Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Evaporation
What is Evaporation?
Evaporation is the escape of the more energetic particles from the surface of a liquid, changing it to a gas; it occurs only at the surface, can happen at any temperature below the boiling point, and cools the liquid left behind because the particles remaining have a lower average kinetic energy.
This definition is part of the Thermal Physics chapter in Cambridge O Level Physics.
Evaporation in context
Heating a substance normally increases the average separation of its particles, so the substance expands — most for gases, less for liquids, least for solids. The energy needed to raise the temperature of one kilogram of a substance by one degree is its specific heat capacity, \(c = \Delta E/(m\,\Delta\theta)\). When a pure substance melts or boils, the energy supplied separates particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; that energy is the latent heat. Evaporation is different again: it happens only at the surface, at any temperature, and it cools the liquid left behind.
Common mistakes with Evaporation
- 7. “Boiling and evaporation are identical.” Why it is wrong: they differ in location, temperature condition, the presence of bubbles and their effect on the liquid's temperature. Corrected model: boiling occurs throughout the liquid at one definite temperature; evaporation occurs at the surface at any temperature and cools the liquid. Exam-safe: “Boiling occurs throughout the liquid at a definite temperature, whereas evaporation occurs only at the surface and at any temperature.”
- 8. “Evaporation occurs throughout a liquid.” Why it is wrong: a particle deep inside is surrounded on all sides by neighbours attracting it and has no free direction in which to escape. Corrected model: evaporation is a surface process; only surface particles can leave the liquid. Exam-safe: “Only particles at the surface can escape, so evaporation takes place at the surface of the liquid.”
- 9. “The coldest particles escape during evaporation.” Why it is wrong: slow particles do not have enough energy to overcome the attractive forces. If the slowest left, the liquid would warm up, not cool. Corrected model: the most energetic surface particles escape, lowering the average kinetic energy of those remaining. Exam-safe: “The most energetic particles escape from the surface, so the average kinetic energy of the remaining particles falls and the liquid cools.”
Examiner tips on Evaporation
- Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
- Five phrases that keep a Topic 2 answer precise “…so the average separation of the particles increases…” (expansion) “…collisions with unit area of the wall become more frequent…” (pressure) “…the fluid expands, so its density decreases, so it rises…” (convection) “…the energy separates the particles, increasing their potential energy rather than their kinetic energy…” (change of state) “…the most energetic particles escape from the surface…” (evaporation)
Questions students ask about Evaporation
What is the difference between boiling and evaporation?
Boiling happens throughout the whole liquid, only at one definite temperature — 100 °C for water at standard atmospheric pressure — with bubbles of vapour forming inside it. Evaporation happens only at the surface, can occur at any temperature below the boiling point, and cools the liquid left behind because the most energetic particles escape, lowering the average kinetic energy of those that remain. The two are not interchangeable.