In the qualitative analysis of Group II cations, hydrogen sulfide (...

$\mathrm{HCl}$ is a strong acid that completely dissociates to give $\mathrm{H}^{+}$ and $\mathrm{Cl}^{-}$ ions.

$\mathrm{HCl} \rightarrow \mathrm{H}^{+} + \mathrm{Cl}^{-}$

Hydrogen sulfide ($\mathrm{H_2S}$) is a weak diprotic acid that ionizes as:

$\mathrm{H_2S} \rightleftharpoons 2\mathrm{H}^{+} + \mathrm{S}^{2-}$

The presence of $\mathrm{H}^{+}$ ions from $\mathrm{HCl}$ suppresses the ionization of $\mathrm{H_2S}$ due to the common ion effect. This significantly decreases the concentration of $\mathrm{S}^{2-}$ ions in the solution. The reduced $[\mathrm{S}^{2-}]$ is sufficient to exceed the extremely low solubility product ($K_{\mathrm{sp}}$) of Group II sulfides, but it is too low to precipitate Group IV sulfides, thereby ensuring selective precipitation.

Answer: decrease the concentration of $\mathrm{S}^{2-}$ ions by common ion effect