When ammonia gas is passed over hot CuO, a gas is evolved. The same gas is evolved by heating

Nitrogen gas is evolved when CuO is treated with ammonia.

$2{\mathrm{NH}}_3+3\mathrm{CuO}\to {\mathrm{N}}_2\uparrow +3\mathrm{Cu}+3{\mathrm{H}}_2\mathrm{O}$

Only NH₄NO₂ releases nitrogen gas when gets heated.

${\mathrm{NH}}_4{\mathrm{NO}}_2\stackrel{∆}{\to }{\mathrm{N}}_2\uparrow +2{\mathrm{H}}_2\mathrm{O}$

Other compounds do not produce nitrogen gas on heating.

$$\begin{gathered} {\mathrm{NH}}_4{\mathrm{NO}}_3 \stackrel{∆}{\to } {\mathrm{N}}_2\mathrm{O}\uparrow + 2{\mathrm{H}}_2\mathrm{O} \\ {\mathrm{NH}}_4\mathrm{Cl} \stackrel{∆}{\to } {\mathrm{NH}}_3\uparrow + \mathrm{HCl} \\ {\mathrm{CH}}_3{\mathrm{COONH}}_4 \stackrel{∆}{\to } {\mathrm{CH}}_3{\mathrm{CONH}}_2 + {\mathrm{H}}_2\mathrm{O} \end{gathered}$$