Group Ii Cation Test

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Black precipitate formed with H2S in acidic medium indicates:

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1. **Systematic Analysis**: In qualitative inorganic analysis, Group II cations are precipitated as sulfides in an acidic medium.
2. **Reagent**: $H_2S$ gas in the presence of dilute $HCl$.
3. **Chemistry**: The low concentration of $S^{2-}$ ions (due to the common ion effect of $H^+$) is only sufficient to exceed the solubility product ($K_{sp}$) of Group II sulfides.
4. **Observations**: **Lead ($Pb^{2+}$)** belongs to Group II and reacts with $H_2S$ to form **$PbS$**, which is a **black precipitate**.
5. **Comparison**: $Zn^{2+}$ is a Group IV cation (precipitates in basic medium as white $ZnS$). $Ca^{2+}$ and $Al^{3+}$ do not form insoluble sulfides under these conditions.

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Group Ii Cation Test

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