When an iron nail is placed in copper sulphate solution, a decrease in the concentration of which ion causes the blue colour of the solution to fade?

Correct answer: Copper(II) ion, \(\mathrm{Cu^{2+}}\)

The blue colour of copper sulphate solution is due to hydrated copper(II) ions, \(\mathrm{Cu^{2+}}\). Since iron is more reactive, it displaces copper(II) ions from the solution: \(\mathrm{Fe(s)+CuSO_4(aq)\rightarrow FeSO_4(aq)+Cu(s)}\). Thus, the concentration of \(\mathrm{Cu^{2+}}\) decreases and the blue colour fades. In contrast, \(\mathrm{Fe^{2+}}\) ions are formed during the reaction; they do not decrease.