Air can be compressed. In a needle-free syringe with a sealed opening, pushing the plunger reduces the space occupied by the trapped air. If the seal holds, the same amount of air occupies a smaller volume.
If the opening is uncovered, air can leave while the plunger moves in. The barrel reading may fall in both setups, so the most important detail in an explanation is whether the air was trapped.
- Look at whether the outlet is open or sealed.
- Use the syringe markings to describe the change in volume.
- Use the sealed setup to explain compression because air is prevented from leaving.
CHAPTER 1 OF 4 · Back to contents
Compare an open and a sealed syringe
Imagine a diagram showing two identical needle-free syringes with their plungers set at 12 mL. The outlet of syringe A is open. The outlet of syringe B is sealed tightly. Both syringes contain air, even though the air is invisible.
When the plunger of A moves in slowly, air can escape through the outlet. When the plunger of B moves in, the sealed outlet keeps the air inside. If B’s reading changes from 12 mL to 8 mL, the trapped air now occupies 8 mL. The change in the reading shows that its volume decreased.
CHAPTER 2 OF 4 · Back to contents
Write a complete answer from the evidence
Question. Why does the reading of syringe B show that air can be compressed?
Sample answer. The outlet of syringe B is sealed, so the air remains trapped inside. When the plunger is pushed in, the volume of this trapped air decreases from 12 mL to 8 mL. This shows that air can be compressed.
A sentence such as “The plunger moved in” gives an observation. Add what the observation tells you about the air. The two pieces of evidence are the sealed outlet and the lower volume reading.
CHAPTER 3 OF 4 · Back to contents
Try a syringe containing water and an air bubble
In a sealed syringe containing water and a visible air bubble, a gentle push makes the bubble smaller. The amount of water stays the same and the water’s volume changes very little under the conditions of this comparison. Most of the visible decrease in volume comes from the trapped air bubble being compressed.
If a question asks what to keep the same in the open-and-sealed comparison, use identical syringes and the same starting volume of air. If you want to compare how far the plungers move for the same push, you would also need the same applied force. The condition you deliberately change is whether the outlet is open or sealed.
Ask your child to finish the sentence “I know the air stayed inside because …” and then to state what happened to its volume. This gives them a way to check whether both parts of the explanation are present.
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Does air in an empty-looking syringe take up space?
Yes. An ordinary syringe with its plunger drawn back contains air unless it has been prepared in a special way. The air occupies the available space inside the barrel.
Does the air lose mass when it is compressed?
In a sealed syringe that does not leak, the amount of air stays the same. Its volume becomes smaller when the plunger moves in.
Why does the plunger sometimes move back when released?
The compressed air pushes on the plunger. If that push overcomes friction, the plunger can move outward and the air expands.
Contents · Official syllabus and next practice
Official syllabus and next practice
Apply the same reasoning to a sealed syringe that starts at 15 mL and ends at 10 mL. Name the evidence that the air remained inside.