Dry ice is placed in the bottom of a large glass tank. When soap bubbles are blown into the tank, they float on the relatively dense layer of carbon dioxide gas in the tank. they also expand slightly.
Curriculum Notes
This is a good demo to use when the density of gases is being discussed. It is also a good example of a physical equilibrium system (bubble in air) that is disrupted when the bubble comes to rest on the layer of carbon dioxide and expands. This demo takes about five minutes to perform, but it must be set up about five minutes in a draft-free area before you perform it so the dry ice can generate a layer of carbon dioxide gas.
One day of lead time is required for this project.
Discussion
There are really two reasons why the bubbles float. First of all, the carbon dioxide is denser than air even if they are the same temperature. In this case though, the CO2 is also colder, increasing its density even further.
When a bubble is in air, the rate of diffusion of exhaled atmospheric gases out of the bubble is roughly equal to the rate of diffusion of atmospheric gases inward. But carbon dioxide diffuses through the bubble membrane much more quickly than atmospheric gases (probably because it is very soluble in the water comprising the bubble membrane). So when the bubble comes to rest on the carbon dioxide layer the bubble slowly increases in size.
Materials
- large glass tank
- 2-3 pounds of dry ice
- one or two bottles of soap bubbles with plastic bubble blowers
- desk lamp (optional)
- background (optional)
Procedure
- place dry ice in tank
- wait for about five minutes for the carbon dioxide gas to be generated
- have a volunteer or two blow bubbles into the tank
- bubbles will be suspended in the tank and grow slightly in size
- side lighting and a dark background improves visibility
Safety Precautions
Dry ice is very cold. It can cause frostbite. Avoid touching it, particularly if your fingers are wet.