Dry ice

Dry ice, carbon dioxide (CO₂) in its solid form. It is a dense, snowlike substance that sublimates (passes directly into the vapor without melting) at −78.5 °C (−109.3 °F). This property of skipping the transitional phase of melting into a liquid has earned it the name dry ice. Dry ice is used as a refrigerant, especially during shipping of perishable products such as meats or ice cream.

Production and characteristics

The production of dry ice takes advantage of the spontaneous cooling that occurs when compressed, liquefied CO₂ at −57 °C (−71 °F) or lower is allowed suddenly to expand to atmospheric pressure. The liquid freezes to a finely divided solid called “snow” that is compacted into cakes weighing about 20 kg (45 pounds). Commercial dry ice is available as blocks, pellets, sticks, or slices.

Dry ice sublimates to produce CO₂ gas in the ratio of about 1:850. Thus a pound (453.6 grams) of dry ice will convert into 250 liters (8.8 cubic feet) of CO₂ at room temperature. Since even a small amount of dry ice produces such a large volume of CO₂, it poses a potential asphyxiation risk, particularly in enclosed and poorly ventilated places. The very low surface temperature of dry ice (−78.5 °C [−109.3 °F]) can burn bare skin, with symptoms similar to frostbite. When dry ice is stored in sealed containers without an adequate escape route for the sublimating gas, high pressure can build up, causing the container to rupture or explode. The fragility of dry ice and the potential health hazards it poses necessitate handling it with extreme caution.

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Applications

The benefit of using dry ice for transporting perishable goods is its ability to maintain extremely low temperatures without producing liquid waste. Besides food items, dry ice is used to transport and store medicines, vaccines, and biological and chemical samples. It is also used in meat processing—the low temperature it produces inhibits bacterial growth that might otherwise cause spoilage.

Pellets of dry ice are used to clean industrial and food-processing equipment and automobiles in a technique called dry ice blasting. When fired at high speed, the pellets physically dislodge dirt and other contaminants from surfaces. The rapid change in temperature of the contaminant particles upon contact with dry ice also causes them to shrink and become brittle, making them easier to remove. Since the rapid sublimation of the dry ice produces expanding CO₂ gas, an additional lifting force helps remove the loosened debris. An advantage of using dry ice in such cleaning operations is the absence of secondary residues, unlike those usually produced when cleaning with liquid chemical reagents. As a mildly abrasive material, dry ice is particularly suitable for cleaning and preserving electronic equipment and heritage masonry.

Beginning in 1960 dry ice was used for cryoamputation, in which a nonviable, infected limb is frozen to provide anesthesia and local infection control, allowing the patient to stabilize before definitive surgical amputation. Dry ice also finds application in the entertainment industry—the combination of dry ice and hot water generates a fog that is used to create special effects in television and theatrical productions.