copper sulphate solution (CuSO4) is blue in colour. When iron nail placed in it , we can observe the following :-
1. reddish brown deposits on iron (these are of copper)
2. colour of the solution turn into blue to light green .
The formula of these reaction is :- Fe+CuSO4 ------ FeSO4 + Cu
Here, Fe more reactive than Cu . So Fe displace Cu to form iron sulphate and cupper .