Oxidation, reduction and redox

Many reactions involve oxidation and reduction happening together. You also need to name the oxidising agent and the reducing agent.

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Two ways to define it

Oxidation: gain of oxygen, or loss of electrons.
Reduction: loss of oxygen, or gain of electrons.
Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Oxidation and reduction always happen together, so these reactions are called redox reactions.

Oxidising and reducing agents

An oxidising agent oxidises something else, and is itself reduced (it accepts electrons or gives away oxygen).
A reducing agent reduces something else, and is itself oxidised (it gives away electrons or takes oxygen).
Copper(II) oxide + hydrogen → copper + water
CuO + H2 → Cu + H2O: CuO loses oxygen (reduced), so it is the oxidising agent. H2 gains oxygen (oxidised), so it is the reducing agent.

Using electrons

In a displacement reaction, Mg + Cu2+ → Mg2+ + Cu:
magnesium loses 2 electrons (oxidised): it is the reducing agent.
copper ions gain 2 electrons (reduced): they are the oxidising agent.
In a blast furnace, carbon monoxide is the reducing agent: Fe2O3 + 3CO → 2Fe + 3CO2.
Worked example

In Zn + CuSO4 → ZnSO4 + Cu, identify the reducing agent.

  1. Zinc atoms lose electrons to become Zn2+: zinc is oxidised.
  2. A substance that is oxidised is the reducing agent.

Answer: Zinc is the reducing agent (it is oxidised, giving electrons to the copper ions).

Key idea

Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons. The oxidising agent is reduced; the reducing agent is oxidised.

Key words
Oxidation
Gain of oxygen, or loss of electrons.
Reduction
Loss of oxygen, or gain of electrons.
Displacement reaction
A reaction where a more reactive element takes the place of a less reactive one in a compound.

All GCSE and International GCSE Chemistry key terms and definitions →

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Extra practice

2 more questions

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  1. What is reduction in terms of electrons?

    1. Gain of electrons
    2. Loss of electrons
    3. Loss of neutrons
    4. Gain of protons
    Show the answer

    Answer: Gain of electrons

    OIL RIG: reduction is gain.

  2. In ZnO + C → Zn + CO, which substance is the reducing agent?

    1. Carbon
    2. Zinc oxide
    3. Carbon monoxide
    4. Zinc
    Show the answer

    Answer: Carbon

    Carbon removes oxygen from zinc oxide.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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