Carbon dioxide, or CO2, is an interesting gas in that it freezes directly from a gas into a solid—dry ice—at normal atmospheric pressures, bypassing the liquid phase entirely. At a pressure of 1 atmosphere, CO2 becomes dry ice at temperatures below –109.3 degrees Fahrenheit. However, you can liquefy CO2 if you increase the pressure on it to 5.1 atmospheres, which is about 75 pounds per square inch, and keep the temperature below minus 69 degrees Fahrenheit. At these conditions, known as the triple point, CO2 coexists in the solid, liquid and gaseous states. On an industrial basis, manufacturers need sophisticated equipment to create and store liquid CO2. However, you can make it yourself at home, although the CO2 will remain in the liquid state for a few seconds only.
Buy some dry ice. You can buy plastic dry ice packs at several home-improvement and grocery stores, such as Walmart and Safeway. You can transport the dry ice home in an ice cooler to help keep it frozen.
Carefully saw open the plastic ice pack over a sturdy work surface and remove a chunk of the dry ice. Be sure to don rubber gloves and safety goggles to protect your hands and eyes from the cold.
Gently hammer the ice chunk into crushed ice.
Cut off the narrow portion of the tip end of a plastic pipette with a scissor so there is a larger opening and scoop up some of the crushed ice to collect it in the bulb end.
Crimp shut the pipette's open end using needle-nose pliers.
Holding the sealed pipette with the pliers, plunge it into a clear container of warm tap water.
Observe the plastic bulb expand as the frozen CO2 thaws. As the pressure builds to above 5.1 atmospheres in the pipette, liquid CO2 will appear in the bulb. After a few seconds, the bulb will burst, and some of the CO2 will revert back to a solid. The rest will become a gas that forms bubbles in the water.
Wear insulated gloves to protect against frostbite and burn when handling dry ice. Be careful not to inhale carbon dioxide fumes directly.
Don't store a dry ice pack in your freezer, because its low temperature will cause the freezer's thermostat to turn off. However, if your freezer suddenly breaks, a pack of dry ice will keep things cold for a few hours.