Assertion : Pressure of a dry gas at any temperature can be calculated by substracting aqueous tension at that temperature from the pressure of the moist gas.
Reason : The pressure of water vapours is called aqueous tension.

Options

A. If both assertion and reason are correct and reason is correct explanation for assertion
B. If both assertion and reason are correct and reason is not correct explanation for assertion
C. If assertion is correct but reason is incorrect
D. If assertion and reason are both incorreect.

Explanation

To solve the question, we need to analyze both the assertion and the reason given.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the pressure of a dry gas at any temperature can be calculated by subtracting the aqueous tension (the pressure of water vapor) from the pressure of the moist gas.
- When a gas is collected over water, it contains both the gas itself (dry gas) and water vapor. The total pressure of this moist gas is the sum of the pressure of the dry gas and the pressure of the water vapor.

2. **Understanding the Reason**:
- The reason states that the pressure of water vapors is called aqueous tension.
- This is a correct statement. Aqueous tension refers to the partial pressure exerted by water vapor in a mixture of gases.

3. **Evaluating the Assertion**:
- The assertion is correct because it accurately describes how to find the pressure of a dry gas when it is collected over water. The formula used is:
\[ P_{\text{dry gas}} = P_{\text{moist gas}} - P_{\text{aqueous tension}} \]
- This means that the pressure of the dry gas can indeed be calculated by subtracting the aqueous tension from the total pressure of the moist gas.

4. **Evaluating the Reason**:
- The reason is also correct, as it correctly defines aqueous tension as the pressure of water vapor.
- However, while both statements are true, the reason does not provide a direct explanation for the assertion. It simply states a fact about water vapor without linking it to the calculation of dry gas pressure.

5. **Conclusion**:
- Since both the assertion and the reason are correct, but the reason does not explain the assertion, the correct answer is:
**Option B**: Both assertion and reason are correct, but reason is not the correct explanation for assertion.