A student leaves an iron nail outside in the rain for several weeks. The nail becomes covered in a reddish-brown solid. The student then places a second iron nail in a bowl of warm water and observes bubbles forming on its surface. Describe the differences between these two processes, stating which is a physical change and which is a chemical change, and giving a reason for each classification.

Cambridge International IGCSE Chemistry (0620) — 6.1 Physical and chemical changes · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The first nail develops a reddish‑brown solid – iron oxide (rust). This is a chemical change because a new substance is formed and the process is irreversible.
The second nail produces bubbles in warm water. This is a physical change because no new substance is formed; the iron remains iron and the bubbles are simply gas released, a reversible change.

Examiner tips

  • Use the word ‘chemical change’ for rust, ‘physical change’ for bubbling.
  • Explain irreversibility for rust and reversibility for gas release.
  • Mention new substance for rust, same substances for bubbling.
  • Keep answer concise and use correct terminology.

Common mistakes

  • Saying both are chemical changes.
  • Confusing rust with oxidation of water.
  • Failing to state that bubbles are gas, not a new substance.

Mark scheme (4 marks)

  1. The formation of reddish-brown solid (rust/iron oxide) on the first nail is a chemical change
  2. Because a new substance (iron oxide / rust) is formed / the change is irreversible / difficult to reverse
  3. Bubbles forming on the second nail in warm water is a physical change
  4. Because no new substance is formed / the change can be reversed / the same substances are still present

Key terms in this question

physical change · chemical change

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