Put a Balloon in the Freezer: Gases and Temperature

Blow up a balloon, measure around its middle with a tape, and put it in the freezer. An hour later it comes out visibly saggy. Take the measurement again and you get a number, and that number is the point of this Charles’s law balloon experiment. The ideal gas law predicts that the circumference should drop by about 5%. Most balloons drop by more than that. The gap is not a measurement error, and working out where it comes from teaches more physics than the demonstration itself.

Key Takeaways

• You’ll measure a balloon’s circumference, chill it in the freezer for an hour, and measure again as it warms back up.
• It teaches Charles’s law, that gas volume is proportional to absolute temperature, and why temperature must be in kelvin.
• Best for ages 6 to 12. Younger children get the saggy balloon; older ones get the arithmetic.
• The usual failure is measuring in the wrong place. Mark a line around the balloon with a marker and always measure on that line.
• Uninflated and burst balloons are a serious choking hazard for children under eight. This is regulated, not folklore.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Latex balloons	3	Supermarket. Check for a latex allergy before you start	Foil party balloons work but stretch far less, so the effect is smaller
Flexible tape measure	1	Sewing box	A length of string plus a ruler: wrap, pinch, lay flat, measure
Permanent marker	1	Stationery drawer	A loop of tape as a reference line
Freezer	1	Kitchen. Most home freezers run near minus 18 °C	A bucket of ice and water gets you to 0 °C, which halves the effect
Fridge thermometer	1	Supermarket or hardware shop	Assume minus 18 °C, the standard setting, and say so in your notes
Bicycle or balloon pump	1	Optional, but it makes Take It Further possible	Skip it; you lose the humidity comparison

Time: prep 5 min, experiment 90 min mostly waiting, cleanup 2 min Cost: everything is probably in your kitchen Mess level: low

Do the Experiment

Step 1 — Inflate to a size you can repeat

Blow the balloon up to about 25 cm across, well short of bursting, and tie it off. A tight balloon is a stiffer rubber shell and gives a smaller result, so stop while it still feels soft. Note how you inflated it, by mouth or by pump, because that matters in Step 6.

Step 2 — Draw a measuring line and record the baseline

With the marker, draw a line all the way around the balloon’s widest part. Wrap the tape on that line and write down the circumference to the nearest millimetre. Measure three times and take the middle value. Also note the room temperature.

Step 3 — Predict, in kelvin

Have your child convert both temperatures to kelvin: room temperature 22 °C is 295 K, a freezer at minus 18 °C is 255 K. Charles’s law says volume scales with absolute temperature, so the volume ratio is 255 ÷ 295 = 0.864. Circumference scales with the cube root of that, which is 0.952. Predict the new circumference before the balloon goes in.

Step 4 — Freeze it for an hour

Put the balloon in the freezer, not touching the walls or an ice pack. Close the door. Set a timer for 20, 40 and 60 minutes and pull it out briefly at each one to measure.

What you should see: a steady shrink that flattens off. Most of the change has happened by 30 minutes and it is essentially finished by an hour. A 60 cm balloon typically comes out between 54 and 57 cm, and it will look much saggier than a 5% number sounds, because a balloon that has lost its tautness wrinkles.

Step 5 — Take it out and watch it recover

Put it on the counter and measure every two minutes.

What you should see: the balloon re-inflates to within a few millimetres of its starting size over 10 to 20 minutes. It may not get all the way back, and condensation often appears on the outside as room air cools against the cold rubber.

Step 6 — Do the whole thing again with a pumped balloon

Inflate a second balloon with a pump instead of your mouth, to the same 25 cm, and repeat.

What you should see: the pumped balloon usually shrinks less than the mouth-blown one. Same temperatures, same rubber, different result, and the reason is the water vapour in your breath.

What’s Actually Going On

Charles’s law says that at constant pressure, the volume of a fixed amount of gas is proportional to its absolute temperature. NASA’s Glenn Research Center states it as V = constant × T and works a numerical example. The word that does all the work is absolute. Using Celsius gives nonsense: 22 °C to minus 18 °C looks like a change from 22 to minus 18, which would predict a negative volume. In kelvin it is 295 to 255, a drop of 13.6%.

