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Step-by-Step Explanation
Step 1: Ionization of H₂S
Hydrogen sulfide gas ($\text{H}_2\text{S}$) in aqueous solution partly ionizes as shown below:
$\text{H}_2\text{S} \rightleftharpoons \text{H}^+ + \text{HS}^-.$
Further, $\text{HS}^-$ can also dissociate:
$\text{HS}^- \rightleftharpoons \text{H}^+ + \text{S}^{2-}.$
Hence, the overall concentration of sulfide ions ($\text{S}^{2-}$) depends on the extent of ionization of $\text{H}_2\text{S}$ in water.
Step 2: Effect of Adding HCl (Common Ion Effect)
When hydrochloric acid (HCl) is added to the solution, it furnishes additional $\text{H}^+$ ions:
$\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-.$
Because the solution gains a higher concentration of $\text{H}^+$ ions, the equilibrium for the ionization of $\text{H}_2\text{S}$ shifts to the left (toward the non-ionized $\text{H}_2\text{S}$) in order to reduce the excess $\text{H}^+$ (Le Chatelier’s principle). This process is known as the common ion effect. As a result, the concentration of free $\text{S}^{2-}$ ions in the solution decreases significantly.
Step 3: Precipitation of Group II Cations
In qualitative analysis, Group II cations such as $\text{Pb}^{2+}$, $\text{Hg}^{2+}$, $\text{Cu}^{2+}$, and others, form sulfides that have very low solubility products ($K_{\text{sp}}$). Even a small concentration of $\text{S}^{2-}$ is sufficient to exceed the solubility product ($K_{\text{sp}}$) of these sulfides, causing them to precipitate out from the solution.
Step 4: Non-precipitation of Group IV Cations
On the other hand, Group IV cations (for instance, $\text{Zn}^{2+}$, $\text{Ni}^{2+}$, $\text{Co}^{2+}$, $\text{Mn}^{2+}$) require a higher concentration of $\text{S}^{2-}$ to form their sulfides (they have relatively higher solubility products). Because the presence of HCl drastically reduces the $\text{S}^{2-}$ ion concentration, it is no longer high enough to precipitate these Group IV sulfides. Consequently, they remain dissolved in the solution under these acidic conditions.
Step 5: Final Reasoning
Therefore, the presence of HCl keeps the sulfide ion concentration sufficiently low to precipitate only the more insoluble sulfides of Group II cations and not those of Group IV. This is why the statement “presence of HCl decreases the sulphide ion concentration” correctly explains why the Group II cations are precipitated while Group IV cations are not.
