Boyle's Law With a Syringe and a Marshmallow

Drop a mini marshmallow into a medicine syringe, plug the tip with your thumb, and push. The marshmallow caves in like a deflating football. Let go and it springs most of the way back. That is the whole of this Boyle’s law experiment kids can run in under ten minutes, and the reason it lands is the second half: ask your child why the marshmallow shrank. Almost every child says “the air got squeezed out.” Nothing got out. The syringe is sealed. What changed is the pressure of the air trapped inside hundreds of tiny bubbles in the sugar. That gap between the obvious answer and the right one is the lesson.

Key Takeaways

• You’ll seal a mini marshmallow in a plunger syringe and compress it to half its volume, then stretch it back out.
• It teaches Boyle’s law, pressure times volume staying constant at a fixed temperature, and the bigger idea that gases compress enormously while liquids and solids barely do.
• Best for ages 8 to 14. Younger children enjoy it but won’t hold the volume-pressure trade-off in their heads.
• The usual failure is a leaky seal. If the marshmallow doesn’t move, air is escaping past your thumb or the nozzle.
• The marshmallow never fully recovers. That imperfection is worth explaining, not hiding.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Oral or catheter-tip syringe, no needle, 20–60 mL	1	Any pharmacy, usually under $2; feeding syringes from a pet shop also work	A bicycle pump with the hose blocked shows the same thing, but you can’t see inside
Mini marshmallows	3–4, since you’ll ruin a couple	Supermarket baking aisle. Contains gelatin; some brands list traces of wheat or soy	A small cube of fresh bread, or a stale mini doughnut. Both have trapped air
Thumb, or a blob of poster putty	1	—	A rubber syringe cap if your syringe came with one
Permanent marker	1	Stationery drawer	Masking tape to mark the barrel
Kitchen scale (optional)	1	—	Skip it; you only lose the density comparison
Small glass of water	100 mL	Tap	Needed only for the “does water squash too?” control

Time: prep 3 min, experiment 10–15 min, cleanup 2 min Cost: under $3, and everything is probably in your kitchen except the syringe Mess level: low

Do the Experiment

Step 1 — Get a syringe that actually seals

Block the tip with your thumb and push. You should feel firm resistance after a couple of centimetres, and the plunger should spring back when you let go. If it slides all the way down, the rubber seal is worn or your thumb isn’t flat against the nozzle. A 60 mL catheter-tip syringe has a wide, blunt tip that a child’s thumb covers easily, which is why it beats the skinny 10 mL kind.

Step 2 — Load one marshmallow

Remove the plunger. Drop a single mini marshmallow into the barrel. Replace the plunger and push it down gently until it just touches the marshmallow without squashing it. Read the volume on the barrel. On a 60 mL syringe you’ll usually be near 40 mL. Mark that line with the marker.

Step 3 — Ask for a prediction before you squeeze

This step is not padding. Ask: “If I block the tip and push the plunger to half that number, what happens to the marshmallow?” Write the answer down. A prediction made after the result is not a prediction.

Step 4 — Seal and compress slowly

Press your thumb flat over the nozzle. Push the plunger down to half your marked volume and hold it for five seconds. Push slowly. A slow squeeze keeps the air at room temperature, which is the exact condition Boyle’s law describes.

What you should see: the marshmallow visibly shrinks, puckers, and goes slightly translucent as its bubbles collapse. At half volume it loses roughly a third of its height. The plunger pushes back hard against your hand the whole time.

Step 5 — Release and watch it re-inflate

Let the plunger go while keeping your thumb on the nozzle. The marshmallow puffs back out in about a second.

What you should see: it recovers most of its size but not all of it. A squeezed marshmallow stays a little denser and a little more wrinkled than it started.

Step 6 — Now go the other way

Push the plunger down to about 10 mL with the tip open, so there’s barely any air left in the barrel. Seal the tip with your thumb and pull the plunger hard toward the 50 mL mark.

What you should see: the marshmallow swells noticeably, sometimes to one and a half times its original width, because you dropped the pressure around it to well under one atmosphere. This half of the demo surprises children far more than the squeeze does.

Step 7 — Run the control that makes the point

Fill the syringe with water instead, leaving no air bubble at all. Seal and push as hard as you can.

What you should see: nothing. The plunger barely budges. Same syringe, same hand, completely different result, because you’re pushing on a liquid now rather than a gas.

What’s Actually Going On in This Boyle’s Law Experiment

Boyle’s law says that for a fixed amount of gas held at a constant temperature, pressure multiplied by volume stays constant: pV = constant. Robert Boyle published the relationship in the 1660s, and NASA’s Glenn Research Center gives the clean version: halve the volume and the pressure doubles. In your syringe, going from 40 mL to 20 mL takes the trapped air from about 1 atmosphere to about 2.

