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Mercury boils at \( 356^{\circ} \mathrm{C} \). However mercury thermometers are made such that they can measure temperatures up to \( 500^{\circ} \mathrm{C} \). This is done by :

1. maintaining vacuum above the mercury column in the steam of the thermometer
2. filling nitrogen gas at high pressure above the mercury column
3. filling nitrogen gas at low pressure above the mercury column
4. filling oxygen gas at high pressure above the mercury column

Correct answer: filling nitrogen gas at high pressure above the mercury column

Solution

Mercury would normally boil at 356°C, so to measure higher temperatures the pressure above it must be increased. High-pressure nitrogen is inert and suppresses boiling, allowing the thermometer to work up to 500°C.

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