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

The blue colour of copper sulphate solution fades when an iron nail is dipped in it because copper sulphate is converted into iron sulphate. This is a classic example of a Single Displacement Reaction. Because iron is more reactive than copper, it displaces copper from its salt solution. Chemical Equation: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s). The original blue colour is due to Cu2+ ions in the copper sulphate solution. As the reaction proceeds, these ions are replaced by Fe2+ ions, forming Ferrous Sulphate (FeSO4), which is light green in colour. This causes the blue to fade. Beyond the fading liquid, you will see a reddish-brown coating of copper metal depositing on the surface of the iron nail.