Dry ice, frozen carbon dioxide, is useful as a coolant due to its low temperature. Small amounts can be made by placing a cloth bag over a liquid carbon dioxide tank’s discharge nozzle. Evaporative cooling is required to produce larger quantities. Dry ice is usually stored in isothermal containers and can be used to produce artificial fog for shows and in conjunction with laser light shows.
Dry ice is frozen carbon dioxide, the same substance that animals exhale and plants need to perform photosynthesis. Because of its low temperature, -109.3°F (-78.5°C), dry ice is useful as a coolant. It’s also relatively easy to make.
Sometimes scientists only need a small amount of dry ice. This can be accomplished by simply opening the valve on a liquid carbon dioxide tank and placing a cloth bag over the discharge nozzle. A mixture of gaseous carbon dioxide and dry ice is produced when some of the escaping gas condenses on the fabric. This approach is the simplest way to make a small amount of dry ice for laboratory use.
Evaporative cooling is required to produce larger quantities. First, liquefied carbon dioxide is produced by compressing carbon dioxide gas. Since the temperature of a substance increases as it is compressed, the gas must be cooled during the compression process to help form a liquid. If the gas inside the vessel is about room temperature, liquefaction begins to occur when the pressure is about 870 pounds per square inch (ppsi), or about 6,000 kilopascals.
The next step is pretty simple. In any given piece of matter, there will be some molecules that will move very fast and some that will move much slower. Their average speed is what we call the temperature. If a vat of liquid nitrogen is allowed to partially evaporate, the more energetic molecules escape, decreasing the average momentum of the molecules in the mixture, and therefore its temperature. After enough heat evaporates, the temperature of the liquid drops below freezing, a phase transition occurs, and we are left with solid carbon dioxide – dry ice.
Dry ice is usually stored in isothermal containers. When exposed to air, it immediately begins to release carbon dioxide, because the temperature difference between the ice and the environment is large enough that it mostly skips the liquid phase and simply turns into gas. Some of the carbon dioxide is turned into vapor, which gives it the appearance of fog. This substance is always used to produce artificial fog for shows. It can also be used in conjunction with laser light shows, scattering the light so it can be easily seen.
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