AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(B) Nitrogen dioxide
When lead nitrate is heated, brown fumes of nitrogen dioxide (NO₂) are produced: $2\text{Pb(NO}_3)_2 \xrightarrow{\Delta} 2\text{PbO} + 4\text{NO}_2 + \text{O}_2$.
Source: Chapter 1, Section 1.2.2, Activity 1.6
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The key fact is that the brown fumes specifically identify NO₂. Although PbO and O₂ are also products of the same reaction, they are not brown fumes — PbO is a yellow solid and O₂ is a colourless gas. Examiners test whether students can distinguish between all the products of this reaction and correctly identify which one appears as brown fumes.