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Oxidation is the loss of electrons from a substance. It is also the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide:

magnesium + oxygen → magnesium oxide

2Mg + O₂ → 2MgO

Reduction is the gain of electrons by a substance. It is also the loss of oxygen from a substance. For example, copper(II) oxide can be reduced to form copper when it reacts with hydrogen:

copper(II) oxide + hydrogen → copper + water

CuO + H₂ → Cu + H₂O

Usually, oxidation and reduction take place at the same time in a reaction. We call this type of reaction a redox reaction (red – reduction, ox – oxidation).

Note that:

• the oxidising agent is the chemical that causes oxidation by providing oxygen
• the reducing agent causes the other chemical to be reduced by removing oxygen

Take a look at the following thermite reaction:

aluminium + iron(III) oxide → iron + aluminium oxide

2Al + Fe₂O₃ → 2Fe + Al₂O₃

Aluminium has been oxidised. This means that the iron(III) oxide is the oxidising agent. We can also see that the iron(III) oxide has been reduced. This means that the aluminium is the reducing agent.