Final answer:
Reagent X is likely Bromine water or a nitrous acid solution, used to distinguish between ethyl amine and aniline by forming a yellow dye only with aniline.
Step-by-step explanation:
To distinguish between ethyl amine and aniline, the reagent X that Parul uses is likely Bromine water or a nitrous acid solution (prepared from sodium nitrite and hydrochloric acid). The reaction with these reagents results in the formation of a yellow dye when they react with aromatic amines such as aniline but do not give the same reaction with aliphatic amines like ethyl amine.
Aniline reacts with nitrous acid to form a diazonium salt which can then couple with other aromatic rings present in the reaction mixture to form a yellow azo compound. In the case of ethyl amine, an aliphatic amine, the reaction with nitrous acid leads to the formation of an alcohol and nitrogen gas, without producing any yellow dye.
Therefore, the tube that formed a yellow dye contained aniline, and the other tube that did not form a yellow dye contained ethyl amine. Reagent X therefore serves as a useful qualitative test to distinguish between aniline and ethyl amine in a laboratory setting.