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When iron nails are placed in copper sulphate solution, the blue colour of copper sulphate solution become light and turns into light green  because iron displaces copper from copper sulphate solution. this is caled displacment reaction
Copper sulphate + Iron → Iron sulphate + Copper
CuSO4 + Fe --> FeSO4 + Cu
cuso4reacts with fe and fe displaces cu to give feso4.

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 .