Oxidation and Reduction

This lesson covers oxidation and reduction (redox reactions) as specified in the AQA GCSE Chemistry syllabus (4.4.1). You need to understand what oxidation and reduction mean in terms of oxygen transfer and electron transfer, identify which substances are oxidised and reduced in a reaction, and recognise oxidising and reducing agents. These concepts are fundamental to understanding chemical changes, extraction of metals, and electrochemistry.

Definitions of Oxidation and Reduction

Oxidation and reduction always occur together — they are two halves of the same process, known as a redox reaction (reduction-oxidation).

In Terms of Oxygen Transfer

Term	Definition	Example
Oxidation	The gain of oxygen	Magnesium + oxygen → magnesium oxide (magnesium is oxidised)
Reduction	The loss of oxygen	Copper oxide + carbon → copper + carbon dioxide (copper oxide is reduced)

In Terms of Electron Transfer

Term	Definition	Memory aid
Oxidation	The loss of electrons	OIL — Oxidation Is Loss
Reduction	The gain of electrons	RIG — Reduction Is Gain

Together: OIL RIG — Oxidation Is Loss, Reduction Is Gain.

Exam Tip: The mnemonic OIL RIG is essential for this topic. Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). This applies to all redox reactions. If a question asks about electron transfer, always use this framework. Many students confuse these, so drill the mnemonic until it is automatic.

Redox Reactions

In every redox reaction, one substance is oxidised (loses electrons or gains oxygen) while another is reduced (gains electrons or loses oxygen). These two processes always happen simultaneously.

graph LR
    A["Substance A<br/>(Loses electrons)"] -->|"OXIDATION"| B["Substance A<br/>(Oxidised form)"]
    C["Substance B<br/>(Gains electrons)"] -->|"REDUCTION"| D["Substance B<br/>(Reduced form)"]
    A -.->|"Electrons transferred"| C

    style A fill:#e74c3c,color:#fff
    style B fill:#c0392b,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#2980b9,color:#fff

Example 1: Displacement Reaction

Magnesium + copper sulfate → magnesium sulfate + copper

Magnesium is oxidised — it loses electrons to form Mg2+ ions.
Copper ions (Cu2+) are reduced — they gain electrons to form copper metal.
Magnesium is the reducing agent (it causes the copper ions to be reduced by donating electrons).
Copper sulfate is the oxidising agent (it causes the magnesium to be oxidised by accepting electrons).

Example 2: Extraction of Iron

Iron(III) oxide + carbon → iron + carbon dioxide

Iron(III) oxide is reduced — it loses oxygen.
Carbon is oxidised — it gains oxygen.
Carbon is the reducing agent.
Iron(III) oxide is the oxidising agent.

Exam Tip: Questions often ask you to identify the oxidising agent and reducing agent. Remember: the reducing agent is the substance that gets oxidised (it reduces the other substance by giving it electrons). The oxidising agent is the substance that gets reduced (it oxidises the other substance by taking electrons away). This is counterintuitive — the reducing agent is oxidised, and the oxidising agent is reduced.

Oxidising and Reducing Agents

Term	What it does	What happens to it
Oxidising agent	Oxidises another substance (takes electrons from it or gives it oxygen)	It is reduced
Reducing agent	Reduces another substance (gives electrons to it or removes its oxygen)	It is oxidised

Common Oxidising Agents

Oxygen (O2) — oxidises metals when they burn or corrode
Chlorine (Cl2) — used in bleach and disinfection
Potassium manganate(VII) — turns from purple to colourless when it oxidises a substance

Common Reducing Agents

Carbon (C) — used in the blast furnace to reduce metal oxides
Carbon monoxide (CO) — reduces iron oxide in the blast furnace
Hydrogen (H2) — can reduce copper oxide to copper when heated
Reactive metals (e.g. magnesium, zinc) — reduce less reactive metal ions in displacement reactions