Question

Why colour of cuso4 solution changes when iron nail put into the solution

Answer 1

Displacement reaction takes place and thus Fe diffuses Cu the colour also change.
CuSO4 + Fe —- FeSO4 + Cu

Answer 2

It is because when an Iron nail is put into CuSO₄ solution, the following reaction takes place.

                        Fe  +  CuSO₄  --------->  FeSO₄  +  Cu

As the reaction proceeds, Iron slowly displaces copper from copper sulphate, and ferrous sulphate is formed. As the concentration of copper sulphate decreases gradually, the blue colour of it also disappears slowly.

Thus the colour of CuSO₄ changes when an iron is put in it.