Understanding Liquid Boiling

Boiling is a specific type of vaporization that occurs within the bulk of a liquid when it is heated. Unlike evaporation, which happens only at the liquid's surface at any temperature, boiling occurs when the vapor pressure of the liquid reaches a critical point relative to the surrounding pressure.

Let's break down the key concepts involved in the boiling of a liquid:

• 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. As a liquid is heated, more molecules gain enough energy to escape into the gas phase, increasing the vapour pressure above the liquid surface.
• Atmospheric Pressure (or Air Pressure): This is the pressure exerted by the weight of the air column above the liquid surface. It is the external pressure acting on the liquid.

The Condition for Boiling

For a liquid to boil, bubbles of vapour must be able to form not just on the surface, but throughout the entire volume of the liquid and rise to the surface. For these vapour bubbles to form and exist against the external atmospheric pressure pushing down on the liquid, the pressure inside the bubbles (which is the vapour pressure of the liquid at that temperature) must be at least equal to the external pressure.

Therefore, the boiling point of a liquid is the temperature at which its vapour pressure is equal to the external pressure (typically atmospheric pressure).

Analyzing the Options for Liquid Boiling

Let's evaluate the given options based on the condition for boiling:

1. When atmospheric pressure becomes equal to the vapour pressure. This statement correctly describes the condition under which a liquid boils. At this point, the vapour pressure is high enough to overcome the external pressure, allowing bubbles of vapour to form within the liquid.
2. When atmospheric pressure becomes less than vapour pressure. If the atmospheric pressure is less than the vapour pressure, the liquid is already boiling or would boil even more readily. This describes a state where boiling is happening, not the threshold condition for it to begin. Lowering the external pressure lowers the boiling point.
3. When atmospheric pressure becomes higher than the vapour pressure. If the atmospheric pressure is higher than the vapour pressure, the external pressure is too great for vapour bubbles to form within the liquid. Boiling cannot occur under this condition.
4. When vapour pressure becomes equal to the air pressure. This statement is equivalent to option 1, as "air pressure" in this context refers to atmospheric pressure. It also correctly describes the boiling condition.

Both option 1 and option 4 describe the same correct condition. Option 1 is typically phrased this way in textbooks.

Summary of Boiling Condition

The essential condition for the boiling of a liquid is that the vapour pressure developed by the liquid at a certain temperature becomes equal to the external pressure acting on it. This external pressure is usually the atmospheric pressure.

Revision Table: Boiling Concepts

Additional Information: Factors Affecting Boiling Point

The boiling point of a liquid is not fixed; it depends on the external pressure.

• Pressure: Increasing the external pressure raises the boiling point, as the liquid needs to reach a higher vapour pressure (and thus a higher temperature) to match the external pressure. Conversely, decreasing the external pressure lowers the boiling point. This is why water boils at a lower temperature at high altitudes (where atmospheric pressure is lower).
• Impurities: Dissolving a non-volatile solute in a liquid generally raises its boiling point (boiling point elevation). This is a colligative property, depending on the concentration of solute particles.