Aqueous solution of sodium hydroxide can be used to test for the presence of : I. Ca$^{2+}$ ,   II. Zn$^{2+}$ ,  III. Cu$^{2+}$

Analysis of the Question

The question asks us to identify which of the given metal ions ($Ca^{2+}$, $Zn^{2+}$, and $Cu^{2+}$) can be identified using an aqueous solution of sodium hydroxide ($NaOH$). In qualitative analysis, $NaOH$ is used as a reagent to form characteristic precipitates with various metal cations.

Explaining the Concept

When aqueous $NaOH$ is added to solutions containing these metal ions, they react to form metal hydroxides. We look for distinct physical changes, such as the color of the precipitate or its solubility in excess $NaOH$.

1. $Ca^{2+}$ (Calcium ion): When $NaOH$ is added, it forms a white precipitate of calcium hydroxide: $Ca^{2+}_{(aq)} + 2OH^-_{(aq)} \rightarrow Ca(OH)_{2(s)}$ (White precipitate)
2. $Zn^{2+}$ (Zinc ion): When $NaOH$ is added, it forms a white precipitate of zinc hydroxide. Crucially, this precipitate is amphoteric and dissolves in excess $NaOH$ to form a colorless solution of sodium tetrahydroxozincate(II): $Zn^{2+}_{(aq)} + 2OH^-_{(aq)} \rightarrow Zn(OH)_{2(s)}$ (White precipitate) $Zn(OH)_{2(s)} + 2OH^-_{(aq)} \rightarrow [Zn(OH)_4]^{2-}_{(aq)}$ (Colorless solution)
3. $Cu^{2+}$ (Copper(II) ion): When $NaOH$ is added, it forms a characteristic pale blue precipitate of copper(II) hydroxide: $Cu^{2+}_{(aq)} + 2OH^-_{(aq)} \rightarrow Cu(OH)_{2(s)}$ (Pale blue precipitate)

Step-by-Step Solution

• Step 1: Test for $Ca^{2+}$: $NaOH$ produces a white precipitate. This confirms the presence of $Ca^{2+}$.
• Step 2: Test for $Zn^{2+}$: $NaOH$ produces a white precipitate that dissolves in excess. This confirms the presence of $Zn^{2+}$.
• Step 3: Test for $Cu^{2+}$: $NaOH$ produces a pale blue precipitate. This confirms the presence of $Cu^{2+}$.

Since all three ions produce distinct, observable reactions with aqueous sodium hydroxide, $NaOH$ is a valid reagent for testing the presence of all three.

Conclusion

Therefore, the correct option is C.

Exam Tip

When dealing with qualitative analysis questions, always remember the "Amphoteric Trio": Zinc ($Zn^{2+}$), Lead ($Pb^{2+}$), and Aluminium ($Al^{3+}$). These three form white precipitates with $NaOH$ that dissolve in excess. Knowing this will help you quickly identify them in both JAMB and WASSCE practical-based theory questions!