Which of the following pairs of cations will turn borax beads blue in an oxidising flame? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: On adding ammonia solution (NH3) to various metal chlorides, they react differently based on their coordination and redox chemistry. Step 2: Mercurous chloride (Hg2Cl2, calomel) undergoes a disproportionation reaction upon treatment with ammonia: Hg2Cl2 + 2NH3 -> Hg↓ (black) + Hg(NH2)Cl↓ (white) + NH4Cl. The finely divided metallic mercury gives a black appearance. Step 3: Therefore, Hg2Cl2 leaves a black residue on the addition of NH3, which corresponds to option (c).