Science and Technology K–6 · Year 5

Squash the syringe: gases compress, liquids do not

Science understanding: Chemical sciences

Practical, model not builtLow risk

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The idea

Gas particles are far apart with empty space between them, so a sealed syringe of air can be pushed to half its volume, while water particles are already touching and the plunger will not move.

What you need

2 plastic syringes, 60 mL, without needles
syringe caps or a rubber bung to seal the tip (a finger works for a quick test)
water
a bathroom scale to push on (optional, to read the force)
recording table

How to do it

Draw 60 mL of air into syringe A and seal the tip. Push the plunger in steadily with the palm of your hand to the 30 mL mark and read the volume; 25 mL is the limit for this syringe, so stop there and push no further. Let go and record what the plunger does.
Draw 60 mL of water into syringe B with no air bubble and seal it. Push with the palm as firmly as it took to reach 30 mL on syringe A, hold for 5 seconds and read the volume.
Stand syringe A tip-down on the bathroom scale, press the plunger to 40 mL, then 30 mL, and read the scale each time.
Record the volume against the reading in a table and graph it.
Draw the particles in each syringe before and after pushing.
Explain why a bicycle pump works with air but a water pistol has no compression stroke.

What you should see

The air syringe compresses to about half its volume with a firm push and springs back when released; the water syringe does not move a visible amount under the same push. Boyle's law: pushing 60 mL of air to 30 mL at 20 degrees Celsius doubles the pressure from 101.3 to 202.6 kPa, which on a 60 mL syringe piston (about 26 to 29 mm across inside, 5.3 to 6.6 cm2) needs about 5.5 to 6.8 kg of push on the scale, plus a little more to overcome the plunger's friction. At the 25 mL limit the pressure is 243 kPa, so the extra push is 141.8 kPa on the same piston, 75 to 94 N (7.7 to 9.6 kg), which is as much as a plastic syringe should be given. Water's compressibility is so small that 1 atmosphere of extra pressure shrinks it by 0.005 percent, invisible in a syringe.

What changes

What you change: volume the air is pushed to
What you measure: force on the scale (pressure)
What you keep the same: temperature
amount of air sealed in
syringe

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Air is nothing, so the syringe is empty.
The plunger stops because it hits the water.
Squashing air makes it heavier (same particles, less space).

Safety card

Low riskLearners carry it out

Hazards

plunger flying out under release
syringe breaking under a very hard push

Controls

keep the tip pointed down
push with the palm, not thumbs, and stop at 25 mL

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-6MW-S ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-DAT-01
Australian Curriculum v9AC9S5U04AC9S5I03AC9S5I05

Sources

The pages the author read to write this activity.

All Concept Studio activities