Conc. $HNO_3$ is added before proceeding to test for group $III$ members. This is to

The qualitative distinction of $ZnSO_4$ and $Al_2(SO_4)_3$ can be done by using the reagent

$A$ solution containing a group $IV$ cation gives a precipitate on passing $H_2S$ gas. $A$ solution of this precipitate in dilute $HCl$ produces a white precipitate with $NaOH$ solution and a bluish-white precipitate with basic potassium ferrocyanide. The cation is:

When $H_2S$ is passed through a mixture containing $Cu^{2+}$,$Ni^{2+}$,and $Zn^{2+}$ in an acidic solution,which ion will precipitate?

$A$ mixture of two salts is used to prepare a solution $S$,which gives the following results:
White precipitate$(s)$ $\xleftarrow{\text{Dilute } NaOH(aq.) \text{ at Room temperature}}$ $S$ (aq. solution of the salts) $\xrightarrow{\text{Dilute } HCl(aq.) \text{ at Room temperature}}$ White precipitate$(s)$ only
The correct option$(s)$ for the salt mixture is(are):
$A$. $Pb(NO_3)_2$ and $Zn(NO_3)_2$
$B$. $Pb(NO_3)_2$ and $Bi(NO_3)_3$
$C$. $AgNO_3$ and $Bi(NO_3)_3$
$D$. $Pb(NO_3)_2$ and $Hg(NO_3)_2$

$A$ colorless aqueous solution contains nitrates of two metals,$X$ and $Y$. When it was added to an aqueous solution of $NaCl$,a white precipitate was formed. This precipitate was found to be partly soluble in hot water to give a residue $P$ and a solution $Q$. The residue $P$ was soluble in aqueous $NH_3$ and also in excess sodium thiosulfate. The hot solution $Q$ gave a yellow precipitate with $KI$. The metals $X$ and $Y$,respectively,are