Water droplets on the outside of a cold cup form when water vapour from the surrounding air cools and condenses on the cup. A double-walled cup with an insulating gap can keep its outer surface warmer than a comparable single-walled cup, so fewer droplets may form.
Follow the heat transfer and the state change separately. Heat travels from the warmer surroundings towards the colder drink. Water vapour near a sufficiently cold outer surface cools and changes from gas to liquid.
- Identify the surrounding air as the source of the water vapour.
- Compare the temperature of the two outer surfaces.
- Keep the drink temperature, surroundings and observation time comparable.
Worked example | Two cups in the same room
A single-walled cup and a double-walled cup contain the same volume of chilled water at the same starting temperature. After five minutes in the same room, the first cup is wet outside while the second has few visible droplets.
The gap between the walls slows heat transfer towards the cold drink. The outer wall can stay warmer, so nearby water vapour may cool less. If that surface is warm enough, very little water vapour condenses there.
Make the comparison fair
Choose cups with similar outer dimensions and outer materials if possible. Wipe both dry before starting and leave them side by side for the same time. Record the outer surface temperatures and compare droplets over similar areas.
Keep the cups away from a fan or sunny window, and avoid splashing the outside when filling them. Repeating the test gives you more than one observation to compare.
Explain a changed result
On a humid day, droplets appear even on the outer wall of the double-walled cup. Explain how this can happen.
Answer. The outer surface can still be cool enough for water vapour in the surrounding air to condense. The extra wall slows heat transfer, and the result still depends on the air and the surface temperature.
Frequently asked questions
Where does the liquid on the outside come from?
It comes from water vapour in the surrounding air, assuming the cup is intact and the water inside has not spilled.
Which state change should I name?
Condensation is the change from water vapour, a gas, to liquid water.
Official information
MOE, 2023 Primary Science Teaching and Learning Syllabus, Primary Five Cycles in Matter and Water. Syllabus checked 11 November 2026. The examples are original.