Why kelvin? Because temperature in the kinetic picture is a measure of the average kinetic energy of the molecules, and zero has to mean zero motion. HyperPhysics’ treatment of the ideal gas sets out the relationship. Cool a gas and its molecules slow down, so they hit the balloon wall less often and less hard, so a smaller balloon is enough to balance the push from outside.

Volume falls 13.6%, but you are measuring circumference, which goes as the cube root of volume. The cube root of 0.864 is 0.952, so a 60 cm balloon should measure about 57.1 cm. That 2.9 cm is the honest prediction, and it is much less dramatic than the sag suggests, because a balloon that loses even a little internal pressure stops being a smooth sphere and starts being a wrinkled bag.

Now the interesting discrepancy. Two things push against the simple prediction, in opposite directions.

Water vapour condenses out. Air from your lungs leaves saturated with water vapour at around body temperature, where the saturation vapour pressure is roughly 5 kilopascals, about 5% of atmospheric pressure. At minus 18 °C, air can hold only a small fraction of that, so nearly all of that vapour condenses or freezes onto the inside wall. Water vapour that becomes liquid is no longer gas, so the balloon loses about 5% of its gas molecules. That makes the shrink bigger than Charles’s law alone predicts.

The rubber is not a constant-pressure container. A balloon’s internal pressure is atmospheric plus whatever the stretched rubber adds. As the balloon shrinks, the rubber relaxes and that extra pressure falls, which by itself would let the gas occupy more volume. This pushes the other way, and it is why Charles’s law is only approximately the right law for a balloon.

Which effect wins depends on how you inflated it, and that is exactly what Step 6 tests. A pump draws in room air at maybe 50% humidity, so there is far less vapour to lose.

How to Explain It to Your Kid

Ages 5–8: Cold air takes up less room

Have them squeeze the balloon before and after. Tell them air is made of millions of pieces too small to see, that warm pieces jump around fast and push the balloon out, and cold pieces move slowly and push less, so the balloon gets smaller without losing any air at all. Then let them hold it while it warms up and feel it tighten in their hands, which is the part that lands.

Ages 9–12: Celsius will lie to you

This is the best possible excuse to introduce the kelvin scale, because here Celsius actively gives the wrong answer and kelvin gives the right one. Do both calculations and compare them to the tape measure. Then show them the cube root step, which is why a 14% volume change looks like a 5% change in the measurement they can take.

Ages 13+: Find out which correction dominates

Set them the real problem: the ideal-gas prediction and the measurement disagree, so quantify both corrections. Estimate the mole fraction of water vapour in breath from saturation vapour pressure, estimate the balloon’s gauge pressure by how hard it is to squeeze, and decide which effect is larger. Then test it with mouth versus pump, three balloons each, and see whether the difference is bigger than the spread in their own measurements.

The question to ask: “If we took this balloon up a mountain instead of putting it in the freezer, would it get bigger or smaller, and would the reason be the same?”

Why Teaching This Early Pays Off

The gas laws are usually a memorisation exercise with three names attached, and the single most common exam error in the whole topic is using Celsius where kelvin is required. A child who has watched Celsius predict a negative volume is unlikely to make that mistake later. That is a narrow payoff, and it is real.

The broader one is the habit in Step 6. The measurement did not match the prediction, so instead of shrugging, you looked for the missing physics and found two candidates pulling opposite ways. Chi and Wylie’s ICAP framework argues that learners who construct and explain outperform those who merely watch (Chi & Wylie, 2014), and this is the version of that worth practising: a disagreement between model and data is information, not a mistake. Smith, diSessa and Roschelle make the related point that intuitions and partial models are the raw material of understanding rather than errors to be deleted (Smith et al., 1994).

No afternoon changes a trajectory, and honest framing beats inflated promises. But “the model was close and here is exactly why it was off” is a sentence a 13-year-old can learn to say, and it is most of what science is.

