dry ice

dry ice, carbon dioxide (CO₂) in its solid form, a dense, snowlike substance known for its ability to sublime—converting from a solid to a gas without becoming a liquid. At standard atmospheric pressure, dry ice sublimates at −78.5 °C (−109.3 °F). It is an effective refrigerant—a substance used to absorb heat and sustain low temperatures—because it maintains very low temperatures and cools without leaving any liquid residue. These properties make it useful in transporting perishable goods such as meats or frozen food.

Formation and properties

To produce dry ice, carbon dioxide gas is pressurized and cooled until it becomes a liquid. The liquid carbon dioxide is released or expanded in a controlled environment, where the sudden change in conditions causes part of it to solidify into carbon dioxide snow. The snow is compressed into pellets or blocks. Solid carbon dioxide is colorless and odorless, though it often appears white because of condensed water vapor or surface frost. It is nonflammable and chemically stable.

Applications

Dry ice is often used in applications that require reliable low-temperature cooling.

• Food preservation: Dry ice is used to preserve the quality of meat, dairy products, and frozen desserts during shipping.
• Industrial cleaning: In dry ice blasting, solid CO₂ pellets are used as a nonabrasive cleaner to remove such contaminants as paint, oil, or mold from surfaces without damaging the underlying material.
• Medicine and research: Dry ice helps to preserve medical samples and to maintain donated organs in a viable state between the organ’s removal from the donor and transplantation. It is also used in dermatology to cool the skin before procedures such as dermabrasion.
• Special effects: When placed in water, dry ice produces dense fog, making it popular in theatrical productions and for entertainment events.

Safety and handling

Dry ice has a low surface temperature (−78.5 °C [−109.3 °F]), and direct contact with skin can result in frostbite. It should be handled using insulated gloves or tools such as tongs. Protective eyewear is recommended when cutting or chipping dry ice. Household freezers are not suitable for the storage of dry ice because its extreme cold may interfere with temperature-control systems.

As dry ice sublimates, it releases CO₂ gas, which is heavier than air and may accumulate in low or enclosed spaces. Inadequate ventilation allows CO₂ to accumulate in higher concentrations, where its displacement of oxygen can create a risk of asphyxiation. Early symptoms of carbon dioxide exposure include dizziness, headache, and shortness of breath. Higher concentrations of the gas may result in unconsciousness. To prevent gas and pressure buildup, dry ice should be stored in a well-insulated container with a loose or vented lid. It should not be placed in confined spaces—such as walk-in freezers or closed vehicles—unless there is sufficient ventilation. Unused dry ice should be left to sublimate in a well-ventilated area, allowing the gas to dissipate naturally. It should not be disposed of in enclosed waste bins, plumbing systems, or trash chutes, where gas buildup can pose safety risks.