Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level 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, how readily an atom gives up electrons to become a positive ion decides where the metal sits in the reactivity series — and that one position predicts its reactions with water, steam and acid, whether it displaces another metal, whether it can protect iron from rusting, and how hard it is to pull out of its ore.
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 is a common and expensive slip. Where Lesson 9.5A.
- 18. “The aluminium window frames have rusted.” 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 because of its oxide layer. Transfer Name two other metals that corrode without rusting. Copper, which goes green; silver, which tarnishes.
Questions students ask about Rusting
What conditions are needed for iron to rust?
Rusting needs both oxygen and water; if either is absent, rust does not form. Salt speeds up rusting but is not a required condition — a nail rusts perfectly well in pure water and air. The product is hydrated iron(III) oxide, and the full name must include both the oxidation state and the water of hydration.