The mechanism is collisions. The air in the barrel is nitrogen and oxygen molecules flying at a few hundred metres per second, and pressure is the summed force of those molecules hammering the walls. NOAA’s primer on air pressure puts it plainly: each impact is far too small to feel, but the total is considerable. Squeeze the same molecules into half the space and each square centimetre of wall gets hit twice as often. Temperature hasn’t changed, so the molecules are not moving faster. There are simply more of them per unit volume.

The marshmallow is the visible indicator, and here is the part most explanations skip. A marshmallow is a foam: a sugar-and-gelatin matrix whipped full of air pockets. When you raise the pressure in the barrel you raise it inside those pockets too, and each pocket shrinks according to the same pV law. The sugar syrup and gelatin around them barely change at all. That difference is enormous. HyperPhysics’ page on bulk elastic properties notes that water at the bottom of the Pacific, under roughly 400 atmospheres, compresses by only about 1.8 percent. Your air halved its volume under 2.

So the marshmallow doesn’t shrink because you crushed the sugar. It shrinks because the sugar is a flexible container for gas, and you compressed the gas.

Two honest caveats. First, Boyle’s law describes an ideal gas at constant temperature. If you ram the plunger down fast, the air heats up a little, which is adiabatic compression, and the pressure you feel is higher than pV = constant predicts. That is why Step 4 says to push slowly. Second, the marshmallow doesn’t fully recover because the gelatin matrix is viscoelastic: some bubble walls tear, and others stay folded. The gas obeys physics perfectly. The candy is a slightly lossy container.

How to Explain It to Your Kid

Ages 5–8: Air is a thing you can squash

Have them push the plunger with the tip open, then with your thumb sealing it. Two completely different feelings from the same hand. Tell them air is made of pieces too small to see, and pushing the plunger crowds those pieces together. The marshmallow is full of air pockets, so when the air gets crowded the pockets get smaller and the marshmallow shrinks with them. Then let them do the pull, which is the part they’ll want to repeat eight times. Don’t correct the vocabulary at this age: “squashed air” is the hook you hang the real word on later.

Ages 9–12: Halve the space, double the push-back

Introduce the words pressure and volume, and the trade between them: when one goes down, the other goes up. Use the marked barrel to make it numerical. Forty millilitres to twenty is half the volume, so about double the pressure. Twenty to ten is half again, so roughly four times the starting pressure. Ask them to feel whether the plunger fights back about twice as hard on the second squeeze. It does, roughly. Then run the water control, because “gases squash, liquids don’t” is a sentence a 10-year-old can carry for years. It explains hydraulic car brakes and the hiss of a bicycle pump.

Ages 13+: Make it quantitative and find where the law breaks

Have them write pV = nRT and identify what is held fixed and what is free to change. Then measure: tape a strip of paper to the barrel and estimate pressure from the marshmallow’s diameter at each stop. Compressing a foam is a crude manometer, but diameter plotted against 1/V should look roughly linear over the first factor of two. The interesting part is finding where it stops being linear and arguing about why: bubble walls touching, gas dissolving into the sugar syrup, or the heat released during compression. Any of those is a real hypothesis.

The question to ask: “A sealed, half-empty water bottle rides with you from the beach up a mountain road. At the top, is it tighter or softer than it was at the bottom, and why?”

Why Teaching This Early Pays Off

Gas laws arrive in school as three named rules to memorise and then confuse. A child who has felt the plunger fight back has a physical referent for every one of them, and that changes what memorising means. The compressibility contrast pre-loads a lot too: why hydraulic brakes use oil instead of air, why your ears pop on a plane, why a diver’s lungs are the dangerous part of surfacing.

The research supports a specific claim here, not a sweeping one. Alfieri and colleagues’ meta-analysis found that unassisted exploration did not improve outcomes, while guided discovery, with an adult explaining alongside the activity, did (Alfieri et al., 2011). Kirschner, Sweller and Clark make the point more bluntly: minimal guidance tends to fail (Kirschner et al., 2006). The syringe is not the active ingredient. The conversation while it is in your child’s hand is. Chi and Wylie’s ICAP framework adds the reason: learners who explain and predict outperform learners who only watch (Chi & Wylie, 2014).

One afternoon will not change your child’s trajectory, and nobody should claim it will. What it does is give a later textbook sentence something to attach to.

