Which of the following compounds changes the color of an aqueous $FeCl_3$ solution?

Passing $H_2S$ gas into a mixture of $Mn^{2+}, Ni^{2+}, Cu^{2+}$ and $Hg^{2+}$ ions in an acidified aqueous solution precipitates

In the group $III$ radicals,in place of $NH_4Cl$,which of the following can be used?

$H_2S$ gas is passed through an acidified solution containing $Ag^+$,$Cu^{2+}$,and $Zn^{2+}$ ions. Which of these will form a precipitate?

$A$ laboratory reagent imparts green colour to the flame. On heating with solid $K_2Cr_2O_7$ and conc. $H_2SO_4$,it evolves a red gas. Identify the reagent.

$A$ 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