The color of copper sulphate changes from blue to pale green when an iron nail is dipped into it. This is due to a chemical reaction between the iron in the nail and the copper sulphate solution. During this reaction, the following steps take place:

1. Oxidation of Iron: The iron metal undergoes oxidation, where its atoms lose electrons. As a result, iron atoms convert into ferrous (Fe2+) ions.

2. Reduction of Copper: In copper sulfate solution, the copper ions (Cu2+) present in the solution gain the electrons lost by the iron atoms. This results in the reduction of Cu2+ ions to copper atoms. These copper atoms then form a coating of metallic copper on the surface of the iron nail.

The overall chemical reaction can be represented as:

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

As iron atoms replace copper atoms in copper sulfate, the deep blue color of the solution fades and a pale green color of ferrous sulfate (FeSO4) solution is visible.