Explain why a liquid evaporates at temperatures below its boiling point.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A puddle of water on a road disappears on a warm day, even though the temperature of the water is well below 100 °C.
Model answer (4 marks)
1. Particles in a liquid possess a range of kinetic energies, not all particles have the same energy.
2. Some surface particles have enough kinetic energy to overcome the forces of attraction to neighbouring particles.
3. These high‑energy particles escape from the surface and become gas/vapour.
4. The average kinetic energy of the remaining liquid particles decreases, so the liquid cools; this can happen continuously at any temperature.
2. Some surface particles have enough kinetic energy to overcome the forces of attraction to neighbouring particles.
3. These high‑energy particles escape from the surface and become gas/vapour.
4. The average kinetic energy of the remaining liquid particles decreases, so the liquid cools; this can happen continuously at any temperature.
Examiner tips
- Use the term ‘kinetic energy’ and ‘forces of attraction’. Show the sequence: range of energies → surface escape → cooling. Keep each point concise to fit 4 marks.
Common mistakes
- Confusing evaporation with boiling; forgetting the role of surface particles. Using vague phrases like ‘heat up’ instead of ‘kinetic energy’. Omitting the cooling effect of evaporation.
Mark scheme (4 marks)
- Particles in a liquid have a range of kinetic energies / not all particles have the same energy
- Some particles at the surface have enough (kinetic) energy to overcome the forces of attraction to neighbouring particles
- These high-energy particles escape from the surface and become gas / vapour
- The average kinetic energy of the remaining liquid particles decreases, so the liquid cools / this can happen continuously at any temperature
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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