Assertion : When helium is allowed to expands into vacuum, heating effect is observed.
Reason : The inversion temperature of helium is very low.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To solve the question regarding the assertion and reason about helium expanding into a vacuum, we can break down the explanation as follows:

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that when helium is allowed to expand into a vacuum, a heating effect is observed. This is true for helium and similar gases (like hydrogen) because they exhibit a heating effect when they undergo free expansion into a vacuum.

2. **Understanding the Reason**:
- The reason provided is that the inversion temperature of helium is very low. The inversion temperature is the temperature above which a gas cools upon expansion and below which it heats up. For helium, this inversion temperature is indeed low, which means that when it expands, it tends to heat up rather than cool down.

3. **Joule-Thomson Effect**:
- The Joule-Thomson effect explains the temperature change of a real gas when it expands. For gases like helium and hydrogen, which have low inversion temperatures, expansion leads to a heating effect. This is because their critical temperatures are low, and they do not behave ideally under expansion.

4. **Conclusion**:
- Since both the assertion and the reason are true, and the reason correctly explains the assertion, we conclude that both statements are valid. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

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