Zinc powder is added to copper(II) sulfate solution. The blue colour of the solution fades and a reddish-brown solid forms. Explain why this reaction is a redox reaction, referring to the changes in oxidation state of both zinc and copper.

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

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

Zinc powder is added to copper(II) sulfate solution. The blue colour of the solution fades and a reddish-brown solid forms.

Model answer (4 marks)

Zinc is oxidised:
1. Zn (0) → Zn²⁺ + 2e⁻ – the oxidation state rises from 0 to +2.
2. Copper(II) ions are reduced: Cu²⁺ + 2e⁻ → Cu (0) – the oxidation state falls from +2 to 0.
The loss of electrons by zinc and the gain of electrons by copper shows that the reaction is a redox process.

Examiner tips

  • Show the electron transfer explicitly; use the +2 and 0 oxidation states.
  • Use the words ‘oxidised’ and ‘reduced’ to match the mark scheme.
  • Mention the colour change as evidence of Cu²⁺ disappearing and Cu forming.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Writing Zn²⁺ → Zn instead of Zn → Zn²⁺ (wrong direction).
  • Forgetting to state the change in oxidation state numbers.
  • Using ‘oxidation’ for both elements instead of ‘oxidised’ for Zn and ‘reduced’ for Cu.

Mark scheme (4 marks)

  1. Zinc loses electrons / is oxidised
  2. Oxidation state of zinc increases from 0 to +2
  3. Copper(II) ions gain electrons / are reduced
  4. Oxidation state of copper decreases from +2 to 0

Key terms in this question

redox · oxidation · oxidation state

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