The salt which responds to dilute and concentrated $H_2SO_4$ is

Which of the following anions cannot be easily removed from its aqueous solution by precipitation?

$[X] +$ 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

The Chromyl chloride test is performed for the confirmation of the presence of which of the following in a mixture?

Sodium sulphite on heating with dilute $HCl$ liberates a gas which

$X$ (salt) + $AgNO_3$ $(aq.) \to \underset{(\text{Yellow ppt.})}{Y \downarrow}$ (soluble in excess of $NH_3$ solution). Salt $X$ does not contain: