It is a displacement reaction. Because an iron nail dropped in the solution is more reactive than copper sulphate and displaces the molecules, the copper sulphate solution changes colour. As the copper sulphate solution turns into iron sulphate solution, the copper deposits on the nail and the colour change from blue to green.
Why does the colour of copper sulphate solution change when an iron nail is kept immersed in it?
Why does the colour of copper sulphate solution change when an iron nail is kept immersed in it?