Describe the differences between boiling and evaporation

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – 2.2.3 Melting, Boiling and Evaporation

2.2.3 Melting, Boiling and Evaporation

Learning Objective

Describe the differences between boiling and evaporation.

Key Concepts

  • Both boiling and evaporation are processes that change a liquid into a gas (vapor).

  • They occur at different temperatures and under different conditions.

  • Energy is required for both processes; the amount of energy supplied per molecule is the latent heat of vaporisation, \$L_v\$.

Boiling

Boiling is a rapid vaporisation that occurs throughout the bulk of the liquid when its temperature reaches the boiling point. The boiling point is the temperature at which the vapour pressure of the liquid equals the external (atmospheric) pressure.

  • Occurs at a specific temperature for a given pressure.

  • Vapour bubbles form inside the liquid and rise to the surface.

  • Requires the entire liquid to be at the boiling temperature.

  • Visible vigorous bubbling.

Evaporation

Evaporation is a slow surface phenomenon that can occur at any temperature below the boiling point.

  • Only molecules at the surface with sufficient kinetic energy escape into the air.

  • Does not require the liquid to reach a particular temperature.

  • Rate depends on temperature, surface area, humidity, and wind speed.

  • No visible bubbles; the liquid surface gradually recedes.

Comparison of Boiling and Evaporation

AspectBoilingEvaporation
Location in liquidOccurs throughout the bulk; bubbles form inside.Occurs only at the surface.
Temperature requirementLiquid must reach its boiling point (\$T_{\text{bp}}\$) for the given pressure.Can occur at any temperature below \$T_{\text{bp}}\$.
Visible signsRapid formation of bubbles and vigorous motion.Gradual reduction of liquid level; no bubbles.
Energy inputHeat supplied raises the entire liquid to \$T{\text{bp}}\$; latent heat \$Lv\$ is used as bubbles form.Only surface molecules need enough kinetic energy to overcome \$L_v\$; no bulk heating required.
Rate dependencePrimarily determined by the amount of heat supplied.Depends on temperature, surface area, humidity, and airflow.

Typical Questions

  1. Explain why water boils at a lower temperature on a mountain than at sea level.

  2. Describe how increasing the surface area of a puddle affects its rate of evaporation.

  3. Why does a pot of water on a stove first heat up before it begins to boil?

Suggested Diagram

Suggested diagram: Side‑view illustration showing (a) bubbling throughout a liquid during boiling and (b) surface molecules escaping during evaporation.

Summary

Boiling and evaporation are both phase changes from liquid to gas, but they differ in where they occur, the temperature required, the visual cues, and the factors that control their rates. Understanding these differences helps explain everyday phenomena such as why a kettle whistles when water boils, while a wet shirt dries slowly on a warm day.