On strongly heating $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2$ crystals, the gas formed is

When solid lead nitrate is heated strongly a it turns yellow due to formation of lead oxide and a reddish brown gas is evolved which is nitrogen dioxide. 

$2\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)\to 2\mathrm{PbO}\left(\mathrm{s}\right)+4{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$