What are oxidation and reduction?
Oxidation: a substance gains oxygen or loses hydrogen. Reduction: a substance loses oxygen or gains hydrogen.
When copper powder is heated in air, its brown surface turns black: 2Cu + O₂ → 2CuO. Copper is oxidised. If hydrogen is passed over the hot black CuO, it turns brown again: CuO + H₂ → Cu + H₂O. Now CuO is reduced and H₂ is oxidised.
Redox reactions, oxidising and reducing agents
Oxidation and reduction happen together in one reaction, so it is called a redox reaction.
- Oxidising agent: gives oxygen (or removes hydrogen); it is itself reduced.
- Reducing agent: takes oxygen (or gives hydrogen); it is itself oxidised.
Examples
ZnO + C → Zn + CO: ZnO is reduced (oxidising agent), C is oxidised (reducing agent).
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂: MnO₂ is reduced, HCl is oxidised (it loses hydrogen).
At higher classes oxidation is defined as loss of electrons; in Class 10 we use gain/loss of oxygen and hydrogen.
Corrosion: rusting of iron
Corrosion is the slow eating away of a metal when it reacts with air, water, acids or gases around it. Examples: iron gets a reddish-brown coat of rust, silver turns black (silver sulphide), copper gets a green coat (basic copper carbonate).
Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O. Iron rusts only when both oxygen (air) and water are present; salt water makes it faster.
Why corrosion is harmful
Rust flakes off and exposes fresh iron, so bridges, ships, car bodies and railings get weak. Huge sums are spent every year on repairs.
Preventing corrosion
- Painting, oiling or greasing (keeps air and water away)
- Galvanising: coating with zinc
- Electroplating with chromium or tin
- Making alloys like stainless steel
Rancidity of food
When fats and oils in food are oxidised by air, their smell and taste change. This is rancidity. Old chips, namkeen and ghee kept open become rancid.
Preventing rancidity
- Adding antioxidants (substances that stop oxidation)
- Filling packets with nitrogen instead of air
- Storing in airtight containers
- Keeping food in a refrigerator and away from light
Board exam tip: "Explain corrosion and rancidity with one example each" and "identify the substance oxidised/reduced" are very common 2–3 mark questions.
Key formulas and definitions
- Oxidation = gain of O or loss of H
- Reduction = loss of O or gain of H
- Oxidising agent is reduced; reducing agent is oxidised
- CuO + H₂ → Cu + H₂O (CuO reduced, H₂ oxidised)
- Rust = Fe₂O₃·xH₂O (needs air + water)
Worked examples
1. In CuO + H₂ → Cu + H₂O, which substance is oxidised and which is reduced?
H₂ gains oxygen to form H₂O, so H₂ is oxidised. CuO loses oxygen to form Cu, so CuO is reduced.
2. In ZnO + C → Zn + CO, name the oxidising agent and the reducing agent.
ZnO gives oxygen to carbon, so ZnO is the oxidising agent. Carbon takes the oxygen, so C is the reducing agent.
3. In MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is oxidised?
HCl loses hydrogen to become Cl₂, so HCl is oxidised. MnO₂ loses oxygen, so it is reduced.
4. Three test tubes have iron nails: A in tap water with air, B in boiled water covered with oil, C with dry calcium chloride (dry air). Where does rust form?
Only in A. B has no air (boiling removes dissolved air and oil stops more entering), C has no water. Rusting needs both.
5. Why are chips packets filled with nitrogen?
Nitrogen is unreactive and pushes out oxygen, so the oils in the chips are not oxidised and do not go rancid.
6. Why does galvanised iron not rust even if the zinc coat is scratched?
Zinc is more reactive than iron, so zinc gets oxidised first and protects the iron below it.
Common mistakes
- Saying the oxidising agent is oxidised. It is the opposite: the oxidising agent gets reduced.
- Thinking iron rusts in dry air or in air-free water. It needs both oxygen and water.
- Calling rancidity 'food going bad due to germs'. Rancidity is oxidation of fats and oils.
- Forgetting the hydrogen rule: losing hydrogen is also oxidation (HCl → Cl₂).