Oxidising Agent – IGCSE Chemistry Definition
IGCSE Chemistry definition of oxidising agent: a substance that oxidises another substance and is itself reduced. Covers examples and electron transfer.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
An oxidising agent is a substance that oxidises another substance — and in doing so, is itself reduced. This concept is tested on the 0620 Supplement syllabus and requires careful identification of which substance gains and which loses electrons in a redox reaction.
The 0620 definition
An oxidising agent is a substance that oxidises another substance. It does this by:
- Accepting (gaining) electrons from the other substance, or
- Providing oxygen to the other substance
The oxidising agent itself is reduced in the process.
The key relationship
The oxidising agent gains electrons (is reduced). The reducing agent loses electrons (is oxidised).
This seems counterintuitive — the oxidising agent is reduced, not oxidised. Remember: the agent is named for what it does to the other substance, not what happens to itself.
Examples
| Reaction | Oxidising agent | Why |
|---|---|---|
| CuO + H₂ → Cu + H₂O | CuO (copper oxide) | Provides oxygen to hydrogen; CuO is reduced to Cu |
| 2Na + Cl₂ → 2NaCl | Cl₂ (chlorine) | Accepts electrons from sodium; Cl₂ is reduced to Cl⁻ |
| Fe₂O₃ + 3CO → 2Fe + 3CO₂ | Fe₂O₃ (iron oxide) | Provides oxygen to CO; Fe₂O₃ is reduced to Fe |
| Mg + CuSO₄ → MgSO₄ + Cu | Cu²⁺ (copper ions) | Accepts electrons from Mg; Cu²⁺ is reduced to Cu |
Common oxidising agents
- Oxygen (O₂)
- Chlorine (Cl₂)
- Potassium manganate(VII) — KMnO₄ (turns from purple to colourless)
- Acidified potassium dichromate(VI) — K₂Cr₂O₇ (turns from orange to green)
- Metal oxides (e.g. CuO, Fe₂O₃)
- Concentrated sulfuric acid
Worked exam question
In the reaction: Zn + CuSO₄ → ZnSO₄ + Cu. (a) Identify the oxidising agent. (1) (b) Explain your answer in terms of electron transfer. (2)
Mark scheme
(a) Copper sulfate / CuSO₄ / Cu²⁺ ions [1]
(b) Cu²⁺ ions accept / gain electrons from zinc [1]; Cu²⁺ is reduced to Cu / the copper ion causes zinc to be oxidised [1]
Common exam mistakes
- Confusing oxidising agent with reducing agent. The oxidising agent causes oxidation in something else — but is itself reduced.
- Naming the wrong substance. The oxidising agent gains electrons. The reducing agent loses electrons.
- Forgetting that the name refers to what the substance does to others, not what happens to it. An oxidising agent oxidises another substance.
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