2.3 Evaporation and Boiling

Introduction

Liquids can change into gases in two different ways:

• Evaporation — happens at the surface, at any temperature.
• Boiling — happens throughout the liquid, at a specific temperature.

Both involve a liquid turning into a gas, but they are not the same process. This section explains both, how they differ, and how pressure affects them.

Definition

Evaporation: Evaporation is the process by which molecules at the surface of a liquid gain enough kinetic energy to escape into the gaseous phase.

Boiling: Boiling is the process in which a liquid turns into vapour when it is heated to its boiling point, resulting in the formation of vapour bubbles within the liquid.

In simple words:

• Evaporation = liquid → gas at the surface, any temperature.
• Boiling = liquid → gas throughout the liquid, at a specific temperature.

Main Idea

Core Concept

• Evaporation happens when fast-moving molecules at the surface escape into the air.
• Boiling happens when the whole liquid reaches its boiling point and vapour bubbles form inside the liquid.
• Both processes require energy to overcome intermolecular forces.
• Both are affected by external pressure.

Detailed Explanation

Evaporation

What happens:

• Only molecules at the surface of the liquid participate in evaporation.
• These molecules must have sufficient kinetic energy to overcome intermolecular forces.
• Energy for evaporation comes from the surrounding environment.
• This absorption makes the surroundings cooler than before.
• Evaporation can occur at any temperature below the liquid's boiling point.

Effect of temperature:

- Higher temperatures increase the rate of evaporation since more molecules have sufficient energy to escape.

Cooling effect:

• As high-energy molecules leave the liquid, the average kinetic energy of the remaining molecules decreases.
• This leads to a cooling effect.
• This is why sweating helps cool the body.

Examples:

• Drying of clothes on a line.
• Evaporation of water from lakes and oceans contributing to the water cycle.
• Sweat evaporating from the skin, providing a cooling effect.

Boiling

What happens:

• Boiling occurs throughout the entire liquid, not just at the surface.
• Vapour bubbles form within the liquid, rise to the surface, and burst, releasing gas.
• Boiling occurs at a specific temperature known as the boiling point.
• This temperature varies depending on the liquid and the external pressure.
• For water at sea level, the boiling point is 100°C (212°F).
• Continuous heat input is required to maintain the liquid at its boiling point.
• The temperature of the liquid remains constant during boiling, as the added energy is used to convert the liquid into gas.
• Boiling is characterized by the vigorous formation of bubbles and steam, making it a highly visible process.

Effect of pressure:

• Higher pressure increases the boiling point.
• Lower pressure decreases the boiling point.
• This is why water boils at lower temperatures at high altitudes.

Effect of Pressure on Evaporation and Boiling

The effect of pressure on evaporation and boiling is primarily related to how it influences the rate at which molecules escape from the liquid's surface into the gas phase.

Effect of Lower Pressure:

• At lower atmospheric pressures, the rate of evaporation increases.
• This is because there is less external pressure exerted on the liquid surface, making it easier for molecules to escape into the vapour phase.
• For example, water evaporates more quickly at high altitudes where the atmospheric pressure is lower than at sea level.
• Lower pressure reduces the boiling point of a liquid, which means the liquid can transition to the gas phase at a lower temperature.
• This indirectly affects evaporation by facilitating the conditions under which it can occur more readily.

Effect of Higher Pressure:

• At higher atmospheric pressures, the rate of evaporation decreases.
• The increased external pressure on the liquid surface makes it more difficult for molecules to escape into the vapour phase.
• For example, in a pressure cooker, the high pressure suppresses evaporation until a higher temperature is reached, allowing food to cook faster.
• Higher pressure increases the boiling point of a liquid, which means the liquid requires a higher temperature to transition into the gas phase.
• This indirectly affects evaporation by making the conditions less favourable for it to occur.
• For example, in a pressure cooker, the high pressure suppresses boiling until a higher temperature is reached, allowing water to boil at a higher temperature. This allows food to cook faster.

Summary Table

Intermediate Level

Evaporation:

• Surface phenomenon.
• Any temperature below boiling point.
• Causes cooling because high-energy molecules leave.
• Energy comes from surroundings.

Boiling:

• Bulk phenomenon (throughout the liquid).
• At boiling point (specific temperature).
• Temperature remains constant during boiling.
• Requires continuous heat input.
• Vapour bubbles form inside the liquid.

Pressure effects:

• Lower pressure → increases evaporation rate, decreases boiling point.
• Higher pressure → decreases evaporation rate, increases boiling point.

Why pressure matters: Pressure determines how hard it is for molecules to escape from the liquid surface. Lower pressure means less resistance, so molecules escape more easily.

Important Points

• - Evaporation happens at the surface, at any temperature below boiling point.
• - Boiling happens throughout the liquid, at the boiling point.
• - Evaporation causes cooling because high-energy molecules leave.
• - Boiling requires continuous heat input.
• - Temperature remains constant during boiling.
• - Lower pressure increases evaporation rate and decreases boiling point.
• - Higher pressure decreases evaporation rate and increases boiling point.
• - At sea level, water boils at 100°C.
• - At high altitudes, water boils at lower temperatures.

Remember

• - Evaporation = surface, any temperature.
• - Boiling = throughout liquid, at boiling point.
• - Evaporation → cooling effect.
• - Boiling → temperature constant.
• - Low pressure → faster evaporation, lower boiling point.
• - High pressure → slower evaporation, higher boiling point.
• - Water boils at 100°C at sea level.
• - Pressure cooker → high pressure → higher boiling point → faster cooking.

Common Mistakes

Quick Revision

Evaporation:

• Process by which molecules at the surface of a liquid gain enough kinetic energy to escape into the gaseous phase.
• Only surface molecules participate.
• Energy comes from surroundings → cooling effect.
• Occurs at any temperature below boiling point.
• Higher temperature → faster evaporation.
• Examples: drying clothes, evaporation from lakes, sweating.

Boiling:

• Process in which a liquid turns into vapour when heated to its boiling point.
• Occurs throughout the entire liquid.
• Vapour bubbles form within the liquid.
• Occurs at a specific temperature (boiling point).
• Temperature remains constant during boiling.
• Requires continuous heat input.
• Boiling point changes with external pressure.
• Water boils at 100°C at sea level.

Effect of Pressure:

• Lower pressure → increases evaporation rate, decreases boiling point.
• Higher pressure → decreases evaporation rate, increases boiling point.
• Example: Pressure cooker cooks food faster.

60-Second Revision

• Evaporation = surface, any temperature, cooling effect.
• Boiling = throughout liquid, at boiling point, temperature constant.
• Low pressure → faster evaporation, lower boiling point.
• High pressure → slower evaporation, higher boiling point.
• Water boils at 100°C at sea level.
• Pressure cooker → high pressure → higher boiling point → faster cooking.
• Sweating cools the body by evaporation.
• ⚠️ Warning: evaporation and boiling are not the same — evaporation happens at the surface at any temperature, while boiling happens throughout the liquid at a specific temperature.