Group reagent: HCl → H+ + Cl– ; H2S ⇌ 2H+ + S2-
Radicals: Cu2+, Bi2+, Sn2+
Here HCl is a strong electrolyte under goes complete dissociation where as H2S is a weak electrolyte under goes partial dissociation and attains equilibrium between it’s ions and molecules.
In this solution, because of common ion (H+) effect supression of degree of dissociation of H2S takes place. As a result [S2-] decreases. Hence the metal radical forms it’s respective sulphides [CuS, SnS, BiS] whose solubility product is less than ionic product. Hence forms precipitate.