Dry ice is a genericized trademark for solid ('frozen') carbon dioxide. The term was coined in 1925.
Dry ice at normal pressures does not melt into liquid carbon dioxide but rather sublimates directly into carbon dioxide gas at −78.5°C (−109.3°F). Hence it is called "dry ice" as opposed to normal "wet" ice (frozen water).
Dry ice is produced by compressing carbon dioxide gas to a liquid form, removing the heat produced by the compression (see Charles' law), and then letting the liquid carbon dioxide expand quickly. This expansion causes a drop in temperature so that some of the CO2 freezes into "snow" which is then compressed into pellets or blocks.
Uses
- Cooling foodstuffs, biological samples, and other perishable items.
- Producing "dry ice fog" for special effects. When dry ice is put into contact with water, the frozen carbon dioxide sublimates into a mixture of cold carbon dioxide gas and cold humid air. This causes condensation and the formation of fog.
- Cleaning, analogous to sandblasting. Tiny pellets of dry ice (instead of sand) are shot at a surface to be cleaned. Dry ice isn't as hard as sand, but it speeds processing by sublimating to nothing and doesn't produce nearly as much lung-damaging dust.
- Increasing precipitation from existing clouds or decreasing cloud thickness by cloud seeding.
- Producing carbon dioxide gas as needed in such systems as the fuel tank inerting system in the B-47 aircraft.
Handling
Because of its particular characteristics, dry ice requires special precautions when handling. It is extremely cold and there should be no direct contact with skin (i.e. wear proper insulating gloves). It is constantly sublimating to carbon dioxide gas, so it cannot be stored in a sealed container as the pressure build-up will quickly cause the container to explode. The sublimated gas must be ventilated, otherwise it may fill the enclosed space and create a suffocation hazard. Special care for ventilating vehicles is needed as well due to the small space. People who handle dry ice should also be aware that carbon dioxide is heavier than air and will sink to the floor.