Which of the following pairs of sulphides are insoluble in dilute HCl? (a) CoS and NiS (b) CoS and M — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Group IV sulfides include CoS, NiS, MnS, and ZnS. Step 2: Although CoS and NiS require basic conditions to precipitate initially, once formed, they undergo a rapid transition (aging) to a highly stable crystalline form that is completely insoluble in dilute HCl. Step 3: ZnS and MnS, on the other hand, easily dissolve in dilute HCl. Therefore, CoS and NiS are the pair insoluble in dilute HCl, matching option (a).