Which of the following statements correctly defines the boiling point of a liquid? The temperature at which the

Step 1: Analyze the question You are asked for the correct definition of the boiling point of a liquid.

Step 2: The concept A liquid boils when bubbles of vapour can form throughout the liquid (not just at the surface) and grow without being crushed by the surroundings. This happens when the pressure of the vapour inside the bubbles, the saturated vapour pressure (SVP), becomes equal to the external pressure on the liquid, usually atmospheric pressure.

• At the normal boiling point, SVP = standard atmospheric pressure (760 mmHg or 101.3 kPa).
• If the external pressure is lower (e.g. on a high mountain), the liquid boils at a lower temperature.
• If the external pressure is higher (e.g. in a pressure cooker), it boils at a higher temperature.

Step 3: Check each option

• A. SVP equals atmospheric pressure. This is the standard definition of boiling point, so it is correct.
• B. Molecules leave at a rate equal to the rate they return. This describes dynamic equilibrium in a closed container, which is a saturated vapour. It has nothing to do with boiling.
• C. Molecules leave at a higher rate than they return. This describes net evaporation. It happens at any temperature, not only at the boiling point.
• D. Molecules leave at a lower rate than they return. This describes condensation, the opposite of boiling.

Step 4: Answer The correct option is A: the boiling point is the temperature at which the saturated vapour pressure of the liquid equals the atmospheric (external) pressure.

Exam tip: Remember "Boiling = Balanced pressure": SVP balances the external pressure. If an option mentions "equal rates of leaving and returning," that is saturation or equilibrium, not boiling.