Chemical reactions glossary 2026

Acid

A substance that produces hydrogen ions (H+) when dissolved in water

Alkali

A soluble base

Anion

A negatively charged ion e.g. Cl-

Atomic number

The number of protons in the nucleus of an atom

Base

A substance that accepts hydrogen ions (H+) or that dissolves to release hydroxide ions (OH-).

Compound

A substance made from two or more different elements that have been chemically joined

Cation

A positively charged ion. E.g. Ca2+

Combination reaction

When two or more substances combine to make one new product

Combustion

A chemical process that occurs when fuel is burned and reacts with oxygen to release energy

Complete combustion

A combustion reaction that occurs when fuel reacts with oxygen to produce carbon dioxide and water

Decomposition reaction

When one single compound breaks down into two or more simpler substances

Electrons

A negatively charged particle found in electron shells the move around the outside of the nucleus

Element

A pure substance made up of only one kind of atom

Ion

A charged (positive or negative) atom or group of atoms

Ionic compound

A substance made of positive and negative ions

Incomplete combustion

A combustion reaction that occurs when fuel reacts with a limited supply of oxygen to produce carbon or carbon monoxide, water and some energy

Insoluble

A substance that does not dissolve in a liquid (solvent)

Law of conservation of mass

Matter cannot be created or destroyed in a chemical reaction - the total mass of reactants equals the total mass of products

Mass number

The total number of protons and neutrons in a nucleus

Nucleus

Central part of an atom that consists of protons and neutrons

Neutrons

A particle found inside the nucleus of an atom, it has no charge ( is neutral)

Neutralisation

A chemical reaction between an acid and a base which produces a more neutral solution (salt and water)

Precipitate

An insoluble solid that forms when ions in a solution are mixed together

Precipitation

When two solutions react to form an insoluble solid called a precipitate

Protons

A positively charged ion. E.g. Ca2+

Product

A chemical substance formed as a result of a chemical reaction

Observation

Something you can see, hear or measure during an experiment e.g. colour change, fizzing, temperature

Reactant

A chemical substance that is present at the start of the chemical reaction and is changed during the reaction

Soluble

A substance that dissolves in a liquid (solvent)

Universal indicator

A pH indicator that shows various colour changes over a wide range pH values to indicate the acidity or alkalinity of solutions

Word equation

A way of showing a chemical reaction using words

e.g. magnesium + oxygen → magnesium oxide.