Why does the blue colour of copper sulphate solution fade and turn green when an iron nail is added?

Correct answer: Iron displaces copper from copper sulphate and forms iron sulphate.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. In this displacement reaction, pale-green iron sulphate forms and copper is deposited as a reddish-brown coating on the nail: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Therefore, the blue solution fades and appears green. Option B is incorrect because copper is less reactive than iron and cannot displace iron.