Shashank was asked to carry out a displacement reaction which would show the following:

(i) Formation of colourless solution

(ii) Black deposits

The reactants he should use are:

To obtain a colourless solution and black deposits via a displacement reaction, we must choose a more reactive metal that can displace a less reactive metal from a colored solution to form a colourless solution.

• Why Option C is correct: Zinc ($\text{Zn}$) is more reactive than copper ($\text{Cu}$). When zinc is added to a blue copper sulphate ($\text{CuSO}_4$) solution, it displaces copper to form a colourless zinc sulphate ($\text{ZnSO}_4$) solution. The displaced copper deposits on the zinc granules as a very finely divided powder, which appears black or dark brown.

$$\text{Zn(s)} + \text{CuSO}_4\text{(aq) [blue]} \rightarrow \text{ZnSO}_4\text{(aq) [colourless]} + \text{Cu(s) [black deposit]}$$

• Why Option A is incorrect: Iron ($\text{Fe}$) is less reactive than aluminium ($\text{Al}$), so no displacement reaction occurs.
• Why Option B is incorrect: Although aluminium is more reactive than iron, aluminium metal has a tough, protective oxide layer ($\text{Al}_2\text{O}_3$) on its surface which prevents it from reacting readily with iron(II) sulphate solution under normal conditions.
• Why Option D is incorrect: Iron ($\text{Fe}$) is less reactive than zinc ($\text{Zn}$), so no reaction occurs.