The temperature at which the vapour pressure of a liquid equals external pressure is called

Options

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

Answer

To answer the question, "The temperature at which the vapor pressure of a liquid equals external pressure is called," we can follow these steps:

### 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. **Identifying External Pressure**:
- External pressure is the pressure exerted by the surroundings on the liquid. This can vary depending on the environment (e.g., atmospheric pressure).

3. **Defining the Temperature Condition**:
- The question asks for the specific temperature at which the vapor pressure of the liquid equals the external pressure. This is a critical point in phase transitions.

4. **Analyzing the Options**:
- **Freezing Point**: This is the temperature at which a liquid turns into a solid. It is not relevant to vapor pressure.
- **Boiling Point**: This is the temperature at which the vapor pressure of the liquid equals the external pressure. This is the correct answer.
- **Melting Point**: This is the temperature at which a solid turns into a liquid. Again, not relevant to vapor pressure.
- **Critical Temperature**: This is the highest temperature at which a substance can exist as a liquid. Above this temperature, the substance cannot be liquefied regardless of pressure.

5. **Conclusion**:
- The temperature at which the vapor pressure of a liquid equals the external pressure is called the **boiling point**.

### Final Answer:
The temperature at which the vapor pressure of a liquid equals external pressure is called the **boiling point**.

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