Before You Start: What Parents Should Know

Best age	8–14. Under about 7 the pressure-volume trade is too abstract to stick, though the squashing is still enjoyable
Supervision	Light. An adult should load the first marshmallow and check the seal
Real hazards	None beyond normal kitchen care. Mini marshmallows are a choking hazard for under-3s, so keep the bag away from toddlers. Marshmallows contain gelatin, and some brands note wheat or soy traces
Don’t	Never use a hypodermic syringe or any syringe with a needle. And don’t reuse a syringe from the medicine cabinet and then eat the marshmallow: a brand-new oral syringe is food-grade, a used one is not

When It Doesn’t Work

• The plunger slides all the way down and the marshmallow never moves. Your seal is leaking. Test it with no marshmallow: thumb on the tip, push, release. If the plunger doesn’t bounce back, flatten your thumb harder or press the tip into a blob of poster putty.
• The marshmallow barely changes. It’s stale. Old marshmallows have lost moisture and their bubble walls have stiffened. Open a fresh bag and keep it closed between runs.
• The marshmallow jams and won’t come out. It expanded against the barrel wall during the pull step. Push the plunger down with the tip open to release the pressure, then poke the marshmallow free with the handle end of a spoon.
• The marshmallow is too big. Mini marshmallows only. A standard one will not fit a 60 mL barrel, and cutting one exposes sticky surfaces that glue themselves to the plunger.
• Your child says “the air got squeezed out.” Expected, not a failure. Ask where the air went. There’s no exit. Smith, diSessa and Roschelle argue that wrong answers like this are raw material rather than errors to stamp out (Smith et al., 1994).

Take It Further: More Boyle’s Law Experiments for Kids

1. Make it a real measurement. Mark the barrel at 60, 50, 40, 30, 20 and 15 mL. At each stop, photograph the marshmallow against graph paper and measure its diameter. Plot diameter against volume. A 13-year-old can then plot diameter against 1/volume and argue about linearity.
2. Fast versus slow. Compress to half volume over ten seconds and note how hard it feels, release, then repeat with a one-second slam. The fast push feels harder because it heats the gas. Change exactly one variable, speed, and record the force, ideally by pushing down against a bathroom scale so you get a number.
3. Swap the indicator. Try a cube of fresh bread, a cheese puff and a marshmallow in turn. Rank them by how much they shrink, then tear each open and look at the bubble size. Pore structure versus compressibility is genuinely open-ended, and it pairs well with the physics of a popcorn kernel, the same story run in reverse at 180 °C.

What to Watch For Over the Next 3 Months

• Week 4: they use “pressure” correctly about something unrelated, like a bike tyre, a ketchup bottle or their ears in a lift.
• Month 2: they predict rather than guess. Someone asks what happens to a balloon taken underwater and your child reasons out loud instead of answering instantly.
• Month 3: they spot the gas-versus-liquid distinction on their own, usually while squeezing something. The follow-up question worth encouraging is “why don’t car brakes use air?”, and the answer is exactly this experiment.

Frequently Asked Questions

Do I need a syringe with a needle?

No, and you should not use one. Oral and catheter-tip syringes have no needle, cost a dollar or two at any pharmacy, and the wide blunt tip is easier for a child’s thumb to seal. A needle adds a puncture risk and nothing else.

Can my child eat the marshmallow afterwards?

Only if the syringe is brand new and unused. A fresh oral syringe is food-grade plastic, so one marshmallow that has been inside it is fine. A syringe that has dosed medicine is not food-grade, however carefully you rinsed it.

Why doesn’t the marshmallow go back to its original size?

Because candy is not an ideal gas. The sugar-and-gelatin walls between the bubbles are viscoelastic, so some stay folded or torn after a hard squeeze. The air inside obeys Boyle’s law exactly. The container doesn’t bounce back perfectly.

Is this the same thing as the can-crushing demo?

Related but inverted. Here you supply the force with your hand. In the can-crushing experiment the atmosphere supplies it once steam condenses inside. The egg-in-a-bottle trick is the same mechanism again, with the egg acting as the plunger.

What school topic does this connect to?

Gas laws and the particle model of matter, usually middle school and again in high-school chemistry. The National Research Council’s framework for K–12 science names “developing and using models” as a core practice, and a syringe is a model of a gas you can hold in one hand (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. “Boyle’s Law.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/boyles-law/
2. NOAA National Weather Service. “Air Pressure.” JetStream: An Online School for Weather. https://www.noaa.gov/jetstream/atmosphere/air-pressure
3. Nave, C. R. “Bulk Elastic Properties.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html
4. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
5. Kirschner, P. A., Sweller, J., & Clark, R. E. (2006). “Why Minimal Guidance During Instruction Does Not Work.” Educational Psychologist, 41(2), 75–86. https://doi.org/10.1207/s15326985ep4102_1
6. 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
7. 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
8. 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