A redox reaction is a reaction where one substance is reduced and another is oxidised. The word redox comes from reduction and oxidation happening at the same time.

What is oxidation and reduction?

There are two ways to define redox reactions:

1. In terms of oxygen:

• Oxidation = gain of oxygen
• Reduction = loss of oxygen

Example:

• Magnesium + oxygen → magnesium oxide
• Mg is oxidised (gains oxygen)

2. In terms of electrons:

• Oxidation = loss of electrons
• Reduction = gain of electrons

💡 Use the acronym OIL RIG: Oxidation Is Loss, Reduction Is Gain

Identifying redox reactions

In a redox reaction:

• One substance loses electrons → it is oxidised
• One substance gains electrons → it is reduced

You can spot redox reactions when:

• A metal reacts with oxygen
• A metal displaces another
• Electrons are transferred

Examples of redox reactions

Example 1: Magnesium + oxygen

2Mg + O₂ → 2MgO

• Mg is oxidised (gains oxygen)
• O₂ is reduced (forms oxide)

Example 2: Iron + copper sulfate

Fe + CuSO₄ → FeSO₄ + Cu

• Iron is oxidised (loses electrons to form Fe²⁺)
• Copper ions are reduced (gain electrons to become copper metal)

This is a displacement reaction and also a redox reaction.

Redox in terms of ions