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Dry ice is composed of carbon dioxide, which at room temperature is a gas. The carbon dioxide used to make dry ice is liquefied and then stored and shipped in highly pressurized tanks.
To make dry ice, the liquid carbon dioxide is withdrawn from the tank and allowed to evaporate at a normal pressure in a porous bag (a porous material is one through which air and water molecules can pass). This rapid evaporation consumes so much heat from the surrounding air that part of the liquid carbon dioxide freezes to a temperature of -109° Fahrenheit (-78° Celsius). The frozen liquid is then compressed by machines into blocks of "dry ice" which will sublimate (return to the gaseous state) when set out at room temperature.
"Metals And Other Materials - How Is Dry Ice Made?." Science Fact Finder. Ed. Phillis Engelbert. UXL-Gale, 1998. eNotes.com. 2006. 20 Oct, 2009
