Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

When an iron nail is dipped into a copper sulfate (CuSO₄) solution, a displacement reaction occurs. Iron is more reactive than copper, so it displaces copper ions from the copper sulfate solution. The chemical equation for this reaction is as follows:

Fe(s)+CuSO₄(aq) → FeSO₄ (aq) + Cu(s)
In words, iron reacts with copper sulfate to form ferrous sulfate (FeSO₄) and copper. The copper produced is in solid form and will appear as a reddish-brown coating on the surface of the iron nail.

The change in color of the copper sulfate solution is due to the formation of this solid copper, which is no longer in the form of copper ions in the solution. The initial blue color of copper sulfate solution fades as copper metal is deposited on the iron surface, leading to a color change in the solution.