Introduction
This question tests the concept of condensation and heat transfer, a common PSLE Science theme where students must explain why water droplets appear on surfaces.
The teacher’s explanation highlights how temperature differences cause fogging on spectacles — a real-life application of condensation.
The Question
Alice and Beatrice were travelling in an air-conditioned car.
When they stepped outside, Alice’s lenses fogged up but Beatrice’s lenses did not.
Explain why this happened.
Step-by-Step Explanation (Following Teacher’s Working)
1. Inside the air-conditioned car, Alice’s spectacles became cold
The teacher writes:
- “Heat from Alice’s head transferred to lenses”
- This means the lenses lost heat and became cooler than the outside air.
2. Outside the car, the air was warm and humid
Warm air contains more water vapour.
Beatrice did not wear her spectacles long enough for them to cool, so her lenses were not as cold.
3. When Alice stepped out, water vapour condensed on her cold lenses
Condensation happens when:
- Warm, moist air touches a cold surface
- Water vapour loses heat
- It changes into tiny water droplets
- This forms fog on the lens
This is the same effect as foggy mirrors after a hot shower.
4. Beatrice’s lenses did not fog
Because her lenses were not as cold, the water vapour did not condense on her lenses.
✅ Final Answer
Alice’s lenses were colder than the warm, humid air outside the car. Water vapour in the air condensed on her cold lenses, forming tiny water droplets. Beatrice’s lenses were not cold, so condensation did not occur.
Key Concepts
| Concept | Why It Matters |
| Condensation | Explains why fog / droplets form |
| Heat Transfer | Cold surfaces absorb heat from moist air |
| Temperature Difference | Required for condensation |
| Water Vapour | Must lose heat to condense |
Tips for Students
✔ Fogging = condensation, not evaporation.
✔ Always mention: warm air → touches cold surface → loses heat → water droplets form.
✔ Compare both people (Alice vs Beatrice) as the exam requires.
✔ Use temperature keywords: warmer, cooler, lose heat.
For Parents
Fogging of spectacles is a common real-world scenario.
Helping your child understand heat transfer and condensation builds confidence for many PSLE topics such as:
- Water cycle
- States of matter
- Air and the environment
- Heat energy
Mastery of these basics leads to stronger explanation-type answers.