Introduction
A small amount of water is added to an aluminum soda can and brought to boiling on a hot plate or with a Bunsen burner. The water gas molecules will occupy all the space inside the can since the air molecules have been pushed out. The hot gas molecules are the same pressure as the air outside the can. When the can is placed in cold water upside down, the hot gas water molecules are cooled very rapidly. Some of the gas molecules are condensed back into liquid water so there are less molecules of water in the gas phase inside the can. The cold water will also cool any remaining gas molecules, decreasing their kinetic energy and therefore decreases the number of collisions with the walls of the can. This decreases the pressure inside the can. Since the air pressure outside the can is stronger than that inside the can, it causes the can to collapse.
H2O(g) à H2O(l)
To Conduct Demonstration
- Place the can containing water on a hot plate (turned to high) or a ring stand with a Bunsen burner underneath.
- Allow several minutes for the water to come to a full boil.
- Steam must displace the air inside the can; wait until you see a steady flow of steam exiting the spout, then immediately remove the can from the heat and place in the ice water bath.
- As the hot steam cools and condenses to water, a vacuum is created inside the can and atmospheric pressure will crush it.
NOTES:
- 250 ml water to a 5 gallon can
- 20 min to boil, 1 or 2 min to collapse. Collapsing will take longer if the can is left to heat longer and it itself gets hot.
- Requires a large hotplate.
Safety
If using a large can do not continue heating the can after inserting the rubber stopper as pressure will increase.