Question

why does the colour of copper sulphate solution change when an iron nail is dipped in it

Answer 1

due to displacement reaction. as iron is highly reactive metal than copper. so iron displaces copper and form FeSO4
following reaction takes place :
Fe+CuSO4 gives FeSO4 +Cu

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)