Before You Start: What Parents Should Know

Best age	6–12. From 6 for the observation, from 10 for the kelvin arithmetic
Supervision	Light, but an adult must control the balloons. Inflate them and keep the packet out of reach
Real hazards	Choking and suffocation. US regulation 16 CFR 1500.19 requires latex balloons to carry a choking-hazard warning, because children under eight can choke on uninflated or broken pieces. Pick up every fragment of a burst balloon immediately. Latex allergy is real and can be severe; use foil balloons if anyone in the house is affected
Don’t	Do not inflate a balloon with a gas canister, do not put a balloon in a freezer that contains dry ice, and do not over-inflate to get a bigger effect. A tighter balloon gives a smaller result and a louder bang

When It Doesn’t Work

• The circumference barely changed. You measured in a different place. Draw the marker line in Step 2 and always measure on it. Balloons are not spheres and the widest point moves.
• It shrank far more than 5%. Check the balloon is not touching the freezer wall or an ice pack, where it loses heat faster and gets colder than the air. Also check whether it simply deflated: mark the knot and see if it has loosened.
• It never recovers its original size. Some loss is normal, because latex is slightly permeable and the balloon leaks a little helium-free air over an hour. If it has lost more than about 2 cm, the knot is leaking.
• Your measurements disagree by a centimetre each time. Normal, and the reason you measure three times and take the middle value. Have the same person measure every time, with the same tension on the tape.

Take It Further With the Charles’s Law Balloon Experiment

1. Build a Charles’s law graph. Measure the circumference in four environments: a warm room, a cold room, the fridge at about 4 °C, and the freezer at about minus 18 °C. Convert each to kelvin, cube the circumference to get something proportional to volume, and plot it against temperature. You should get something close to a straight line that extrapolates toward zero near minus 273 °C. Extrapolating to absolute zero from kitchen measurements is a genuinely impressive thing for a 13-year-old to have done.
2. Mouth versus pump, properly. Three balloons of each kind, same size, same hour. Average each group and compare the difference against the scatter within each group.
3. Go the other way. Warm a balloon with a hairdryer on its lowest setting, held 30 cm away, and measure. Then pair it with the syringe-and-marshmallow version of Boyle’s law, which holds temperature fixed and changes pressure instead, or with proving air has mass on a balloon balance.

What to Watch For Over the Next 3 Months

• Week 4: they notice a car tyre looking flat on a cold morning and connect it, unprompted, to the balloon.
• Month 2: they use kelvin voluntarily, or at least ask whether a temperature needs converting before it goes into a calculation.
• Month 3: they ask what happens at absolute zero. That question opens the door to the third law of thermodynamics, and it is worth a long answer.

Frequently Asked Questions

Will the balloon pop in the freezer?

No. It is shrinking, not stretching, so the rubber is under less tension than when you tied it. Latex does stiffen in the cold, so handle a frozen balloon gently and do not squeeze it hard, but bursting from cold alone is not something you should expect.

Can I use the fridge instead of the freezer?

Yes, with a smaller effect. A fridge at 4 °C is 277 K, so the volume ratio is 277 ÷ 295 = 0.94 and the circumference drops only about 2%. That is close to the limit of what a sewing tape can resolve, which is why the freezer is the better choice.

Why does the balloon feel cold but not look much smaller?

Because circumference changes by the cube root of volume. A 14% volume loss is a 5% change in the number you measure, and 5% of 60 cm is 3 cm. The visible sagging comes from the loss of tautness, not from the size change.

Is a helium balloon better?

No, and it introduces a problem. Helium diffuses through latex quickly, so your balloon shrinks for two reasons at once and you cannot separate them. Plain air from your lungs or a pump is the right choice here.

What school topic does this prepare?

The gas laws and the kelvin scale, in middle-school science and again in high-school chemistry and physics. The National Research Council’s framework for K–12 science names “analysing and interpreting data” as a core practice, and comparing a prediction to a tape measure is precisely that (NRC, 2012).

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. NASA Glenn Research Center. “Charles and Gay-Lussac’s Law.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/charles-and-gay-lussacs-law/
2. Nave, C. R. “Ideal Gas Law.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/idegas.html
3. NOAA National Weather Service. “Vapor Pressure Calculator.” https://www.weather.gov/epz/wxcalc_vaporpressure
4. U.S. Consumer Product Safety Commission. 16 CFR 1500.19, “Misbranded toys and other articles intended for use by children.” Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-16/chapter-II/subchapter-C/part-1500/section-1500.19
5. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
6. Smith III, J. P., diSessa, A. A., & Roschelle, J. (1994). “Misconceptions Reconceived: A Constructivist Analysis of Knowledge in Transition.” Journal of the Learning Sciences, 3(2), 115–163. https://doi.org/10.1207/s15327809jls0302_1
7. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165