Marshmallows have air inside of them, much as you do. In fact, what makes marshmallows, shaving cream, and whipped cream fluffy is the air. Now at any given moment, there are several pounds per square inch of pressure on your body and on just about everything you see in the room. The reason why you do not implode under the pressure is because there is like pressure pushing outwards from the air everywhere else. However, if you were to evacuate the air on the outside suddenly, all the air inside of you would suddenly exert all that pressure with nothing to counter it. Thus, it explodes if done suddenly enough. And that's why marshmallows (and people) expand.

Marshmallows have very many tiny air pockets in them.When you have a marshmallow sitting on your desk (what we call atmospheric pressure) the air in the pockets is pushing out on the marshmallow and the air surrounding the marshmallow is pushing in with equal force. We can say the marshmallow is at equilibrium.

When you put the marshmallow in a vacuum (less pressure outside the marshmallow) the air trapped in the marshmallow is still pushing out with the same force, but now there is less force being exerted by the outside air so the net force causes the air pockets to expand. As it expands the actual marshmallow material stretches but keeps the marshmallow from exploding.

Have you ever put a partially filled balloon in a vacuum?? If you did, you would see it expand as well. It expands for exactly the same reason. There is less force on the outside of the balloon and more pushing on the inside, so the balloon stretches and expands. A marshmallow is like thousands (or more) tiny balloons all stuck together.

Actually, it is not correct that there is no pressure. Under normalconditions, it is subjected to atmospheric pressure -- 14.7lbs/sq. in.Since it is not contracting under such conditions, it must be respondingwith an opposing pressure on its surface. If you put the marshmallow ina vacuum, you remove the external pressure, but not the internalpressure (largely due to millions of small air bubbles in the candy).The bubbles still have 14.7 lbs/sq. in of pressure inside -- and nothingoutside the mallow to push back -- so it expands. It will continue to doso until the pressures equalize (there is a bit of pressure from thestrain of the material surrounding the bubbles) -- otherwise it wouldexpand to fill the bell jar...

A marshmallow has a lot of air trapped inside. The air is in tiny pockets, like microscopic bubbles. If you reduce the pressure outside the marshmallow, the air will expand, making the marshmallow larger. The same thing would happen with a balloon. If you take a balloon from sea level up to the top of a mountain, where the air pressure is lower, the balloon would get a little bit larger. If you took *all* the outside pressure away, using a vacuum pump, the balloon could expand so much that it pops. A marshmallow won't pop, though, because as it expands, the little pockets of air start to leak, and the air comes out. You could also think of this the other way around: If you start with a balloon and *add* pressure around the outside, you will squeeze it to a smaller size, and the same with the marshmallow.

As long as you keep the amount of air constant (no air leaking out) and the temperature doesn't change, then the volume of the balloon times the pressure inside the balloon will be constant. In other words, if you drop the pressure in half, then the marshmallow will double in volume.If you're interested, chemists write this as
P*V = n*R*T
where P is pressure, V is volume, T is temperature (measured in Kelvin: Celsius plus 273 degrees), n is the number of molecules of gas, and R is a constant (8.31 Joules per Kelvin per mol of atoms). With this equation, you can calculate almost anything related to gases--even how much the pressure in your tires will go up from driving on a hot day.

Because there are tiny air cavities inside of the marshmallow that do have an internal pressure, and they push the sugar-flour matrix outward if there is no outside force pushing back.

We can answer this question by thinking of the world as made up of atoms (which is true, the world is made of atoms!). "Pressure" is a measure of the amount of atoms in a given volume of space. In a "low pressure" atmosphere, such as up in mountains, there are less atoms in the air in a given volume of space than there are atoms in the same volume of space in a "high pressure" atmosphere, such as at sea-level. (This is why people need to 'catch their breath' often when they first go up into mountains. We breathe oxygen. Oxygen is made of oxygen atoms. So in low pressure atmospheres, like in the mountains, there are fewer atoms in the air, which means there is less oxygen!)

So, if "low pressure" means LESS atoms in a given space, then "no pressure" means... NO atoms in a given space. This situation is also referred to as a "vacuum". Outer space is close to being a vacuum, and often-times in laboratories we humans create near-vacuums inside glass containers to do experiments under a "no pressure" environment.

Marshmallows are made of atoms too, of course. Marshmallows want to expand when they are in a no pressure environment because of a property of matter call entropy. Entropy means that when atoms are in empty space, the atoms want to move apart from each other and spread out to fill all the available space, if they can. A marshmallow expands in a no pressure environment because the atomic bonds that hold the marshmallow atoms together are relatively weak compared to other solids, so when the marshmallow atoms feel the force of entropy, which makes the atoms want to take up space, the atoms inside the marshmallow begin moving apart from one another, and try to fill up the empty space. Atoms inside a marshmallow would not feel such a strong force of entropy in a pressurized environment, because the gas (or air) atoms in the environment would already be taking up the space around the marshmallow (so the space wouldn't be empty). Also, if you put a stronger solid into a no pressure environment, such as a piece of metal (say, copper or gold), the solid would not expand like a marshmallow because the atomic bonds inside of this type of solid are stronger than the bonds inside a marshmallow. Even though the metal would feel the same force of entropy in a no pressure environment as the marshmallow would, the strong atomic bonding inside the metal would prevent it from expanding like a marshmallow.