Question

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

Answer 1

The colour of copper sulphate soltuion changes because as iron is more reactive than copper it displace Cu and and forms FeSO4 due to this the colour of the solution changes
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Answer 2

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When an iron nail dipped in the copper sulphate solution than iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes.

The reaction involved here is:

Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)