Cambridge IGCSE Physics 0625 – 2.2.3 Melting, Boiling and Evaporation
2.2.3 Melting, Boiling and Evaporation
Objective
Describe evaporation in terms of the escape of more‑energetic particles from the surface of a liquid.
What is Evaporation?
Evaporation is a cooling process that occurs at temperatures below the boiling point of a liquid. It involves the loss of molecules from the liquid’s surface into the surrounding air.
Particle‑Model Explanation
According to the kinetic particle model, the particles in a liquid have a distribution of kinetic energies. Most particles have moderate speeds, but a few have very high kinetic energy.
Particles deep inside the liquid are constantly colliding with neighbours, which prevents them from escaping.
Particles at the surface experience fewer collisions on the upward side, so a high‑energy particle can overcome the attractive forces of the liquid.
When such a particle succeeds, it leaves the liquid and becomes a vapour molecule.
Thus, evaporation is the net loss of the most energetic particles from the surface.
Energy Considerations
The escaping particles take away kinetic energy. The average kinetic energy of the remaining liquid particles therefore decreases, which we perceive as a drop in temperature.
Mathematically, the average kinetic energy per particle is related to temperature by
where \$k_{\mathrm{B}}\$ is Boltzmann’s constant and \$T\$ is the absolute temperature.
Factors Affecting the Rate of Evaporation
Temperature of the liquid – Higher temperature means a larger proportion of particles have enough energy to escape.
Surface area – A larger exposed surface provides more opportunities for particles to leave.
Air movement (wind) – Moving air removes vapour molecules from the surface, reducing the chance of re‑condensation.
Humidity of the surrounding air – Low humidity means fewer vapour molecules are already present, so the net escape rate is higher.
Nature of the liquid – Liquids with weaker intermolecular forces (e.g., alcohol) evaporate more readily than those with strong forces (e.g., water).
Evaporation vs. Boiling
Aspect
Evaporation
Boiling
Temperature
Occurs below the boiling point
Occurs at the boiling point
Location in liquid
Only at the surface
Throughout the bulk (formation of bubbles)
Energy requirement
Only the most energetic surface particles escape
All particles receive enough energy to form vapour bubbles
Cooling effect
Significant cooling of the remaining liquid
Little immediate cooling; temperature remains at boiling point
Suggested Diagram
Suggested diagram: Illustration of high‑energy particles escaping from the surface of a liquid while lower‑energy particles remain, showing the kinetic‑energy distribution.
Key Points to Remember
Evaporation is the escape of the most energetic particles from the liquid surface.
It results in a cooling effect because the remaining liquid loses kinetic energy.
The rate of evaporation increases with temperature, surface area, wind speed, and low humidity, and decreases with strong intermolecular forces.
Unlike boiling, evaporation does not require the liquid to reach its boiling point and occurs only at the surface.