Rusting of Iron
Section: 9. Metals | Syllabus: Cambridge IGCSE Chemistry (0620)
Conditions Required for Rusting
Rusting is the corrosion of iron and steel to form hydrated iron(III) oxide, Fe₂O₃·xH₂O - a flaky, brown/orange solid. Rusting only happens when both oxygen and water are present; if either is missing, iron does not rust.
4Fe(s) + 3O₂(g) + xH₂O(l) → 2Fe₂O₃·xH₂O(s)
Investigating the Conditions for Rusting
An iron nail is placed in four test tubes with different conditions:
| Test tube | Conditions | Oxygen? | Water? | Rusts? |
|---|---|---|---|---|
| 1 | nail in air and water | yes | yes | yes |
| 2 | nail in boiled water, sealed under a layer of oil (excludes air) | no | yes | no |
| 3 | nail in dry air, with a drying agent (e.g. calcium chloride) | yes | no | no |
Only the nail with access to both oxygen and water (tube 1) rusts
Worked Example
- Question: an iron nail is placed in boiled, cooled water with a layer of oil on top. Explain why the nail does not rust. [2]
- Answer: boiling removes dissolved oxygen from the water [1]; the oil layer stops oxygen from the air re-dissolving into the water, so the nail has water but no oxygen, and rusting needs both [1]
Preventing Rusting with Barrier Methods
Common barrier methods protect iron and steel by physically excluding oxygen and/or water from the surface:
| Method | Example uses |
|---|---|
| Painting | railings, bridges, car bodies |
| Greasing/oiling | tools, bicycle chains, moving machine parts |
| Coating with plastic | dishwasher racks, garden furniture |
All three work the same way: they form a continuous physical barrier over the iron, so oxygen and water in the air cannot reach the metal surface - and without both of these, rusting cannot start.
Common Mistakes
- ❌ Saying a barrier coating "stops the iron reacting" without explaining how
- ✓ always state what is being excluded - a barrier method works specifically by keeping oxygen and/or water away from the iron surface, not by some other mechanism
- ❌ Assuming a barrier method still protects the iron once it is scratched or damaged
- ✓ a barrier method only works while the coating is intact - once broken, the exposed iron is free to rust (unlike sacrificial protection, covered in the next topic)
Interactive revision notes, videos and practice questions load below.