In qualitative analysis of cation of second group, H2 S gas is passed in presence of HCl, sulphide ions obtained is sufficient for the precipitation of second group cations in form of their sulphides due to lower value of their solubility product (Ksp). Hence, fourth group cations are not precipitated because they require for exceeding their ionic product to their solubility products and higher sulphide ions concentration due to their higher Ksp which is not obtained here due to common ion effect. common ion Caution HCl is added before H2 S gas during qualitative analysis of second group radicals because the concentration of sulphide ions is suppressed by HCl due to common ion effect. The already present H+ions suppress the dissociation of H2 S by moving the reaction in the backward reaction.