Crushing Cans with Air
[Shopping List: hot plate; extension cord; heavy gloves; aluminum cans; tongs; bucket for ice water; ice]
- Place ~1-2 cms of water inside each can. Heat on hot plate until the water boils, and allow to fill with steam for a few minutes. Don't let the water completely boil away before proceeding!
- Grasp the can with tongs (palm facing up) and quickly invert the can and plunge it into a bucket of ice water. Make sure that the opening to the can is upside down and under water so that no air can enter it when you plunge it into the ice water bath.
What's Happening: Air pressure is a force exerted by the many molecules of air that surround us – each molecule is constantly moving around and bumping into objects and other air molecules, and the additive effect of the millions of air molecules colliding around us constitutes an impressive force. We don't typically notice air pressure because it is pushing on us from all sides- as well as inside our bodies- balancing itself out. Air pressure at the Earth's surface can exert a very strong force (~14.7 lbs/in2 at sea level).
With the can crush, you created a state of imbalance in air pressure and thereby observed the true force of air pressure. After the water in the can has been boiling on the hot plate for a few minutes, the space above the liquid water, which had been filled by air, is now filled with steam (the gas form of water). When the can is inverted into an ice water bath, this steam quickly cools and condenses back into liquid water. As steam takes up approximately 1,600 times more space than an identical mass of liquid water, the condensation of steam into liquid water in the can results in a large amount of empty space inside the can. With the can inverted, the only thing that could enter the opening to fill this empty space is water – a very dense substance. So the can contains only a small amount of liquid water (condensed from the steam) and no air molecules to push out on the walls of the can. Meanwhile, air outside the can is still pushing on the walls. With such an imbalance, it is impossible for the thin aluminum can to remain intact, and the air molecules outside the can push in and crush it. Liken this demonstration to a tug-of-war when one team has neglected to show up: the air molecules that are actually exerting a force will win the battle to push on the walls of the can.
Variations and Related Activities: Lift the can with tongs, then grasp it with a gloved hand for the inversion procedure. This will prove to kids that you weren't crushing it with the tongs.
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