Question

why does the colour of cuso4 change when iron nail is kept emerged in it?

Answer 1

Iron is much higher than copper on the reactivity series, i.e. it is more reactive.
In this case it replaces copper in the CuSO4 to form FeSO4 (ferrous sulphate).
In simpler terms it sort of kicks out the copper which doesn't resist as it's less reactive and takes its place in the sulphate throne.
Ferrous sulphate is green in nature so the copper sulphate (CuSO4) solution turns green.