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The Big Idea
Dry ice is the name for carbon dioxide in its solid state. At room temperature, it will go from a solid to a gas directly. While carbon dioxide gas is invisible, the very cold gas causes water vapor in the air to condense into water droplets, thus creating fog.
The temperature of the surface of a block of solid carbon dioxide (dry ice) is -78.5 degrees C (-109.8 degrees F). Once it gets to this temperature, carbon dioxide bypasses the liquid state and goes directly into a gas in a process called sublimation. One pound of dry ice makes 250 liters of carbon dioxide gas! If you heat a block of dry ice, by putting it in warm water for instance, it will sublimate more quickly. The fog that seems to be coming off a block of dry ice is actually not the carbon dioxide itself but rather water vapor from the air, condensing due to the cooling effect of the carbon dioxide. This effect is especially pronounced in the moist air above a warm container of water; thus, putting dry ice in warm water not only accelerates its sublimation but creates a lot of fog.
In any closed container above -78.5 degrees C, sublimating dry ice will create a lot of pressure. (It is important not to let this pressure build up: see safety notes below.) In the apparatus in the video, the pressure forces gas out of a nozzle that has been dipped in a soap solution, thus creating a fog and carbon dioxide filled bubble.
The soap bubble, like all soap bubbles , is made of a layer of water molecules sandwiched between two layers of soap molecules. Soap molecules have a hydrophilic ("water-loving") end, and a hydrophobic ("water-fearing") end, and they are arranged in the bubble with the hydrophobic ends pointing both ways out of the bubble film (both into the bubble center and out into the world). The hydrophilic ends point into the middle of the bubble layer where the water is trapped.
Materials and Set-up
The apparatus can be constructed from schedule 40 PVC fittings. Use a 3" or 4" Y for the bottom with a cleanout plug for introducing water and dry ice. Reduce the size to 1" or 1 1/2" PVC as you go up, and then narrow to a 1/2" or 3/4" nozzle. This apparatus can also be made with a soda bottle and tubing; just cut a door in the top-side of the soda bottle and use modeling clay to secure the tubing. If you use a soda bottle, the door does not have to seal perfectly for this to work, though taping it closed will mean that more of the carbon dioxide will be force out the nozzle.
Put warm water in the base of your apparatus, add a ping pong ball-sized chunk of dry ice, and close it up, allowing the pressure to push the carbon dioxide and fog out of the nozzle. Dip the nozzle into the cup with the soap mixture and watch the bubbles form. Once a bubble is large enough, it will fall on its own and break, releasing the carbon dioxide and water droplets it contained. You will have to periodically dip the nozzle into the soap to keep bubbles forming. When the dry ice chunk is gone, reset the apparatus with new warm water and a new chunk of dry ice.
Safety IssuesDry ice will freeze your skin and cause frostbite if handled directly. Use tongs or insulated gloves.
If you are driving with dry ice in the car, keep the windows open, as elevated carbon dioxide levels in the car can be dangerous; the same is true if you have dry ice in a small enclosed room.Never put dry ice into a closed container. The container may explode with catastrophic results.