Find the number of compounds which liberate NO₂ gas as one of the products on heating.

${\mathrm{NH}}_4{\mathrm{NO}}_3, {\mathrm{NH}}_4{\mathrm{NO}}_2, \mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2, \mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2$

$\mathrm{Hg}{\left({\mathrm{NO}}_3\right)}_2,{ \mathrm{AgNO}}_3,{ \mathrm{LiNO}}_3, \mathrm{Al}{\left({\mathrm{NO}}_3\right)}_3,{ \mathrm{KNO}}_3$

[ If your answer is 4 then write the answer as 0004.]

The following reactions are obtained when compound is heated. 

${\mathrm{AgNO}}_3\underset{5000\mathrm{C}}{\stackrel{\text{ below }}{\to }}{\mathrm{NO}}_2+{\mathrm{O}}_2+\mathrm{Ag}$

$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\stackrel{\mathrm{\Delta }}{\to }\mathrm{ZnO}+{\mathrm{NO}}_2\uparrow +\frac{1}{2}{\mathrm{O}}_2\uparrow$

$\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\stackrel{\mathrm{\Delta }}{⟶}\mathrm{Pb}\downarrow +2{\mathrm{NO}}_2\uparrow +{\mathrm{O}}_2\uparrow$

$\begin{array}{l}{\mathrm{KNO}}_3\underset{{500}^{\circ }\mathrm{C}}{\stackrel{\text{ below }}{⟶}}{\mathrm{KNO}}_2+\frac{1}{2}{\mathrm{O}}_2\\ {\mathrm{NH}}_4{\mathrm{NO}}_{3\left(\mathrm{s}\right)}\to {\mathrm{N}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}\end{array}$

$\begin{array}{l}{\mathrm{NH}}_4{\mathrm{NO}}_2\left(\mathrm{s}\right)\to {\mathrm{N}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\\ \mathrm{Ca}\left(\mathrm{NO}3\right)2\to \mathrm{CaO}+2{\mathrm{NO}}_2+1/2\mathrm{O}2\end{array}$

$\mathrm{Hg}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)\to \mathrm{HgO}\left(\mathrm{s}\right)+{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$

${\mathrm{KNO}}_3$ and ${\mathrm{NH}}_4{\mathrm{NO}}_2\left(\mathrm{s}\right)$ does not give ${\mathrm{NO}}_2$on heating.

Hence  the number of compounds which liberate ${\mathrm{NO}}_2\left(\mathrm{g}\right)$ gas as one of the products on heating are 0006.