Boiling occurs when the vapour pressure of a liquid is

Analysis

The question asks for the specific physical condition that must be met for a liquid to reach its boiling point.

Concept Explanation

Boiling is a phase transition from liquid to gas that occurs throughout the entire volume of the liquid. For bubbles of vapour to form and rise within the liquid, the internal pressure of the vapour must be strong enough to push back against the surrounding atmospheric pressure (or external pressure) pushing down on the surface of the liquid.

• Evaporation: This occurs only at the surface of a liquid at any temperature.
• Boiling: This occurs when the vapour pressure of the liquid becomes equal to the external atmospheric pressure. If the vapour pressure is lower than the atmospheric pressure, the bubbles will collapse before they can reach the surface.

Step-by-Step Solution

1. Define Vapour Pressure: This is the pressure exerted by the vapour molecules in equilibrium with the liquid at a given temperature.
2. Define Atmospheric Pressure: This is the pressure exerted by the weight of the atmosphere above the liquid.
3. The Condition for Boiling: As a liquid is heated, its vapour pressure increases. Boiling begins the moment the vapour pressure rises to match the atmospheric pressure. At this point, the liquid can no longer contain the vapour, and bubbles form freely throughout the liquid.
4. Conclusion: If the vapour pressure were less than the atmospheric pressure, the liquid would not boil. If it were greater, the liquid would have already reached its boiling point and surpassed it. Therefore, the exact point of boiling is defined by equality.

Identification of Answer

The correct option is D.

Exam Tip

Remember the "Pressure Balance" rule: Boiling = Vapour Pressure equals Atmospheric Pressure. If you are asked about boiling at high altitudes (like on a mountain), remember that atmospheric pressure is lower there, which is why water boils at a temperature lower than 100°C!