TutorForAll

Science and Technology K–6 · Year 5

Balloon on a bottle: air expands when it is warmed

Science understanding: Chemical sciences

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Warming a gas makes its particles move faster and spread out, so air in a bottle expands and inflates a balloon, and cooling reverses it, without any air being added.

Safety card

Low riskAn adult supervises

Hazards

  • hot water
  • a thin bottle softening in hot water
  • latex allergy (balloons are natural rubber latex)

Controls

  • teacher pours and moves the bottle
  • rigid PET bottles only, no glass
  • check allergy records; a learner with a latex allergy does not handle the balloons, and the teacher fits them

Note

Hot water above 50 degrees Celsius is poured by the teacher. Risk assessment before the lesson using the NSW Department of Education Chemical Safety in Schools package (CSIS 1.7 Risk Assessment) or Primary RiskAssess.

What you need

  • an empty 500 mL rigid PET bottle (not glass, which can crack when moved from hot to iced water)
  • a balloon
  • a bowl of hot water at about 60 degrees Celsius (teacher pours) and a bowl of iced water
  • thermometer, tape measure, timer

How to do it

  1. Stretch the empty balloon over the neck of the bottle at room temperature; it hangs limp.
  2. Stand the bottle in the hot water and watch for 2 minutes. Measure the balloon's circumference every 30 seconds.
  3. Move the bottle to the iced water and watch for 2 minutes, measuring again.
  4. Record the water temperatures and the largest and smallest circumferences.
  5. Repeat with a 1.25 L PET bottle and compare the size of the balloon.
  6. Explain with particles why no air was added yet the balloon grew, and why it shrank again.

What you should see

The balloon inflates in the hot water and slumps, then pulls inward, in the iced water. Charles's law: 500 mL of air warmed from 20 to 60 degrees Celsius expands to 568 mL, so at most 68 mL moves into the balloon (the air rarely reaches the full water temperature in 2 minutes), a small but clear bulge; the 1.25 L bottle gives 170 mL and a bigger balloon. Cooling to 5 degrees shrinks the 500 mL to 474 mL and the balloon is sucked inward.

What changes

What you change
water temperature (and bottle volume)
What you measure
balloon circumference
What you keep the same
  • same balloon
  • time in the water
  • starting temperature

Common misconceptions

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

  • Hot air is added from the water.
  • The balloon inflates because heat is a gas.
  • The air particles themselves get bigger when hot.

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-SST3-1WS-S
  • Australian Curriculum v9AC9S5U04AC9S5I03AC9S5I04

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S5U04
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. primaryconnections.org.au/v84-sequences/whats-matter
  5. edu.rsc.org/primary-science/gases-primary-science-video-demonstrations/916.article

All Concept Studio activities