Created by Dale Olive and Randy Scoville
Future Flight Hawai`i instructors
Demonstration at a glance:
A heated can is placed upside down into a beaker of water where it instantly crushes.
Since air pressure is crushing the can you need to limit the size of the opening to the can as much as possible. I try to pull the tab back up through the opening with a paper clip and bend the pop tab over the hole to further block it. Squirt a couple ml of water into the can and heat on a hot plate until steam is seen coming from the can. You may have to add water periodically to keep the can from drying out. When steam is seen coming out, pick up the can with beaker tongs and turn upside down into a clear container of water.
Heating the can with a little water inside fills the can with stream. When the can is inverted into the cooler water the air inside the can is cooled and the steam condenses back to a liquid from a gas. What takes the place of the steam? Nothing, its an empty space called a vacuum. If there is little or no air to push on the insides of the can but 14.7 pounds of pressure per square inch on the outside you see why the can crushes inside. When the can is lifted out of the water you will notice considerably more water in the can then you put in. This is due to the fact that the pressure on the water forced it into the vacuum inside the can as well. The fact that liquid is more dense than air means it can't get into the can fast enough to fill the entire space before the can crushes.
This demo can be done on a larger scale with a metal gallon can. Be sure the can is empty and clean of all products as empty fuel cans are often used! Heat the can with a small amount of water inside on a hotplate (with the lid removed.) Remove the can from the heat and replace the lid. The can will slowly crush inwards until the vacuum is filled with can or until a hole is made in the can to allow air to fill the void. To speed up the reaction, cool water can be splashed on the outside of the can. Another safer variety can be done with a two-liter plastic soda bottle. Swirl some hot water inside the bottle for a while and empty. Quickly replace the lid and watch the bottle crush as the air in the bottle contracts as it cools creating lower pressure. This too can be sped up with cool water sprinkled on the outside.
Communications: Hawaii Space Grant Office
This activity is featured in Future Flight Hawaii, a K-12 Education Project of Hawaii Space Grant Consortium.
FEB 27 2001.