equilibrium

When the rate of the forward reaction equals the rate of the reverse reaction.

Homogeneous Reaction

A reaction where all reactants and products are in the same state.

Le Châtelier's Principle

A principle stating that a system at equilibrium will shift in response to changes in conditions.

Homologous Series

A family of organic compounds with the same general formula and similar chemical properties.

Hydrocarbon

A compound composed only of hydrogen and carbon.

Cracking

The breakdown of larger saturated hydrocarbons into smaller more useful ones.

Functional Group

A reactive group in a molecule.

Combustion

The reaction of a fuel with oxygen producing oxides and heat.

Saturated Compound

A compound containing no C=C bonds.

Unsaturated Compound

A compound containing at least one C=C bond.

Polymerisation

The process of creating a long chain molecule from smaller molecules.

Titration

A method for reacting two solutions to determine the number of moles of a solute in one.

Avogadro's Law

Equal volumes of gases contain the same number of particles under the same conditions.

Atom Economy

The mass of desired product divided by total mass of products, multiplied by 100.

Electrolysis

The decomposition of a liquid electrolyte using direct current of electricity.

Electrolyte

The liquid or solution that conducts electricity and is decomposed by it.

Cathode

The negative electrode in electrolysis.

Anode

The positive electrode in electrolysis.

Inert Electrodes

Electrodes that do not take part in electrolysis.

Bauxite

Aluminium ore.

Exothermic Reaction

A reaction that releases heat to the surroundings.

Endothermic Reaction

A reaction that absorbs heat from the surroundings.

Clean Fuel

A fuel that produces non-polluting products when burned.