Liquid Carbon Dioxide
This is an excellent demo to use when phase diagrams and triple points are being considered. It can also be used when states of matterand changes of state are first being introduced. Unless it is a small class, this demo should be projected. This demo takes about ten minutes to perform. One day of lead time is required for this project.
This is an excellent demo to use when phase diagrams and triple points are being considered. It can also be used when states of matterand changes of state are first being introduced. Unless it is a small class, this demo should be projected. This demo takes about ten minutes to perform.
People are around liquid CO2 all of the time (CO2 fire extinguisher, soda fountain CO2 tanks, etc.) they just can't see it because it is usually stored in steel tanks to maintain the high pressure. The triple-point pressure of CO2 is 5.1 atmospheres, so the pressure inside the tube must be at least that. The conditions inside the tube probably approach triple-point conditions, but theoretically it is not at the triple-point because the system is not at equilibrium.1 1"Green chemical processing in the teaching laboratory: a convenient liquid CO2 extraction of natural products," Lallie C. McKenzie, John E. Thompson, Randy Sullivan, James E. Hutchison, Green Chemistry,2004, 355-358
- large, shatter-proof, clear cylinder half-filled with warm water (The one I use is custom-made from thick-walled polycarbonate tubing.)
- cooler with some crushed dry ice
- several Corning 15-mL polypropylene centrifuge tubes with caps
- tongs for retrieving the tubes
- a dark backgound box (optional)
- fill a centrifuge tube with crushed dry ice
- cap the tube tightly (Sometimes the threads will strip out at this point. If so, start over with a different tube.)
- immediately drop the tube into the warm water
- after about 15 or 20 seconds, the dry ice will begin to melt (Sometimes if the seal at the cap isn't good, you will hear the tube begin to vent before dry ice begins to melt. If that occurs, carefully remove the cylinder and slowly remove the cap to vent it, and start over from the beginning using another tube.)
- the liquid CO2 will begin to boil, after which you should soon hear the tube begin to vent
- let the tube remain in the water until all of the liquid CO2 has boiled away
- carefully remove the tube and slowly remove the cap to vent it
- As soon as you cap the tube, the contents are under high pressure and there is a possibility that it could explode. Handle the capped centrifuge tubes with caution when they are outside of the shatter-proof cylinder. Always wear goggles when doing this demo. Do not allow spectators to approach too near when a tube is outside of the cylinder. Do not allow spectators to lean over the cylinder when a sealed tube is in the cylinder.
- Dry ice is very cold. Handling dry ice could result in frostbite, especially if your hands are wet.
Dry-ice is available at science stores. They also have a crusher that you can use.