Assertion : Physisorption of a gas adsorbed at low temperature may change into chemisorption at a high temperature.
Reason : Usually low pressure is also favourable for chemisorption

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. if both assertion and reason are false

Explanation

To solve the question, we need to analyze the assertion and the reason provided:

**Assertion:** Physisorption of a gas adsorbed at low temperature may change into chemisorption at a high temperature.

**Reason:** Usually, low pressure is also favorable for chemisorption.

### Step-by-Step Solution:

1. **Understanding Physisorption and Chemisorption:**
- Physisorption is a physical adsorption process that occurs at low temperatures. It involves weak van der Waals forces and does not involve any significant energy changes.
- Chemisorption, on the other hand, is a chemical adsorption process that occurs at higher temperatures. It involves the formation of strong chemical bonds and is characterized by higher energy changes.

2. **Temperature Influence:**
- At low temperatures, gases tend to be adsorbed onto surfaces through physisorption.
- As the temperature increases, the energy of the molecules increases, which can lead to the breaking of weak physisorption bonds and the formation of stronger chemisorption bonds.

3. **Assertion Evaluation:**
- The assertion states that physisorption can change into chemisorption when the temperature is increased. This is correct, as increasing temperature can facilitate the transition from weak physical bonds to stronger chemical bonds.

4. **Pressure Influence:**
- The reason states that low pressure is favorable for chemisorption. However, chemisorption typically occurs at higher pressures because higher pressure increases the likelihood of gas molecules colliding with the surface, thus facilitating the adsorption process.

5. **Conclusion:**
- The assertion is true because physisorption can indeed convert to chemisorption with an increase in temperature.
- The reason is false because chemisorption is generally favored at higher pressures, not low pressures.

### Final Answer:
- **Assertion:** True
- **Reason:** False
- Therefore, the correct option is **C**: Assertion is true, but the reason is false.