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Why does the colour of copper sulphate solution changes when an iron nail stepped in it?

Solution :
The change in colour occurs due to a displacement reaction in which an iron nail when dipped in copper sulphate solution, a brown coating of copper is formed on the surface of iron because iron displaces copper from the solution of copper sulphate as iron is more reactive than copper and colour of copper sulphate solution changes from blue to pale green.
CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)

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When an iron nail is immersed in the solution of copper sulphate then, iron displaces copper from the solution of copper sulphate because iron is more reactive than copper. Therefore copper sulphate solution colour changes from blue to pale green.
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CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)
In this reaction following two processes take place:

  • Cu²⁺ + 2e = Cu⁰ (re­duc­tion process, Cu²⁺ is the ox­i­dizing agent)
  • Fe⁰ – 2e = Fe²⁺ (ox­i­da­tion process, Fe⁰ is the re­duc­ing agent)

The reaction can be a double displacement reaction as well as a redox reaction (oxidation and reduction both take place simultaneously).

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