During thermal dissociation of a gas, the vapour density.

Options

A. Remains the same
B. Increases
C. Decreases
D. Increases in some cases and decreases in others

Answer

### Step-by-Step Solution:

1. **Understanding Thermal Dissociation**:
- Thermal dissociation refers to the process where a molecule (let's say A) breaks down into smaller molecules (like B and C) upon heating. For example, a molecule A with a molar mass \( M \) dissociates into products B and C with molar masses \( m_1 \) and \( m_2 \) respectively.

2. **Analyzing Molar Mass Changes**:
- In the process of thermal dissociation, the original molecule A has a higher molar mass \( M \) compared to the products B and C, which have lower molar masses \( m_1 \) and \( m_2 \). Thus, we can state that \( M > m_1 \) and \( M > m_2 \).

3. **Understanding Vapor Density**:
- Vapor density is defined as the mass of a gas relative to the mass of an equal volume of hydrogen at the same temperature and pressure. It is directly proportional to the molar mass of the gas. Therefore, if the molar mass decreases, the vapor density will also decrease.

4. **Determining the Effect of Thermal Dissociation on Vapor Density**:
- Since the thermal dissociation results in the formation of products with lower molar masses, the overall molar mass of the system decreases. Consequently, since vapor density is directly proportional to molar mass, the vapor density will also decrease.

5. **Conclusion**:
- Therefore, during the thermal dissociation of a gas, the vapor density decreases. The correct answer to the question is that the vapor density decreases.