When we heat lead nitrate [Pb(N03)2] in a boiling tube, we observe the emission of brown fumes. Which one of the following is the brown gas?

When lead(II) nitrate [Pb(NO3)2] is heated in a boiling tube, it undergoes a thermal decomposition reaction. This process breaks down the white crystalline solid into lead(II) oxide (a yellow solid residue), oxygen gas, and nitrogen dioxide gas [1]. The balanced chemical equation for this reaction is 2Pb(NO3)2 → 2PbO + 4NO2 + O2 [1]. The characteristic observation during this experiment is the emission of reddish-brown fumes, which are specifically identified as nitrogen dioxide (NO2) gas. While oxygen is also evolved, it is a colorless gas and does not contribute to the visible fumes. Nitrogen dioxide is toxic and its distinct brown color serves as a primary indicator of the nitrate's decomposition. Therefore, the brown gas observed is NO2.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Activity 1.6 > p. 9