Explain the following `:`
i) Lead `(Pb^(2+))` is placed in the first as well as second group of qualitative analysis.
ii) The colour of mercurous chloride, `Hg_(2),Cl_(2), ` changes from white to black when treated with ammonia.
iii) During the qualitative analysis of a mixture containing `Cu^(2+)` and `Zn^(2+)` ions, `H_(2)S` gas is passed through an acidified solution containing these ions in order to test `Cu^(2+)` alone. Explain briefly.

Explanation

i) `PbCl_(2)` is partly soluble in water and hence `Pb^(2+)` ions pass to the first group filtrate , i.e. to the II group .
ii) Due to the formation of finely divided mercury.
iii) `a) K_(sp)(CuS)` is less than `K_(sp)(CdS)`.
b) Ionization of `H_(2)S` is further suppressed in presence of acid (common ion effect).
`H_(2)S hArr 2H^(+)+S^(2-)`
So, when `H_(2)S` gas is passed through acidified solution containing `Cu^(2+)` and `Zn^(2+)`, only `Cu^(2+)` ions will be precipitated due to low concentration of `S^(2-)` ions.