Dry ice
Dry ice is a genericized trademark for 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. Hence it is called "dry ice" as opposed to normal "wet (water) ice".
It is used for:
- cooling
- 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 a fog.
- cleaning: shooting tiny dry ice pellets at a surface cools the dirt and causes it to pop off. This is analogous to sandblasting with the advantage that the abrasive agent in this case, dry ice, sublimates to nothing.
- increasing precipitation from existing clouds or decreasing cloud thickness by cloud seeding
- Producing carbon dioxide gas where needed in such odd systems as the fuel tank inerting system in the B-47 aircraft
Dry ice is produced by compressing carbon dioxide gas to a liquid form, removing excess heat, and then letting the liquid carbon dioxide expand quickly. This expansion causes a drop in temperature so that some of the CO2 freezes to "snow" which is then compressed.
Dry ice's unique characteristics requires special precautions when handling. It is obviously cold and there should be no direct contact with skin. It is constantly subliminating to carbon dioxide gas. Thus it cannot be stored in a sealed container as the pressure build up may cause the container to explode. The subliminated gas needs to be ventilated, otherwise it may overwhelm the enclosed space with carbon dioxide gas as the gas is poisonous at high enough concentrations. Special care for ventilating vehicles is needed as the enclosed space in a vehicle is small. Special care for ventilating basements and boat cargo holds is needed as the carbon dioxide gas is heavier than air and tends to sink and be trapped in such areas.
