A student heats lead nitrate powder in a test tube and observes brown fumes along with a colourless gas. Write the balanced chemical equation for this reaction and identify every product formed.
Generated by claude-sonnet-4-6 · 2026-06-26 00:54 · grounding rag
Model Answer
When lead nitrate is heated, it undergoes thermal decomposition. The balanced chemical equation is:
$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$
Products identified:
- Lead monoxide (PbO) — yellow solid residue formed in the test tube.
- Nitrogen dioxide (NO₂) — brown/reddish-brown fumes observed.
- Oxygen (O₂) — colourless, odourless gas released.
This is a thermal decomposition reaction (endothermic).
Source: Chapter 1, Chemical Reactions and Equations
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Explanation
- The examiner expects the correctly balanced equation (1 mark), identification of all three products (1 mark each, or grouped as 1 mark), and the type of reaction (often implied or explicitly asked).
- The most common mistake is missing O₂ as a product — students notice brown fumes (NO₂) but forget oxygen is also released.
- Remember: 2 formula units of Pb(NO₃)₂ give 2 PbO + 4 NO₂ + 1 O₂ — verify atom count on both sides.
- Brown fumes → NO₂; colourless gas → O₂; yellow residue → PbO.