Rusting – IGCSE Chemistry Definition
IGCSE Chemistry definition of rusting: the corrosion of iron requiring water and oxygen. Covers conditions, prevention methods, and exam tips for Cambridge 0620.
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.
Rusting is one of the most practical topics in IGCSE Chemistry. Examiners test the conditions required, the methods of prevention, and the controlled experiment that proves both water and oxygen are needed. This last point — the experimental proof — is a favourite Paper 4 and Paper 6 question.
The 0620 definition
Rusting is the corrosion of iron (or steel). It requires both water and oxygen. The product is hydrated iron(III) oxide, Fe₂O₃·xH₂O.
The word equation: iron + water + oxygen → hydrated iron(III) oxide (rust)
The controlled experiment
Three test tubes, each containing an iron nail:
| Tube | Conditions | Result |
|---|---|---|
| A | Water and air (open tube, water half-covering nail) | Rusts |
| B | Water only (boiled water with oil layer to exclude air) | No rust |
| C | Air only (dry air with anhydrous calcium chloride to absorb moisture) | No rust |
This proves both water and oxygen are necessary — removing either one prevents rusting.
Methods of prevention
| Method | How it works | Example use |
|---|---|---|
| Painting / coating | Barrier — keeps water and oxygen away | Car bodies, bridges |
| Oiling / greasing | Barrier | Bicycle chains, machinery |
| Plastic coating | Barrier | Fridge shelves, paper clips |
| Galvanising | Zinc coating — barrier + sacrificial protection | Gates, buckets, roofing |
| Sacrificial protection | A more reactive metal corrodes instead | Ships’ hulls, pipelines |
| Stainless steel | Chromium forms a protective oxide layer | Cutlery, sinks |
Why galvanising is better than painting
If the coating is scratched:
- Paint: water and oxygen reach the iron → rusting begins at the scratch
- Zinc (galvanising): zinc is more reactive than iron, so zinc corrodes preferentially (sacrificial protection). The iron is still protected even at the scratch.
Worked exam question
An iron fence is galvanised to prevent rusting. (a) State what is meant by galvanising. [1] (b) Explain why galvanising still protects the iron even if the coating is scratched. [2]
Mark scheme
(a) Coating iron/steel with a layer of zinc [1]
(b) Zinc is more reactive than iron [1]; zinc corrodes/reacts preferentially instead of the iron / sacrificial protection [1]
Common exam mistakes
- Saying only water is needed for rusting — both water and oxygen are required.
- Confusing barrier protection (paint) with sacrificial protection (zinc). The examiner wants you to know the difference.
- Describing rust as “iron oxide” without “hydrated” — the correct formula includes water of crystallisation.
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