Reducing Agent – IGCSE Chemistry Definition
IGCSE Chemistry definition of reducing agent: a substance that reduces another substance and is itself oxidised. Covers examples and electron transfer.
Published by IGCSEChemistry.com.my
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.
A reducing agent is a substance that reduces another substance — and in doing so, is itself oxidised. Like the oxidising agent, this is a Supplement concept on the 0620 syllabus, tested in redox identification questions and in the context of metal extraction.
The 0620 definition
A reducing agent is a substance that reduces another substance. It does this by:
- Donating (losing) electrons to the other substance, or
- Removing oxygen from the other substance
The reducing agent itself is oxidised in the process.
The key relationship
The reducing agent loses electrons (is oxidised). The oxidising agent gains electrons (is reduced).
Again, the agent is named for what it does to the other substance.
Examples
| Reaction | Reducing agent | Why |
|---|---|---|
| CuO + H₂ → Cu + H₂O | H₂ (hydrogen) | Removes oxygen from CuO; H₂ is oxidised to H₂O |
| Fe₂O₃ + 3CO → 2Fe + 3CO₂ | CO (carbon monoxide) | Removes oxygen from Fe₂O₃; CO is oxidised to CO₂ |
| 2Na + Cl₂ → 2NaCl | Na (sodium) | Donates electrons to chlorine; Na is oxidised to Na⁺ |
| Mg + CuSO₄ → MgSO₄ + Cu | Mg (magnesium) | Donates electrons to Cu²⁺; Mg is oxidised to Mg²⁺ |
Common reducing agents
- Carbon (C) and carbon monoxide (CO) — used in metal extraction from ores
- Hydrogen (H₂) — reduces metal oxides
- Reactive metals (e.g. Na, Mg, Al, Zn) — donate electrons readily
- In electrolysis, the cathode acts as a source of electrons (reducing agent equivalent)
Reducing agents in metal extraction
Carbon is used as a reducing agent to extract metals that are below carbon in the reactivity series:
ZnO + C → Zn + CO
Fe₂O₃ + 3C → 2Fe + 3CO
Carbon removes the oxygen from the metal oxide (reducing the oxide to the metal), and carbon is oxidised to CO or CO₂.
Metals above carbon in the reactivity series (e.g. aluminium) cannot be extracted this way — electrolysis is needed instead.
Worked exam question
In the blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂. (a) Identify the reducing agent. (1) (b) Explain why CO is the reducing agent. (2)
Mark scheme
(a) Carbon monoxide / CO [1]
(b) CO reduces the iron oxide / removes oxygen from Fe₂O₃ [1]; CO is itself oxidised (gains oxygen to become CO₂) [1]
Common exam mistakes
- Confusing reducing agent with oxidising agent. The reducing agent is oxidised (loses electrons or gains oxygen). The oxidising agent is reduced.
- Stating that the reducing agent “is reduced”. No — the reducing agent is oxidised. It causes reduction in the other substance.
- Forgetting that the name describes what the substance does to others, not what happens to it.
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