Droplets on the outside of a cold cup can form when water vapour in the surrounding air loses heat near the cold surface and condenses into liquid water. The vapour comes from the air outside the cup.
The MOE Primary Science syllabus covers condensation in the water cycle. This PSLE Science guide compares a cold cup with a room-temperature cup and shows how to use observations to support an explanation.
- State where the water vapour was before the change.
- Explain that it loses heat near the colder surface.
- Name the change from water vapour to liquid water.
- Use a comparison to support your explanation.
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Describe what happens outside the cup
The room air contains water vapour. When the air next to the cold cup cools enough, some water vapour in it changes into liquid water. The droplets appear on the outer wall because that is where the surrounding air meets a cold surface.
The cold liquid inside helps cool the cup wall. Keep that role separate from the source of the droplets. For the condensation explanation, water vapour starts outside and the liquid droplets form outside.
A short exam answer can follow three steps. Water vapour in the surrounding air loses heat near the cold cup. It condenses into liquid water. The liquid collects as droplets on the outer surface.
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Worked example | Compare two dry cups
Imagine two identical transparent cups filled to the same height with blue-tinted water. Cup A holds chilled water and Cup B holds room-temperature water. Both outer surfaces are wiped dry, the cups are covered, and no water is spilled. They stand next to each other for the same time.
| Cup | Water inside | Outside observation |
|---|---|---|
| A | Chilled and blue | Clear droplets form |
| B | Room temperature and blue | Surface stays dry |
Only the cold cup develops clear droplets. This supports the idea that the cold outer surface caused water vapour in the air to condense. The colour is supporting evidence in this setup. The cool surface and the comparison cup supply the explanation for why droplets formed on A.
When a question asks for an observation, write that clear droplets formed on Cup A. When it asks for an explanation, use the heat loss and change of state.
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CHAPTER 3 OF 4 · Back to contents
Practise with a cold surface that holds no drink
A pupil compares two identical dry metal spoons. One was chilled in a dry container and one was kept at room temperature. Both are placed in the same room. Predict what you might see if the chilled spoon is cold enough for condensation.
The chilled spoon can develop droplets. Water vapour in the surrounding air loses heat near the spoon and condenses. The room-temperature spoon can remain dry under the same conditions.
The spoon example removes the drink entirely, so it helps you test your explanation. If you can still explain the droplets without a drink present, you have identified the source of water vapour correctly.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Does every cool object become wet?
Droplets form when the surface cools nearby air enough for condensation. The amount of water vapour in the air and the temperature both matter.
What happens after the cup warms up?
The droplets may evaporate over time. In evaporation, liquid water changes into water vapour.
What can a parent ask to check the explanation?
Ask where the water vapour was before the droplet formed, and what change of state happened beside the cup.
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Sources and next reading
SEAB PSLE Science examination syllabus, applicable from 2026