Learning outcomes
By the end of this lesson, students should be able to:
- distinguish evaporation from boiling
- describe evaporation as escape of higher-energy surface particles
- explain evaporative cooling using average kinetic energy
- describe how temperature, surface area and air movement affect evaporation
- apply evaporation ideas to everyday and experimental situations
8.1 Evaporation compared with boiling
Evaporation occurs at the surface of a liquid and can take place at any temperature. No vapour bubbles are required inside the liquid. Boiling occurs throughout the liquid at a definite boiling temperature for a given pressure, with bubbles forming within the liquid.
Evaporation is usually slower because only surface particles can escape and only a fraction have enough energy at any instant. Boiling can be rapid because vapour formation occurs throughout the liquid.
8.2 Particle explanation of evaporation
Liquid particles have a range of kinetic energies. Some particles at the surface move fast enough to overcome attractive effects and escape into the gas phase. Because the faster, higher-energy particles are more likely to leave, the remaining liquid has a lower average kinetic energy.
A lower average kinetic energy means a lower temperature. This is evaporative cooling. Energy is then transferred from the surroundings or the object in contact with the liquid, which can continue the evaporation process.
Figure 17. Original KG2UNI diagram.
8.3 Factors affecting the rate
Higher temperature increases the fraction of particles with enough energy to escape. A larger surface area exposes more particles at the surface. Faster air movement removes vapour from above the liquid, maintaining a concentration difference and reducing the chance that escaped particles return.
Although humidity is not explicitly named in the syllabus list, drier air generally increases evaporation because it contains less vapour of that liquid. In an examination, prioritise the factors stated in the question and the three required factors: temperature, surface area and air movement.
8.4 Applications
Sweating cools the body because water evaporates from the skin and removes energy. Wind increases the cooling effect by moving humid air away. Loose, porous water containers can cool their contents in dry climates because water seeps to the outer surface and evaporates.
Wet clothes dry faster when spread out, placed in warm conditions and exposed to moving air. A volatile liquid on the skin feels cold because it evaporates quickly. Refrigeration uses evaporation and condensation of a working fluid, although the full refrigeration cycle is beyond the required detail here.
Figure 18. Original KG2UNI diagram.
8.5 Fair tests of evaporation
To compare evaporation rates, change only one factor. Use equal initial masses or volumes of the same liquid in identical containers, keep the measurement interval fixed and measure mass loss. When testing surface area, use containers with different exposed areas but keep temperature and airflow as similar as possible.
Open flames must not be used with flammable liquids. Water is the safest choice for a classroom investigation. A balance measuring mass loss provides more objective data than judging whether a surface “looks dry.”
Worked examples
Sweating in wind
Wind removes water vapour from near the skin. Evaporation becomes faster, higher-energy molecules leave the sweat, and the skin loses thermal energy more rapidly.
Spreading clothes
Spreading clothes increases the exposed water surface area. More surface particles can escape each second, so drying is faster.
Cooling after spirit
A volatile liquid evaporates rapidly. The escaping particles take energy from the liquid and skin, reducing their temperature.
Practical focus
Investigation
Place equal masses of water in a shallow dish and a narrow beaker. Keep both beside each other and measure mass after a fixed time. The shallow dish should lose more mass because of its larger surface area. Use a draught-free location if surface area is the variable.
Examination guidance
- Use “higher-energy particles escape” and “average kinetic energy decreases” for full cooling explanations.
- Do not say evaporation happens only at the boiling point.
- In a fair-test question, state both the independent variable and important controls.
Check your understanding
- Where does evaporation occur?
- Why does evaporation cool a liquid?
- State three factors that increase evaporation.
- Why do clothes dry faster in wind?
- Give one difference between boiling and evaporation.
Answers
- At the surface.
- Higher-energy particles escape, reducing the average kinetic energy and temperature of the remaining liquid.
- Higher temperature, larger surface area and greater air movement.
- Moving air removes water vapour from near the clothes, so evaporation continues faster.
- Boiling occurs throughout the liquid at a definite temperature; evaporation occurs only at the surface and can occur at any temperature.
