The temperature at which the vapour pressure of a liquid becomes equals to the external (atmospheric) pressure is its

Options

A. Freezing point
B. Boiling point
C. Melting point
D. Critical temperature

Answer

To solve the question, "The temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure is its:", we will analyze the options provided and determine the correct answer step by step.

### Step-by-Step Solution:

1. **Understanding Vapor Pressure**:
- Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It increases with temperature.

2. **Defining Boiling Point**:
- The boiling point of a liquid is defined as the temperature at which its vapor pressure equals the external atmospheric pressure. At this point, the liquid begins to convert into vapor.

3. **Analyzing the Options**:
- **Option A: Freezing Point**: This is the temperature at which a liquid turns into a solid. It does not relate to vapor pressure equating to atmospheric pressure.
- **Option B: Boiling Point**: This is the correct definition as explained above.
- **Option C: Melting Point**: This is the temperature at which a solid turns into a liquid. Similar to freezing point, it does not relate to vapor pressure.
- **Option D: Critical Temperature**: This is the temperature above which a substance cannot exist in the liquid state, regardless of pressure. It is not related to the vapor pressure equating to atmospheric pressure.

4. **Conclusion**:
- Based on the definitions and analysis of the options, the correct answer is **Option B: Boiling Point**.

### Final Answer:
The temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure is its **boiling point**.