Which ion produces a brown gas when the mixture is heated with $MnO_2$ and concentrated $H_2SO_4$?

Explain the brown ring test for nitrate ions.

Chromyl chloride vapours are dissolved in $NaOH$ and acetic acid and lead acetate solution is added,then

Among the following observations, the correct one that differentiates between $SO_{3}^{2-}$ and $SO_{4}^{2-}$ is:

$A$ mixture when heated with conc. $H_2SO_4$ and $MnO_2$ produces brown fumes due to the presence of:

$[X] +$ dil. $H_{2}SO_{4} \longrightarrow [Y]:$
Colourless, suffocating gas
$[Y] + K_{2}Cr_{2}O_{7} + H_{2}SO_{4} \longrightarrow$
Green colouration of solution
Then, $[X]$ and $[Y]$ are