On passing a gas, ' X ', through Nessler's reagent, a brown...

Nessler's reagent is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide. Its chemical formula is ${K}_{2}{HgI}_{4}$. It is used to detect the presence of ammonia. It turns into a pale brown colour in the presence of ammonia.

Nessler’s Reagent Reaction:

$2\mathop{\mathop{{K}_{2}{HgI}_{4}}_{⏟}}_{Nessler ' sreagent} + {NH}_{3} + 3KOH\overset{}{\to}\mathop{\mathop{HgO . Hg\left({NH}_{2}\right)I}_{⏟}}_{IodideofMillon ' sbase\left(brownprecipitate\right)} + 7KI + 2{H}_{2}O$