From Diazonium Salts

$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{H}_2\text{O} \xrightarrow{\text{Warm}} \text{Phenol} + \text{N}_2\uparrow + \text{HCl}$$

Concept

Benzenediazonium chloride undergoes hydrolysis when its aqueous solution is warmed or heated with dilute acid. The diazonium group ($$\text{--N}_2^+$$) acts as an exceptional leaving group, departing as highly stable nitrogen gas ($$\text{N}_2$$) to form a phenyl cation/like intermediate, which reacts with water to yield phenol.