Anu heated some lead nitrate crystals in a boiling tube. She recorded the evolution of:

Options

A. Colourless gas
B. Odourless gas
C. Brown gas
D. Violet gas

Explanation

To solve the question regarding the heating of lead nitrate crystals and the gases evolved, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Identify the Compound**:
- Anu is heating lead nitrate, which has the chemical formula \( \text{Pb(NO}_3\text{)}_2 \).

2. **Understand the Reaction**:
- When lead nitrate is heated, it undergoes thermal decomposition. This means it breaks down into simpler substances.

3. **Write the Decomposition Reaction**:
- The decomposition of lead nitrate can be represented by the following chemical equation:
\[ \text{2Pb(NO}_3\text{)}_2 \xrightarrow{\text{heat}} \text{2PbO} + \text{4NO}_2 + \text{O}_2 \]
- Here, lead oxide (\( \text{PbO} \)), nitrogen dioxide (\( \text{NO}_2 \)), and oxygen gas (\( \text{O}_2 \)) are produced.

4. **Identify the Gases Evolved**:
- From the reaction, we see that nitrogen dioxide (\( \text{NO}_2 \)) and oxygen gas (\( \text{O}_2 \)) are the gases evolved.
- Notably, nitrogen dioxide is a brown gas.

5. **Conclusion**:
- Therefore, when Anu heated the lead nitrate crystals, she recorded the evolution of nitrogen dioxide gas, which is brown in color.

### Final Answer:
The gas evolved when Anu heated lead nitrate crystals is nitrogen dioxide (\( \text{NO}_2 \)), which appears as a brown gas.

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