Assertion : During an adiabatic process, heat energy is not exchanged between system and its surroundings.
Reason : The temperature of a gas increases when it undergoes andn adiabatic expansion.

Options

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

Explanation

To solve the question, we will analyze the assertion and the reason step by step.

### Step 1: Understand the Assertion
**Assertion:** "During an adiabatic process, heat energy is not exchanged between the system and its surroundings."
- An adiabatic process is defined as one in which there is no heat transfer (Q = 0) between the system and its surroundings. This is true because the system is perfectly insulated from its surroundings.

**Hint for Step 1:** Remember that in an adiabatic process, the defining characteristic is the absence of heat exchange.

### Step 2: Analyze the Reason
**Reason:** "The temperature of a gas increases when it undergoes adiabatic expansion."
- In an adiabatic expansion, the gas does work on the surroundings. According to the first law of thermodynamics, the change in internal energy (ΔU) is given by ΔU = Q + W. Since Q = 0 in an adiabatic process, we have ΔU = W.
- For expansion, work done (W) is negative (since the system is doing work on the surroundings). This means that the internal energy of the gas decreases (ΔU < 0).
- The internal energy of an ideal gas is directly proportional to its temperature. Therefore, if the internal energy decreases, the temperature of the gas must also decrease.

**Hint for Step 2:** Recall that in an adiabatic expansion, the gas does work, which leads to a decrease in internal energy and consequently a decrease in temperature.

### Step 3: Conclusion
- The assertion is true: during an adiabatic process, no heat is exchanged.
- The reason is false: during adiabatic expansion, the temperature of the gas actually decreases, not increases.

### Final Answer
The correct answer is: **Option C** - Assertion is true, but the reason is false.