Introduction
Ever wondered what really happens when you push down a plunger trapping air inside a container? This PSLE Science air compression question is a classic way to test your understanding of air as a compressible material. In this post, we break down the diagram and the teacher’s working clearly so you can confidently pick the correct option.
The Question
A container has air trapped above water and a plunger is pushed down.
Question: What will happen to the mass and volume of the air inside the container when the plunger is pushed down?
Options (as shown):
(1) mass remains the same, volume increases
(2) mass increases, volume increases
(3) mass decreases, volume decreases
(4) mass remains the same, volume decreases ✅
Correct Answer: Option (4)
Step-by-Step Working (Teacher Working)
Step 1: Identify what is being compressed
When the plunger is pushed down, the air space becomes smaller.
That means the air is compressed.
Step 2: Decide what happens to volume
Compressed air takes up less space.
So the volume of air decreases.
Step 3: Decide what happens to mass
Even though the air is compressed, the air is still trapped inside the container (it does not escape).
So the amount of air (and therefore the mass of air) remains the same.
Final Choice
✅ Mass remains the same
✅ Volume decreases
➡️ Option (4) is correct.
Key Concepts
1) Air is compressible
- Solids: not easily compressed
- Liquids: not easily compressed
- Gases (air): easily compressed
2) Mass vs Volume (Don’t mix them up!)
- Mass changes only if air enters or leaves the container
- Volume changes when air is compressed or expanded
3) What pushing a plunger does
- Pushing down reduces the space → higher pressure → smaller air volume
Tips for Students
- Always ask: “Can the air escape?”
- If no, mass usually stays the same
- Look for words like “pushed down” or “compressed”
- That usually means volume decreases
- Don’t rush: many students choose “mass decreases” by mistake—mass only decreases if air leaves!
For Parents
This PSLE Science air compression question trains children to read diagrams carefully and apply core concepts (matter, gases, compression). These questions are common because they test:
- scientific reasoning
- cause-and-effect thinking
- exam accuracy under time pressure
With guided practice, students learn to answer faster and with fewer careless errors.
Conclusion
To solve a PSLE Science air compression question, focus on two checks:
- Is the air trapped? → mass stays the same
- Is the air compressed? → volume decreases
That’s exactly why the correct answer is Option (4). If you want more step-by-step practice, watch the video tutorial and try similar questions to build speed and confidence.