Puffing Up Marshmallows
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 appropriate 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!
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.
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Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Gas, pressure, temperature, volume, gas laws
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