Demos: 2A-03 Vacuum Demos

With the vacuum pump and bell jar as the basic apparatus, a series of “fun” demonstrations enable one to show the effects of air pressure in a dramatic way. These include: blowing up balloons, expanding marshmallows, and a shaving soap “snake.”

Directions: (For best viewing in a large room, set up the video camera to record what goes on inside the bell jar.) Make sure the shaving soap “snake” is the last of the demos because it leaves a horrendous mess! Begin by placing small balloons (blown up to near bursting) under the bell jar and turning on the vacuum pump. The balloons will likely burst. Next place the “Michelin Man” (an aluminum skeleton threaded with marshmallows) under the bell jar and activate the pump. Watch carefully so that you can turn off the pump when the marshmallows have reached their peak size. Open the valve to let air back in the jar. The marshmallows shrink down to a very small size! Finally, place some food coloring in the bottle and fill it about half-full with shaving soap (do this slowly and carefully). Place the bottle under the bell jar and turn on the vacuum. A very dramatic action takes place: The shaving soap snakes its way out of the bottle and fills up the bell jar. Then open the valve and watch the soap collapse in a messy heap at the bottom of the jar! (The demo staff will clean it up later.)

Suggestions for presentation: Ask what would happen if the pressure outside a balloon diminished. Have a few very small balloons (blown up to near bursting) available to place under the bell jar. After they have burst, examine the pieces. You might burst one of the balloons in air to show the difference. The balloon in the bell jar bursts into lots of tiny pieces, whereas the one burst in air essentially rips. The difference is that the former does not have to push against air on the outside.

For the Michelin Man, ask if the students know how a marshmallow is made. It consists of lots sugar and many, many tiny air pockets. This is illustrated by the fact that after expanding, the marshmallows collapse to a small wad of mostly sugar!

The shaving soap snake is a real crowd-pleaser! Shaving soap is mostly air bubbles. The food coloring simply makes it more visible. (If it is near Christmas time, use both red and green food coloring.)

Applications: Foods that are expanded by the use of trapped air.

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Last Updated: May 9, 2016 11:44 AM