Introduction
This PSLE Science question checks student understanding of the composition of air.
Many students memorise the wrong percentages or forget that water vapour is also a part of air.
The teacher explains the correct components and their approximate proportions.
The Question
Which gases are found in air?
Options included:
- A: Oxygen
- B: Nitrogen
- C: Carbon dioxide
- D: Water vapour
Students must identify all gases present naturally in Earth’s atmosphere.
Step-by-Step Explanation (Matching Teacher’s Working)
1. Air is made up of several gases
The teacher highlights the typical composition:
- 78% Nitrogen
- 21% Oxygen
- 0.03% Carbon dioxide
- Variable amount of Water vapour (depends on humidity)
2. Check each option provided
✔ Oxygen – Yes, 21%
✔ Nitrogen – Yes, 78%
✔ Carbon dioxide – Yes, small amount (0.03%)
✔ Water vapour – Yes, varies by weather
All four gases listed are found in air.
Final Answer
Option (4): A, B, C and D
(All four gases are found in air.)
Key Concepts
| Concept | Why It Matters |
| Composition of air | Foundation for many PSLE Science topics |
| Water vapour | A gas that varies with humidity |
| Carbon dioxide | Essential for photosynthesis |
| Nitrogen | Most abundant gas in air |
Tips for Students
✔ Memorise the approximate percentages of gases in air
✔ Remember that water vapour is always part of air
✔ Don’t confuse “air” with “oxygen” — oxygen is only 21%
✔ Carbon dioxide is small in amount but still important
For Parents
This question forms the basis for topics like:
- Respiration
- Photosynthesis
- Heat and temperature
- Water cycle
A strong understanding of the composition of air helps your child build confidence for multiple sections of the exam.