Question

why does colour of copper sulphate solution change when an iron nail is dipped in it

Answer 1

The colour of copper sulphate solution change when iron nail is dipped in it because iron displaces copper from it's salt solution and combines with sulphate forming iron sulphate and leaving copper behind.
Iron is more reactive than copper.Thus it displaces copper.

Answer 2

The colour of copper sulphate solution changes when an iron nail is dipped in it. this is because iron displaces copper from Cooper sulphate solution and the solution becomes of iron sulphate .

Fe+. CuSo4 ==>. FeSo4. + Cu
Iron. copper sulphate. Iron sulphate.