What Are Oxidation and Reduction?

In many chemical reactions, substances gain or lose electrons. These reactions are called redox reactions, which stands for reduction–oxidation reactions.

Oxidation and reduction always occur together in the same reaction. When one substance is oxidised, another substance must be reduced.

Understanding redox reactions helps explain many important chemical processes such as rusting, combustion, electrolysis, and metal extraction.

What Is Oxidation?

Oxidation is defined as the loss of electrons during a chemical reaction.

Another common definition used in O Level Chemistry is:

• Gain of oxygen
• Loss of hydrogen

Example:

Mg → Mg²⁺ + 2e⁻

Magnesium loses electrons, so it is oxidised.

What Is Reduction?

Reduction is defined as the gain of electrons during a chemical reaction.

Another definition is:

• Loss of oxygen
• Gain of hydrogen

Example:

Cu²⁺ + 2e⁻ → Cu

Copper ions gain electrons, so they are reduced.

Redox Reactions

A redox reaction occurs when oxidation and reduction happen at the same time.

Example reaction:

Zn + Cu²⁺ → Zn²⁺ + Cu

In this reaction:

• Zinc loses electrons → oxidation
• Copper ions gain electrons → reduction

This means the reaction is a redox reaction.

Oxidising Agents and Reducing Agents

In redox reactions, some substances cause oxidation or reduction.

Oxidising Agent

An oxidising agent is a substance that causes another substance to lose electrons.

It gains electrons itself and is therefore reduced.

Example:

Oxygen often acts as an oxidising agent.

Reducing Agent

A reducing agent is a substance that causes another substance to gain electrons.

It loses electrons itself and is therefore oxidised.

Example:

Carbon is often used as a reducing agent in metal extraction.

Example of a Redox Reaction

Reaction between iron oxide and carbon monoxide:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

In this reaction:

• Iron oxide loses oxygen → reduction
• Carbon monoxide gains oxygen → oxidation

This reaction is used in the extraction of iron in a blast furnace.

Everyday Examples of Redox Reactions

Redox reactions occur in many everyday situations, including:

• Rusting of iron
• Burning fuels
• Respiration in living organisms
• Photosynthesis in plants

These processes involve transfer of electrons and energy.

Why Redox Reactions Are Important

Understanding redox reactions helps chemists explain:

• Corrosion of metals
• Industrial metal extraction
• Electrochemical reactions
• Energy production in biological systems

Redox reactions are fundamental to many chemical and biological processes.

Exam Tip (5070)

Students are often asked to:

• Define oxidation and reduction
• Identify oxidising and reducing agents
• Recognise redox reactions in equations

Example exam question:

Define oxidation in terms of electrons.

Answer:

Oxidation is the loss of electrons.

Practice Question

In the reaction:

Zn + Cu²⁺ → Zn²⁺ + Cu

Which substance is oxidised?

Answer

Zinc is oxidised because it loses electrons.

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