A student takes a warm drink from the fridge. The drink is in a thin glass cup. The student notices that the outside surface of the glass quickly becomes wet, even though no liquid has been spilled. Explain why the outside of the glass becomes wet.

Eduqas A-Level Physics — 3.4 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. The glass is cold, so its outer surface is at a lower temperature than the surrounding air.
2. The surrounding air contains water vapour.
3. When the vapour contacts the cold surface it loses thermal energy.
4. The vapour condenses, changing from gas to liquid on the glass.
5. Liquid water droplets form on the outside of the glass.

Examiner tips

  • Use the exact sequence of points from the mark scheme; each point counts as one mark.
  • Show the cause (cold surface) and effect (condensation) clearly.
  • Mention that the droplets are liquid water, not ice.
  • Avoid unnecessary words or explanations beyond the five points.

Common mistakes

  • Confusing condensation with evaporation or freezing.
  • Failing to state that the air contains water vapour.
  • Using vague terms like "moisture" without linking to vapour.

Mark scheme (5 marks)

  1. The glass is cold / the outside surface of the glass is at a lower temperature than the surrounding air
  2. Water vapour is present in the air / the air contains water vapour (moisture)
  3. Water vapour in the air loses thermal energy / cools down when it contacts the cold surface
  4. The water vapour condenses / changes state from gas to liquid on the surface of the glass
  5. This forms liquid water (droplets) on the outside of the glass

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