In qualitative analysis, to identify the IIA group sulphides, HCl is added to salt solution before the addition of H2S. Because

HCl is added before adding H2S, So that the dissociation of H2S is suppressed in presence of HCl due to common ion effect. This is desired as the solubilities of group II Cations is less and a low concentration of S-2 ions is sufficient to precipitate their sulphides.If HCl is not added, then S-2 ion concentration will be high enough to precipitate group IV cations.