Which of the following pairs of cations can be separated by using on adding NaOH solution? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Sodium hydroxide (NaOH) reacts with many metal cations to precipitate their hydroxides, some of which are amphoteric and dissolve in excess NaOH. Step 2: Cu2+ forms a light blue precipitate of Cu(OH)2 which is basic and completely insoluble in excess NaOH. Zn2+ forms Zn(OH)2 which is amphoteric and dissolves in excess NaOH to form soluble zincate ([Zn(OH)4]2-). Step 3: This allows complete separation of Cu2+ (precipitated) from Zn2+ (dissolved). In contrast, the other pairs (Pb2+/Al3+, Sn2+/Pb2+, Zn2+/Pb2+) consist of two amphoteric species that both dissolve in excess NaOH. Thus, option (a) is correct.