Air Pressure and Phyical Shape and Appearance of Objects

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Updated on Nov 15, 2010

The Idea

The next time you are sitting around the campfire cooking up a batch of s'mores, be sure to point out to your friends that a marshmallow is simply a colloidal suspension of air in a solid. Because the air in the marshmallow is in equilibrium with the atmosphere, the volume of the marshmallow is stable at standard air pressure. However, it's a different story if we disturb the equilibrium conditions by taking away the atmospheric pressure.

What You Need

  • marshmallow
  • bell jar
  • vacuum pump


  1. Place a marshmallow on the base of the bell jar.
  2. Assemble the bell jar and apply a vacuum.
  3. Observe what happens.

Expected Results

The marshmallow grows in volume, as you can see in Figure 41-1.

Why It Works

The pressure of the air trapped in each marshmallow causes the marshmallow to expand when the pressure outside the marshmallow is reduced.

Blowing up a marshmallow. Less is s'more. Why astronauts do not use shaving cream in space.

Other Things to Try

Try this with shaving cream. It will increase in volume.

Try this with hot water, just under the boiling point. The water should begin to boil again.

The Point

Common objects depend on air pressure for them to maintain their physical shape and appearance.