Green edge flame test tells presence of borate ion. — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Alkali metal sulfides like Na2S are highly ionic and completely soluble in water (very high solubility). Step 2: Transition metal sulfides (CuS and ZnS) are highly insoluble. Their solubilities are determined by their solubility products (Ksp). Step 3: CuS has an extremely small solubility product (Ksp ~ 10^-36) compared to ZnS (Ksp ~ 10^-23), meaning ZnS is more soluble than CuS. Thus, the order of decreasing solubility is Na2S > ZnS > CuS, corresponding to option (d).