Question

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

Answer 1

It changes from blue to pale green because iron being more reactive than copper, displaces copper from it's aqueous salt solution. As a result colour of solution changes and copper gets deposited on iron nail.

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