When lead(II) nitrate is heated, it undergoes thermal decomposition to form lead oxide, nitrogen dioxide, and oxygen. The brown fumes are of nitrogen dioxide (NO₂).
$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\text{Heat}} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$
Source: Chapter 1, Section 1.2.2 Decomposition Reaction
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The examiner expects: (1) identification of the brown fumes as NO₂, and (2) the balanced equation. For 1 mark, either naming NO₂ with a brief reason or writing the equation is sufficient, but including both makes the answer complete and safe.