Oxidation – IGCSE Chemistry Definition
IGCSE Chemistry definition of oxidation: gain of oxygen, loss of hydrogen, or loss of electrons. Covers OILRIG, examples, and oxidation number changes.
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.
Oxidation has three definitions at IGCSE level, each applying in different contexts. The 0620 syllabus tests all three, and you need to identify oxidation in reactions involving oxygen gain, hydrogen loss, or electron transfer. Oxidation always occurs alongside reduction — together they form a redox reaction.
The 0620 definitions
Core definitions
- Gain of oxygen: e.g. 2Mg + O₂ → 2MgO (magnesium is oxidised)
- Loss of hydrogen: e.g. CH₃CH₂OH → CH₃CHO + H₂ (ethanol is oxidised — it loses hydrogen)
Supplement definition
- Loss of electrons: e.g. Na → Na⁺ + e⁻ (sodium is oxidised — it loses an electron)
The mnemonic OILRIG ties the electron definition together:
- Oxidation Is Loss (of electrons)
- Reduction Is Gain (of electrons)
Examples of oxidation
| Reaction | What is oxidised | How we know |
|---|---|---|
| 2Mg + O₂ → 2MgO | Magnesium | Gains oxygen |
| CuO + H₂ → Cu + H₂O | Hydrogen (H₂) | Gains oxygen (becomes H₂O) |
| Fe → Fe²⁺ + 2e⁻ | Iron | Loses electrons |
| 2Cl⁻ → Cl₂ + 2e⁻ | Chloride ions | Lose electrons |
| CH₄ + 2O₂ → CO₂ + 2H₂O | Carbon in methane | Gains oxygen |
Oxidation in electrolysis
At the anode, anions are oxidised — they lose electrons:
2Br⁻ → Br₂ + 2e⁻ (bromide ions are oxidised)
4OH⁻ → 2H₂O + O₂ + 4e⁻ (hydroxide ions are oxidised)
The substance that causes oxidation
The oxidising agent is the substance that oxidises another substance. In doing so, the oxidising agent itself is reduced. For example, in CuO + H₂ → Cu + H₂O, copper oxide is the oxidising agent (it oxidises hydrogen) and is itself reduced (loses oxygen).
Worked exam question
In the reaction 2Na + Cl₂ → 2NaCl: (a) Which substance is oxidised? (1) (b) Explain your answer in terms of electrons. (1) (c) Write the half-equation showing the oxidation. (1)
Mark scheme
(a) Sodium [1]
(b) Sodium atoms lose electrons / each Na atom loses one electron [1]
(c) Na → Na⁺ + e⁻ [1]
Common exam mistakes
- Confusing oxidation with reduction. Oxidation is loss of electrons (or gain of oxygen). Reduction is the opposite.
- Identifying the wrong substance as being oxidised. The substance that gains oxygen or loses electrons is oxidised — not the one that provides the oxygen.
- Forgetting that combustion is oxidation. Burning a fuel in oxygen is an oxidation reaction.
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