Understanding Boiling and Vapor Pressure

The question asks about the physical process that occurs when the vapour pressure of a liquid becomes greater than the external pressure above its surface at a given temperature.

Let's understand the terms involved:

• Vapour Pressure: This is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is a measure of the tendency of a material to transition into the gaseous state.
• External Pressure: This is the pressure exerted on the surface of the liquid, typically atmospheric pressure when the liquid is in an open container.

Now let's consider the options provided:

• Freezing: This is the process where a liquid changes into a solid. It occurs when the temperature drops to the freezing point and the kinetic energy of molecules decreases enough to form a fixed structure. It is not related to vapor pressure exceeding external pressure.
• Condensation: This is the process where a gas or vapor changes into a liquid. It occurs when the temperature drops below the dew point or the pressure increases, causing the vapor molecules to lose energy and clump together. It is the opposite of vaporization or boiling.
• Boiling: This is a phase transition from the liquid state to the gaseous state, typically occurring when a liquid is heated to its boiling point. Boiling happens when the vapour pressure of the liquid equals the external pressure surrounding the liquid. When the vapour pressure exceeds the external pressure, bubbles of vapour can form within the bulk of the liquid, not just at the surface, and rise to the surface, which is characteristic of boiling. The condition where vapour pressure is *more than* the external pressure strongly drives this transition throughout the liquid.
• Gauging: This is a term related to measurement, often measuring volume or distance, particularly in barrels or tanks. It is not a phase transition process.

The specific condition described – where the vapour pressure is more than the pressure above a liquid surface at a given temperature – is the driving force behind the rapid vaporization that occurs during boiling. While boiling technically starts when vapor pressure equals external pressure, exceeding this pressure accelerates the process and allows boiling to occur vigorously throughout the liquid.

Therefore, the phenomenon that occurs when the vapour pressure is more than the pressure above a liquid surface at a given temperature is boiling.