The Sulphide which is insoluble in dilute hydrochloric acid is

Analyze the question: We need the sulphide that does not dissolve (does not react) in dilute HCl.

Concept: Metal sulphides react with dilute HCl to release hydrogen sulphide gas (H₂S) if the sulphide is soluble in acid. This happens when the sulphide's solubility product (Ksp) is relatively high. Sulphides with very low Ksp do not dissolve in dilute acid.

• Group I/II sulphides (e.g. Na₂S): soluble in water and react readily with acid.
• Sulphides of Fe and Zn: dissolve in dilute HCl, releasing H₂S.
• Sulphides of Cu, Pb, Cd, Hg, As: very insoluble, so they are not dissolved by dilute HCl. This is the basis of Group II in qualitative analysis, where H₂S in acidic solution precipitates them.

Solution:

• A. FeS + 2HCl → FeCl₂ + H₂S↑. It dissolves. (This is the lab method for preparing H₂S.)
• B. CuS: Ksp is extremely small (about 10⁻³⁶), so it does not dissolve in dilute HCl. Insoluble.
• C. ZnS + 2HCl → ZnCl₂ + H₂S↑. It dissolves in dilute acid.
• D. Na₂S: it is soluble in water and reacts with HCl to give NaCl + H₂S.

Answer: The sulphide insoluble in dilute hydrochloric acid is CuS (B).

Tip: Remember that CuS, PbS, CdS and HgS are the "black sulphides" that are not dissolved by dilute HCl. They precipitate even in acidic medium. FeS and ZnS only precipitate in alkaline or neutral conditions, and they dissolve in dilute acid.