Cambridge IGCSE Chemistry · Syllabus 0620 · Metals
Rusting
What is Rusting?
The corrosion of iron and steel, which happens only when both oxygen and water are present and produces hydrated iron(III) oxide. Salt speeds rusting up but is not one of the required conditions, and the word rust applies to iron and steel alone, not to the corrosion of other metals.
This definition is part of the Metals chapter in Cambridge IGCSE Chemistry.
Rusting in context
Everything in Topic 9 hangs on one chain. A metal is a lattice of positive ions in a sea of delocalised electrons, and that structure gives the physical properties. Those properties decide what each metal is used for. Separately, metals can be put in order of how readily they react — the reactivity series — and that one position predicts a metal’s reactions with water, steam and acid, whether it can protect iron from rusting, and how hard it is to pull out of its ore. Extended candidates add a second layer to the same chain: reactivity is the tendency of an atom to form a positive ion, and that is what displacement reactions and sacrificial protection actually measure.
Common mistakes with Rusting
- 7. “Iron needs oxygen, water and salt to rust.” Repair The two required conditions are oxygen and water. Salt speeds rusting up but is not necessary — a nail rusts perfectly well in pure water and air. Listing salt as a condition contradicts the very experiment that establishes the conditions. Where Lesson 9.5A. Core 9.5.1
- 18. “The aluminium window frames have rusted.” Core 9.5.1 Defect Uses “rust” for a metal that cannot rust. Reason Rusting is specific to iron and steel. Other metals corrode. Repair “The aluminium has corroded” — and in fact aluminium resists corrosion well, which is why it is used for food containers (Core 9.2.1). Transfer Name two other metals that corrode without rusting. Copper, which goes green; silver, which tarnishes. Extended Supplement 9.4.5 Extended candidates can add why aluminium resists corrosion: a thin, tough, continuous oxide layer keeps air and water off the metal. Core candidates are not asked for it.