A few drops of NaOH solution was added to an unknown salt forming a white precipitate which is insoluble in excess solution. The cation likely present is

Analysis of the Question

The question asks us to identify a metal cation based on its reaction with Sodium Hydroxide (NaOH). Specifically, we are looking for a cation that forms a white precipitate that remains insoluble when excess NaOH is added.

Concept Explanation: Qualitative Analysis of Cations

In chemistry, we use NaOH to identify metal ions in a solution. When NaOH is added to a solution containing certain metal ions, it forms a metal hydroxide precipitate. The key to this question is the behavior of these precipitates in excess NaOH:

• Amphoteric Hydroxides: Some metal hydroxides (like those of $Zn^{2+}$, $Pb^{2+}$, and $Al^{3+}$) are amphoteric. This means they can react with both acids and bases. When you add excess NaOH to these, the precipitate dissolves to form a soluble complex (e.g., a zincate or aluminate).
• Non-amphoteric Hydroxides: Some metal hydroxides (like those of $Ca^{2+}$ and $Mg^{2+}$) are basic and do not react with excess NaOH. Therefore, the white precipitate remains visible even after adding more NaOH.

Step-by-Step Solution

1. Evaluate $Zn^{2+}$ (Option A): Adding NaOH to $Zn^{2+}$ forms a white precipitate of $Zn(OH)_2$. This precipitate is amphoteric and dissolves in excess NaOH to form a colorless solution of sodium tetrahydroxozincate(II).
2. Evaluate $Pb^{2+}$ (Option B): Adding NaOH to $Pb^{2+}$ forms a white precipitate of $Pb(OH)_2$. This is also amphoteric and dissolves in excess NaOH to form a colorless solution of sodium tetrahydroxoplumbate(II).
3. Evaluate $Ca^{2+}$ (Option C): Adding NaOH to $Ca^{2+}$ forms a white precipitate of $Ca(OH)_2$. Calcium hydroxide is not amphoteric; it does not react with excess NaOH, so the white precipitate remains insoluble.
4. Evaluate $Al^{3+}$ (Option D): Adding NaOH to $Al^{3+}$ forms a white gelatinous precipitate of $Al(OH)_3$. This is amphoteric and dissolves in excess NaOH to form a colorless solution of sodium tetrahydroxoaluminate(III).

Conclusion

Since $Zn^{2+}$, $Pb^{2+}$, and $Al^{3+}$ all form precipitates that dissolve in excess NaOH, only $Ca^{2+}$ fits the description of forming a white precipitate that is insoluble in excess NaOH.

Exam Tip

Remember the acronym "ZAP" for cations that form precipitates which dissolve in excess NaOH: Zinc ($Zn^{2+}$), Aluminium ($Al^{3+}$), and Plead ($Pb^{2+}$). If you see these in a question about "excess NaOH," you know they will dissolve!