Jump to main content

Puffing Up Marshmallows


Key Concepts
Gas, pressure, temperature, volume, gas laws
Sabine De Brabandere, PhD, Science Buddies
A marshmallow puffed up after cooking in the microwave on a plate


With the peak of camping season behind us, any leftover marshmallows can be offered up to science exploration! Did you realize that this sticky, tasty treat is nothing more than air trapped in a stretchy substance? Have you ever tried to expand a marshmallow without getting your hands all sticky? How did you do it? And how big did it get? Blowing up marshmallows is what this activity is about. You might not “see” a gas like air, but could it help puff up a marshmallow? Be ready to have some fun and be surprised!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


In everyday life, we observe materials in a solid, liquid or gas state. In solids and liquids, the particles making up the material are densely packed. They cannot get much closer when pushed together, making the volume of a solid or liquid more or less fixed.

On the other hand, the particles making up a gas spread out and occupy all the space they are given. There is often plenty of opportunity to squeeze these particles together and contain them in a smaller volume, or to give them more space and allow them to occupy a larger volume. When confined to a smaller volume, these particles bang more often into the wall, creating more pressure on the walls (known as Boyle’s law). When you keep the same confinement, but increase the temperature, these particles start moving faster, banging more violently into the walls, again increasing the pressure on the walls (Gay-Lussac’s law).  When you manage to keep the pressure constant, the increase in temperature (or the more violent collisions) will cause the walls to move outward and the volume to expand (Charles’ law).

Now, what does this have to do with marshmallows? This sticky treat has air—which is a gas—trapped in a stretchy substance. This stretchiness allows the gas bubbles to expand and contract with changing pressure in the bubbles. Will the gas laws help us create a giant marshmallow? Do the activity to find out!


  • Marshmallows, regular size works best. If using a narrow-necked bottle, you will also need a bag of mini marshmallows.
  • Glass jar with lid; alternatively, a glass bottle with a neck that fits the wine conserver pump.
  • Vacuum pump. A kitchen vacuum pump or wine conserver pump both work well.
  • Microwavable plate
  • Knife to cut open a marshmallow
  • Microwave


  1. When using the glass jar with lid and the wine pump, ask an adult to make a hole in the jar lid so the vacuum pump fits tightly. This will allow you to remove air from the jar with the wine conserver pump. We found that first making a smaller hole with a nail and then enlarging that hole by screwing a blade of scissors in the small hole worked well.
  2. Place marshmallows in the glass jar or bottle until the jar or bottle is about half full. Use the regular size marshmallows if they fit through the neck of your jar; mini size will do, if needed.
  3. Place the lid back on the glass jar.
  4. If you will later be using a wine conserver pump, add the bottle stopper that came with your wine conserver pump in the hole in your lid, or in the bottle neck if you are using a narrow-necked bottle. Set it aside.


  1. Place a marshmallow on the plate and cut it in two. What does the substance look like? Do you see how the marshmallow is a foam, or a gas (air in this case) trapped in a solid?
  2. Air is a gas, and air is all around the marshmallows in the jar or bottle and, as we just discovered, also inside the marshmallows. The air (or any gas) is a collection of tiny particles zooming around inside the space they are given. As they move, they bang into the walls of their confinement, pushing them outward. What do you think will happen if you take away some air (or zooming particles) from around the marshmallows? Will fewer air particles bang into the marshmallow? What might happen to the marshmallow as a result? Test and see!
  3. Ask an adult to remove air from the jar or bottle with the vacuum pump. Why do you think the pump only takes air particles from the air around the marshmallow and not from the air trapped inside the marshmallow? Can you see the marshmallows change and if so, how?
  4. Now, let some air stream back into the jar. What do you observe now? Can you explain why these changes happen?
  5. In the next steps, you will investigate something different: what happens if you heat the marshmallow in a microwave. What is your prediction, and why do you think this will happen?
  6. Place a regular-size marshmallow on a microwavable plate. Ask an adult to place the microwave on high and heat the marshmallow for 20-30 seconds. Look through the microwave window while the marshmallow heats up. Can you see the marshmallow change and if so, how? Is this what you expected?
  7. Open the microwave door and watch as the marshmallow cools. Careful, do not touch the marshmallow, as it can be very hot! How does the marshmallow change as it cools? Why do you think this is the case?

Extra: Can you find other foamy substances in the kitchen that can serve as a substitute for the marshmallow in this activity? Maybe chocolate mousse, or the dough of yeast bread? How do you expect these to change when you remove air from around them, or as you heat them?

Extra: The next time you roast a marshmallow, look more carefully and examine in detail. Does it expand everywhere, like it did in the microwave, or only at the sides? Why do you think this happens? Could you find a reason why roasting a marshmallow can make it appear brown, while heating it in a microwave keeps it white? What could cause this change of color? Does it become white again as the roasted marshmallow cools, or does it stay colored after cooling? If you know the differences between physical and chemical changes, could you tell if this browning indicates a physical or a chemical change?

Extra: Ask an adult if you can deflate a bicycle tire. What do you observe? What is coming out of the valve, and what happens to the tire as a result? Use a bicycle pump to pump air back into the tire. What happens now? Can you see how a vacuum pump and a bicycle pump are very different, but related? Look at the links in the “Explore More” section to find instructions on how to convert a bicycle pump into a vacuum pump.

Observations and Results

Did you see the marshmallow puff up and expand? This is what the gas laws predict, as explained below.

Marshmallows are gas bubbles trapped by frothing a solution of sugars and water into gelatin. The gelatin makes the marshmallow stretchy. The gas in the bubbles pushes the gooey substance outward, while the gas (or air) around the marshmallow pushes the substance inward, reaching stability at exactly the size of the marshmallow you popped out of the bag. 

By putting marshmallows in a jar or bottle, and using the vacuum pump, you can remove gas around the marshmallows. The gas in the bubbles keeps pushing outward, while less and less gas around the marshmallows pushes back. The gas bubbles expand, and the marshmallow puffs up. When air flows back into the jar or bottle, the gas bubbles need to give in under the increasing push on the outside walls of the marshmallow, so they shrink back to their original size. This illustrates Boyle’s law, which states that a gas expands if the pressure decreases while all other variables are kept constant.

When heating a marshmallow in a microwave, some moisture inside the marshmallow evaporates, adding gas to the bubbles. In addition, a warmer gas pushes outward with more force. Both adding gas and heating cause the gas bubbles to expand and the marshmallow to puff up.  This is to be expected, knowing the following two gas laws: Dalton’s law states that a gas—when no other variables are changed—expands when more gas is added, and Charles’ law states that a gas—when no other variables are changed—expands when its temperature is increased.

icon scientific method

Ask an Expert

Curious about the science? Post your question for our scientists.


  1. The marshmallows are safe to eat if you wish to do so. Wash all equipment with soapy water.

Additional Resources

Free science fair projects.