When lead nitrate is heated, it decomposes to form lead oxide, nitrogen dioxide (brown fumes), and oxygen gas.Reaction:$2\mathrm{Pb}(\mathrm{NO}_3)_2 \xrightarrow{\mathrm{heat}} 2\mathrm{PbO} + 4\mathrm{NO}_2 + \mathrm{O}_2$Observation: Brown fumes of nitrogen dioxide $(\mathrm{NO}_2)$ are seen.