Question

Which of the following statements is correct for boiling point of a liquid?

• A. Temperature at which a liquid boils at any pressure.
• B. Temperature at which solid is in equilibrium with its liquid.
• C. Temperature at which vapour pressure equals the applied pressure.
• D. Temperature at which applied pressure is greater than vapour pressure of liquid.

Hint:

This is why water boils at a lower temperature at high altitudes (like on a mountain). The applied atmospheric pressure is lower, so the liquid doesn't need to be heated as much for its vapour pressure to catch up and equal it.

Answer 1

The Correct Option is C

Step 1: Recall what happens when a liquid heats up.
As a liquid gets hotter, its molecules move faster. More of them escape into the air as vapour.

Step 2: Meet vapour pressure.
The escaping molecules push back as a pressure above the liquid. This is called the vapour pressure of the liquid.

Step 3: Note the outside pressure.
The air above also presses down on the liquid. This is the applied or atmospheric pressure.

Step 4: Find the boiling condition.
Boiling starts only when the vapour pressure pushing up equals the applied pressure pushing down.

Step 5: Why this matters.
At that point bubbles of vapour can form inside the liquid and rise up freely. That is true boiling.

Step 6: Conclusion.
So the boiling point is the temperature where vapour pressure equals the applied pressure. This is also why water boils faster on a high mountain, where the air pressure is low. \[ \boxed{\text{Vapour pressure equals applied pressure (Option 3)}} \]