Mercury boils at `367^(@)C`. However, mercury thermometers are made such that they can measure temperature up to `500^(@)C`. This is done by

Options

A. maintaining vacuum aboves mercury column in the stem of the thermometer
B. filling nitrogen gas at high pressure above the mercury column
C. filling oxygen gas at high pressure above the mercury column
D. filling nitrogen gas at low pressure above the mercury column

Answer

To solve the question regarding how mercury thermometers can measure temperatures up to 500 °C despite mercury boiling at 367 °C, we need to analyze the options provided and understand the principles of boiling and vapor pressure.

### Step-by-Step Solution:

1. **Understanding Boiling Point**:
- The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. For mercury, this boiling point is 367 °C at standard atmospheric pressure.

2. **Impact of Pressure on Boiling Point**:
- If the external pressure is increased, the boiling point of the liquid also increases. This means that by increasing the pressure above the mercury, we can raise the temperature at which mercury will boil.

3. **Evaluating the Options**:
- **Option 1**: Maintaining a vacuum above the mercury column.
- This would lower the pressure, which would decrease the boiling point of mercury. Therefore, this option is incorrect.

- **Option 2**: Filling nitrogen gas at high pressure above the mercury column.
- By filling the space above the mercury with nitrogen gas at high pressure, we increase the external pressure. This will raise the boiling point of mercury, allowing the thermometer to measure temperatures up to 500 °C. This option is correct.

- **Option 3**: Filling oxygen gas at high pressure above the mercury column.
- While this may also increase the boiling point, oxygen is a reactive gas and can react with mercury, making this option unsuitable. Hence, this option is incorrect.

- **Option 4**: Filling nitrogen gas at low pressure above the mercury column.
- Lowering the pressure would decrease the boiling point of mercury, making this option incorrect.

4. **Conclusion**:
- The correct method to allow mercury thermometers to measure temperatures up to 500 °C is by **filling nitrogen gas at high pressure above the mercury column**.

### Final Answer:
The correct option is **Option 2: Filling nitrogen gas at high pressure above the mercury column**.

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