When crystals of lead nitrate are heated strongly in a dry test tube :

Question

Accepted Answer

When crystals of lead nitrate are heated strongly in a dry test tube, a yellow residue is left. This is because lead nitrate undergoes thermal decomposition when heated. The chemical reaction can be represented as follows:

$$ \text{2Pb(NO}_3\text{)}_2 \rightarrow \text{2PbO} + \text{4NO}_2 + \text{O}_2 $$

1. **Explanation for Option 4 (Correct Answer):**
   - When lead nitrate is heated, it decomposes to form lead(II) oxide (PbO), nitrogen dioxide (NO₂), and oxygen (O₂). The lead(II) oxide is a yellow solid, which is the residue left in the test tube. This yellow residue is characteristic of lead(II) oxide, confirming that Option 4 is correct.

2. **Explanation for why other options are incorrect:**
   - **Option 1: A brown residue is left**
     - This option is incorrect because the residue left after the decomposition of lead nitrate is yellow, not brown. The brown color observed during the reaction is due to the release of nitrogen dioxide gas (NO₂), which is a brown gas, but it does not form a residue.
   
   - **Option 2: White fumes appear in the tube**
     - This option is incorrect because the decomposition of lead nitrate does not produce white fumes. The gases released are nitrogen dioxide (NO₂), which is brown, and oxygen (O₂), which is colorless. There are no white fumes involved in this reaction.
   
   - **Option 3: Crystals immediately vaporise**
     - This option is incorrect because lead nitrate does not vaporize upon heating. Instead, it decomposes into solid lead(II) oxide and gaseous products. The solid residue left is yellow lead(II) oxide, not vaporized crystals.

In conclusion, the correct answer is Option 4, as the decomposition of lead nitrate results in a yellow residue of lead(II) oxide, supporting the given correct answer.