Iron rusts when it is exposed to both oxygen and water. Describe what happens to the iron atoms during rusting in terms of oxidation and reduction, and identify which substance is reduced.

Cambridge International IGCSE Chemistry (0620) — 6.4 Redox · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Rusting is an example of a redox reaction in which iron is converted to iron(III) oxide.

Model answer (4 marks)

Iron atoms lose electrons and are oxidised to Fe³⁺.
Oxygen atoms gain the electrons lost by iron and are reduced to O²⁻.
The reduced substance is oxygen.

Examiner tips

  • Use the word ‘oxidised’ for iron and ‘reduced’ for oxygen. Show the electron transfer: Fe → Fe³⁺ + 3e⁻, O₂ + 4e⁻ → 2O²⁻. Mention that iron is the oxidised species and oxygen the reduced species.
  • common_mistakes
  • :
  • Confusing iron as the reduced species. Forgetting to state that oxygen gains electrons. Using ‘oxidised’ for oxygen or ‘reduced’ for iron.

Mark scheme (4 marks)

  1. Iron atoms lose electrons during rusting
  2. Iron is oxidised
  3. Oxygen gains electrons
  4. Oxygen is the substance that is reduced

Key terms in this question

oxidation · reduction

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