Which reagent is used to distinguish between silver and lead salts?

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:

$MgSO_4$ on reaction with $NH_4OH$ and $Na_2HPO_4$ forms a white crystalline precipitate. What is its formula?

Consider the following sequence of reactions:
$M^{2+}(aq.)$ $\xrightarrow{NH_4Cl(s) + (NH_4)_2CO_3 \, sol.} Q \downarrow$ $\xrightarrow[\text{Followed by addition of } (NH_4)_2C_2O_4]{CH_3COOH} R \downarrow$
Which of the following cations can form precipitate $Q$ but does not form precipitate $R$?

Addition of $SnCl_2$ to $HgCl_2$ gives a precipitate of which color?

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