Explain why iron rusts more quickly when it is in contact with salt water compared with pure water.

Cambridge International IGCSE Chemistry (0620) — 9.5 Corrosion of metals · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Rusting needs oxygen and water. In salt water the dissolved Na⁺ and Cl⁻ ions make the solution an electrolyte, so it conducts electricity and ions can move freely. This accelerates the electrochemical oxidation of iron to Fe²⁺ and Fe³⁺, producing rust. Pure water has very few ions and is a poor conductor, so the oxidation process is slower.

Examiner tips

  • Mention both oxygen and water as reactants
  • Explain that salt water is an electrolyte that increases conductivity
  • Show the link between conductivity and faster oxidation
  • Keep answer concise and use correct terminology

Common mistakes

  • Failing to state that salt water is an electrolyte
  • Confusing salt water with just more water
  • Using vague terms like ‘more oxygen’ instead of ‘electrolyte conductivity’

Mark scheme (4 marks)

  1. Rusting requires both oxygen (from air) and water
  2. Salt water acts as an electrolyte / conducts electricity / allows ions to move
  3. This speeds up the electrochemical / oxidation process that causes rusting
  4. Pure water is a poor conductor / has very few ions, so rusting is slower

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