IGCSE Physics 0625

2.2.3 Melting, Boiling and Evaporation

This section explores the phase changes of matter, focusing on melting, boiling, and evaporation. This note specifically addresses the differences between boiling and evaporation.

Evaporation

Evaporation is a surface phenomenon where liquid molecules gain enough kinetic energy to escape into the gaseous phase. This occurs at the surface of the liquid, and the rate of evaporation depends on several factors.

• Temperature: Higher temperature increases the kinetic energy of the molecules, leading to a faster rate of evaporation.
• Surface Area: A larger surface area allows more molecules to escape, increasing the rate of evaporation.
• Humidity: Higher humidity (more water vapor in the air) decreases the rate of evaporation because the air is closer to being saturated.
• Airflow: Airflow removes water vapor from the surface, maintaining a lower concentration and promoting evaporation.

Boiling

Boiling is a process that occurs throughout the entire volume of the liquid. When the vapor pressure of the liquid equals the surrounding atmospheric pressure, bubbles of vapor form within the liquid and rise to the surface.

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure.

Differences between Boiling and Evaporation

The key differences between boiling and evaporation are summarized in the table below:

Feature	Evaporation	Boiling
Occurrence	Occurs only at the surface of the liquid.	Occurs throughout the entire volume of the liquid.
Temperature	Can occur at temperatures below the normal boiling point.	Occurs at the boiling point of the liquid.
Bubble Formation	No bubbles are formed.	Bubbles of vapor form throughout the liquid.
Rate	Generally slower than boiling.	Generally faster than evaporation.
Pressure	Not directly related to atmospheric pressure.	Occurs when vapor pressure equals atmospheric pressure.

In summary: Evaporation is a surface phenomenon that can occur at any temperature, while boiling is a process that occurs throughout the entire liquid volume at a specific temperature (the boiling point).