Q.:-Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Iron is more reactive than copper. So, when an iron nail is dipped in a copper sulphate solution, iron displaces copper from its solution to form iron sulphate, which is green in color.
Fe (s) + CuSO4 (aq) –––> FeSO4 (aq) + Cu (s)
Hence, the blue color of copper sulphate solution changes into green color because of this displacement reaction.