Iron rusts when it reacts with oxygen and water. A student leaves an iron nail in moist air and notices that the mass of the nail increases over time. Explain why the mass of the nail increases, and describe what would happen to the total mass of the nail and the surrounding air as the reaction takes place.

Edexcel GCSE Chemistry (1CH0) — 1.8 Calculations involving masses · Explain · 4 marks · View as Markdown

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

Iron can react with oxygen in moist air to form iron oxide (rust). The chemical formula of rust is Fe₂O₃.

Model answer (4 marks)

The iron nail reacts with oxygen (and water) from the moist air to give iron(III) oxide, Fe₂O₃. Oxygen atoms are added to the nail, so its mass increases. The surrounding air loses the same amount of mass that the nail gains, so the total mass of nail plus air remains unchanged – mass is conserved.

Examiner tips

  • Use the word ‘reacts’ and mention oxygen and water as reactants
  • Explain that atoms are added to the nail, not created
  • State that the total mass of nail + air is unchanged
  • Show understanding of conservation of mass

Common mistakes

  • Saying the nail’s mass increases because it ‘absorbs’ oxygen without mentioning the reaction
  • Claiming the air’s mass increases instead of decreases
  • Ignoring the role of water or the conservation of mass

Mark scheme (4 marks)

  1. Oxygen (and water) from the air join with the iron to form iron oxide (rust)
  2. The mass of the nail increases because atoms of oxygen (and water) from the air are added to it
  3. The total mass of the nail plus the surrounding air stays the same / is conserved
  4. Because atoms are neither created nor destroyed during a chemical reaction (conservation of mass)

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