IN a chemical reaction Aniline is treated with NaNO_2 and HCl to...

Aniline ($\text{Ar-NH}_2$) reacts with $\text{NaNO}_2$ and $\text{HCl}$ at $0-5^{{}^{\circ}}\text{C}$ to undergo diazotization, forming benzene diazonium chloride ($\text{Ar-N}_2^+\text{Cl}^-$) as compound X.

$\text{Ar-NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0-5^{{}^{\circ}}\text{C}} \text{Ar-N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}$

When the diazonium salt X is hydrolyzed by warming with water (supplying heat), it decomposes to form phenol ($\text{Ar-OH}$) as compound Z, along with the evolution of nitrogen gas.

$\text{Ar-N}_2^+\text{Cl}^- + \text{H}_2\text{O} \xrightarrow{\Delta} \text{Ar-OH} + \text{N}_2 + \text{HCl}$

Thus, compound Z is $\text{Ar-OH}$.

Answer: Ar-OH