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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


Marshmallow Man
Figure I. Figure II. Figure III.

Description

Equipment: Marshmallow man (Arrangement of marshmallows on several wires depicting a person)
Airtight glass bell jar
Vacuum pump

Step 1: The marshmallow man is shown to the audience and the demonstrator explains that the marshmallow man is simply normal marshmallows arranged on several wires. The marshmallow man is located inside the bell jar and is of normal proportions (See Figure I.)

Step 2: The vacuum pump is turned on, and to the amazement of the audience, the marshmallow man swells to very large dimensions. (See Figure II.)

Step 3: The vacuum pump is turned off and air is let into the bell jar. This time, the marshmallow man shrinks to very small proportions. (See Figure III.)

Description

Basic Ideas: A marshmallow is made of mostly sugar and air. The sugar molecules are arranged such that there are many pockets of air trapped inside a marshmallow. These pockets of air can be thought of as many little balloons made of sugar.

Step 1: Since the marshmallow man is under normal circumstances (the bell jar is full of air), the marshmallows are of normal proportions. The air outside the marshmallows is exerting a certain amount of pressure on the marshmallows, trying to press them inwards. At the same time, however, the air inside the marshmallows is exerting the same amount of pressure on the marshmallows, trying to expand them outwards. These two pressures exactly cancel out.

Step 2: The vacuum pump has been turned on and is starting to suck the air out of the bell jar. This causes there to be less air surrounding the marshmallows, and therefore less air pressure exerted on the marshmallows from the surrounding air. The amount of air inside of the marshmallows is the same, however, and so the pressure exerted by the air inside the marshmallows is the same as in Step 1. The two pressures do not cancel out, and the marshmallows expand.

Step 3: In this step, the vacuum pump is turned off and air is allowed back into the glass container. Once the air has returned to the glass container, the air pressure exerted on the marshmallows by the surrounding air is the same as in Step 1. One would therefore expect that the marshmallows would return to their normal size, as in Step 1. However, when the marshmallows were expanding, the air pressure inside the marshmallows was so great that much of the air in the marshmallows escaped. When the bell jar was refilled with air in this step, the marshmallows deflated. The marshmallows no longer contained as much air as before.

Description

Basic Ideas: The chemical structure of a marshmallow is such that there are many pockets of air trapped by sugar molecules. These pockets of air are constantly applying pressure to the sugar molecules in the amount of approximately 15 pounds per square inch.

A gas exerts pressure on all sides of the container which holds the gas. The amount of pressure is related to the energy of the gas and the amount of gas. The higher the energy, the more pressure is exerted, and the more gas is contained, the more pressure is exerted.

On an average day, the air pressure pressing on all things at the surface of the Earth is approximately 15 pounds per square inch.

Step 1: The marshmallow man is under normal circumstances. The air pressure inside the marshmallows exactly equals the air pressure around the marshmallows.

Step 2: The vacuum pump is turned on and the air inside the bell jar is slowly sucked out. This causes a decrease in air pressure around the marshmallows. Since the air pressure inside the marshmallows has not changed, there is now a difference in pressure. The force on the sugar molecules from the air inside is now greater than the force on the sugar molecules from the outside. The marshmallows expand and air eventually leaves the marshmallows.

Step 3: When the air was pumped out of the bell jar in the previous step, the force inside the marshmallows caused the marshmallows to expand. The force became so great, however, that much of the air inside the marshmallows escaped. When the air was allowed to flow back into the container, the air inside the marshmallows had decreased, so the marshmallows deflated.

Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia