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9.2.2Metals

Rusting of Iron — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 11 Unit quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Part 1 of 4What is rusting

The reaction
  • The reaction Rusting is the reaction of iron with both oxygen and water.
  • The word equation isiron + oxygen + water → hydrated iron(III) oxide.
Rust
  • Rust Rust is reddish-brown hydrated iron(III) oxide.
    • It is flaky and weak, so it flakes off and exposes fresh iron underneath.
Oxidation
  • Oxidation The iron is oxidised during rusting because it gains oxygen.
Only iron and steel
  • Only iron and steel Steel is mostly iron, so it rusts too.
    • Other metals that break down in air are said to corrode, not rust.

Part 2 of 4Results and conclusion

TubeWaterOxygenResult
1YesYesNail rusts
2YesNoNo rust
3NoYesNo rust

Part 3 of 4Preventing rust by barriers

Barrier methods
Painting
  • Painting Used on bridges, cars and railings. The paint keeps water and oxygen away from the surface.
Oiling and greasing
  • Oiling and greasing Used on moving parts such as chains and tools, where paint would wear off.
Plastic coating
  • Plastic coating Used on items like dish racks and wire fences; it forms a tough, waterproof layer.
Limitation
  • Limitation If a barrier is scratched or worn away, the iron is exposed to oxygen and water again and starts to rust.

Part 4 of 4Why the controls matter

1Only one condition changes
  • Only one condition changes Between tubes, a single factor (water or oxygen) is removed.
2Same metal and time
  • Same metal and time Nails are identical and left for the same period.
3Fair comparison
  • Fair comparison Any difference in rusting can be linked to the condition that changed.
4Valid conclusion
  • Valid conclusion Both water and oxygen are needed; neither alone causes rust.

Key terms

TermMeaning
RustingThe oxidation of iron or steel in the presence of water and oxygen.
RustHydrated iron(III) oxide, a flaky reddish-brown solid.
CorrosionThe gradual breakdown of a metal by reaction with substances in its surroundings.
Barrier methodA coating that keeps oxygen and water away from the metal surface.
Drying agentA substance such as calcium chloride that absorbs water vapour.

Avoid these mistakes

1
Saying that all metals rust. Only iron and steel rust; other metals corrode.
2
Forgetting that both oxygen and water are required. Either one alone is not enough.
3
Not explaining the purpose of the oil layer in the experiment: it stops air re-entering the boiled water.
4
Assuming a coating protects iron for ever. Once scratched, the metal rusts again.

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