IGCSE Physics 0625 – Melting, Boiling and Evaporation
2.2.3 Melting, Boiling and Evaporation
Objective
Describe how temperature, surface area and air movement over a surface affect evaporation.
Key Concepts
Evaporation is the change of state from liquid to vapour occurring at temperatures below the boiling point.
It takes place at the surface of a liquid where molecules with sufficient kinetic energy escape into the air.
The rate of evaporation depends on several factors that influence how many molecules can leave the surface.
Factors Influencing the Rate of Evaporation
Factor
How it Affects Evaporation
Explanation
Temperature
Higher temperature → faster evaporation
Increasing temperature raises the average kinetic energy of the liquid molecules. More molecules reach the energy required to overcome intermolecular forces and escape.
Surface Area
Larger surface area → faster evaporation
A greater area provides more molecules that are directly exposed to the air, increasing the number that can evaporate per unit time.
Air Movement (Wind)
Stronger air flow → faster evaporation
Moving air removes saturated vapour molecules from the surface, maintaining a larger concentration gradient between the liquid surface and the surrounding air.
Quantitative Description
The rate of evaporation \$E\$ can be expressed qualitatively by the relation
\$\$
E = k \, A \, \left( P{\text{sat}}(T) - P{\text{air}} \right) \, f(v)
\$\$
\$k\$ – constant of proportionality (depends on liquid properties)
\$A\$ – surface area of the liquid
\$P_{\text{sat}}(T)\$ – saturation vapour pressure at temperature \$T\$ (increases with \$T\$)
\$P_{\text{air}}\$ – partial pressure of the vapour in the surrounding air
\$f(v)\$ – function representing the effect of air velocity \$v\$ (greater \$v\$ → larger \$f(v)\$)
Detailed Effects
1. Temperature
Raising the temperature increases \$P_{\text{sat}}(T)\$ exponentially (Clausius‑Clapeyron relation).
More molecules have kinetic energy \$>E_{\text{escape}}\$, so the number escaping per second rises.
Practical example: Water left in a sunny window evaporates faster than in a shaded window.
2. Surface Area
Evaporation occurs only at the liquid–air interface.
Doubling the exposed area roughly doubles the number of molecules that can leave per unit time, assuming other conditions unchanged.
Practical example: Spreading a spill thinly on a tray speeds up drying compared with a deep puddle.
3. Air Movement
Still air becomes quickly saturated with vapour near the surface, reducing the concentration gradient.
Wind or a fan continuously replaces saturated air with drier air, maintaining a larger gradient.
Practical example: Clothes dry faster on a breezy day than on a calm day.
Summary Table
Condition
Effect on Evaporation Rate
Reason
High temperature
Increase
Higher kinetic energy → more molecules escape
Low temperature
Decrease
Fewer molecules have sufficient energy
Large surface area
Increase
More molecules are at the interface
Small surface area
Decrease
Fewer molecules exposed
Strong air flow
Increase
Vapour removed, gradient maintained
Still air
Decrease
Vapour builds up, reducing gradient
Suggested diagram: Sketch showing (a) a shallow pan of water with a large surface area, (b) a deep bowl with a small surface area, (c) airflow over a water surface, and (d) temperature gradient effect on molecular motion.
Exam Tips
When asked how a factor affects evaporation, always link the change to kinetic energy, surface exposure, or concentration gradient.
Use the qualitative formula \$E = kA(P{\text{sat}}-P{\text{air}})f(v)\$ to justify answers.
Remember that boiling is rapid evaporation throughout the liquid, while evaporation (the focus here) occurs only at the surface.