Mark the compound which turns black with $NH_4OH$.

The ion that cannot be precipitated by both $HCl$ and $H_2S$ is

Consider the sulphides $HgS$,$PbS$,$CuS$,$Sb_{2}S_{3}$,$As_{2}S_{3}$ and $CdS$. Number of these sulphides soluble in $50\% \ HNO_{3}$ is .... .

An aqueous solution of a mixture of two inorganic salts,when treated with dilute $HCl$,gave a precipitate $(P)$ and a filtrate $(Q)$. The precipitate $P$ was found to dissolve in hot water. The filtrate $(Q)$ remained unchanged when treated with $H_2S$ in a dilute mineral acid medium. However,it gave a precipitate $(R)$ with $H_2S$ in an ammoniacal medium. The precipitate $R$ gave a coloured solution $(S)$ when treated with $H_2O_2$ in an aqueous $NaOH$ medium.
$1.$ The precipitate $P$ contains
$(A)$ $Pb^{2+}$ $(B)$ $Hg_2^{2+}$ $(C)$ $Ag^{+}$ $(D)$ $Hg^{2+}$
$2.$ The coloured solution $S$ contains
$(A)$ $Fe_2(SO_4)_3$ $(B)$ $CuSO_4$ $(C)$ $ZnSO_4$ $(D)$ $Na_2CrO_4$

$A$ is:
$\text{Salt } A \text{ (Colourless)} \xrightarrow{Cu} \text{Blue solution}$
$\text{Salt } A \text{ (Colourless)} \xrightarrow{HCl_{(aq)}} \text{White ppt}$
$\text{Salt } A \text{ (Colourless)} \xrightarrow{\text{Conc. } H_2SO_4} \text{Brown fumes}$

Nessler's reagent is