Air is bad conductor of heat or partly conducts heat, still vacuum is to be placed between the walls of the thermos flask because

Options

A. it is difficult to fill the air between the walls of thermo flask
B. due to more pressure of air, the flask can crack
C. by convection,heat can flow through air
D. None of the above

Explanation

To answer the question regarding why a vacuum is placed between the walls of a thermos flask, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Heat Transfer**:
- Heat can be transferred in three ways: conduction, convection, and radiation. In the context of a thermos flask, we are primarily concerned with conduction and convection.

2. **Role of Air in Heat Transfer**:
- Air is a poor conductor of heat, meaning it does not transfer heat very efficiently. However, it can still facilitate heat transfer through convection, which occurs when warmer air rises and cooler air sinks, creating a circulation of air.

3. **Vacuum as an Insulator**:
- A vacuum is a space devoid of matter, including air. Since there are no particles in a vacuum, there is no medium for convection to occur. This means that heat cannot be transferred by convection between the inner and outer walls of the thermos flask.

4. **Purpose of the Vacuum**:
- The vacuum between the walls of the thermos flask serves to minimize heat transfer. By preventing convection (and also conduction to a lesser extent), the vacuum helps maintain the temperature of the contents inside the flask, keeping hot liquids hot and cold liquids cold.

5. **Conclusion**:
- Therefore, the vacuum is placed between the walls of the thermos flask specifically to prevent heat transfer by convection, ensuring that the temperature of the contents remains stable.

### Final Answer:
The vacuum is placed between the walls of the thermos flask to prevent heat transfer by convection, as there is no medium in a vacuum to facilitate this process.

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