Ward's World Activity Guides

Under Pressure

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Under Pressure (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.wardsci.com Procedure: 1. Fill an empty doll's head with shaving cream from the neck opening. Ensure openings/holes are present at the eyes and ears. 2. Tape the neck opening closed. 3. Place the doll's head in a bell jar connected to the vacuum pump. 4. Evacuate the chamber of air. 5. Other items that can be placed in the bell jar include soda water, a small balloon, or a marshmallow. Expected Results: Results may vary based on materials used, setup, procedure, and other factors, however, here are a few examples on what to expect: As the surrounding pressure decreases, the shaving cream or marshmallow expands. When the vacuum pump is turned on, and the vacuum chamber is evacuated, the pressure inside the chamber decreases. The lower pressure on the shaving cream or marshmallow causes its volume to increase according to Boyle's Law: P1 × V1 = P2 × V2 P1 = first pressure P2 = second pressure V1 = first volume V2 = second volume The air bubbles trapped in the cream or marshmallow are no longer at atmospheric pressure, expanding like tiny balloons. As the pressure outside these air bubbles (within the chamber) is reduced, the bubbles will expand many times their original volume to equilibrate the pressure on either side of the bubble wall. So, as the pressure decreases, volume increases in an inverse relationship. Follow-up Teaching Notes: • Shaving cream is a foam (colloid) that consists of a gas evenly distributed in a liquid. As the surrounding pressure is decreased, the gas expands in size. 1. Expand on other concepts related to gas laws including, Hooke's law, Charles' law, or the solubility of gases. 2. How do these concepts relate to decompression sickness (aka the bends) experienced by deep-sea divers? 3. We are under a lot of pressure on Earth—air pressure or atmospheric pressure. At sea level, the Earth's atmosphere presses with a force of 14.7 pounds per square inch. How do gas laws help us understand atmospheric pressure? Disposal/Clean-up: • The marshmallow and shaving cream can be disposed of in the garbage. • The soda water can be washed down the drain.

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