Which of the following salts is insoluble in water at room temperature but soluble in boiling water?

An inorganic compound $X$ on treatment with concentrated $H_{2}SO_{4}$ produces brown fumes and gives a dark brown ring with $FeSO_{4}$ in the presence of concentrated $H_{2}SO_{4}$. Also,compound $X$ gives a precipitate $Y$ when its solution in dilute $HCl$ is treated with $H_{2}S$ gas. The precipitate $Y$ on treatment with concentrated $HNO_{3}$ followed by excess of $NH_{4}OH$ further gives a deep blue coloured solution. Compound $X$ is:

Which of the following reagents is commonly used to detect the presence of sulfide ions?

$A$ reagent used to test the presence of $Fe^{2+}$ ion is

Assign $A, B, C, D$ from the given type of reaction.
$FeCl_3 + NH_3 \text{ (excess)} \longrightarrow Fe(OH)_3 \downarrow$

Addition of sodium thiosulphate solution to a solution of silver nitrate gives $X$ as a white precipitate, which is insoluble in water but soluble in excess thiosulphate solution to give $Y$. On boiling in water, $Y$ gives $Z$. $X$, $Y$, and $Z$ respectively are: