Question

Why does the colour of copper sulphate solution change when an iron nail is kept immersed in it ?

Answer 1

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.Therefore, the blue colour of copper sulphate solution fades and green colour appears.

$CuS{O}_4+Fe⟶FeS{O}_4+Cu$