Mark the compound which turns black with $NH_4OH$.

Concentrated aqueous sodium hydroxide can separate a mixture of

$(Yellow\,ppt)\,T \xleftarrow{{{K_2}Cr{O_4}}} X$ $\xrightarrow{{dil.\,HCl}} Y(Yellow\,ppt) + Z \uparrow (pungent\,smelling\,gas)$. If $X$ gives green flame test,then $X$ is:

$A$ black sulphide is formed by the action of $H_2S$ on

$A$ 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

Which compound is used to distinguish between $Fe^{2+}$ and $Fe^{3+}$ ions?