Dry ice is a Halloween party staple and a popular special effect for theaters trying to set a spooky mood. But what is the thick white fog that dry ice produces? And why doesn't dry ice melt into a puddle like regular ice?
Unlike the ice cubes in a cold drink , dry ice doesn't melt to become liquid at all. Instead, at room temperature, it changes directly from a solid to a gas a process called sublimation.
Dry ice is the solid form of carbon dioxide , the molecule that animals breathe out when we exhale and plants take in when they do photosynthesis. Carbon dioxide is a gas at room temperature, and it freezes solid at a much lower point than water: -109 degrees Fahrenheit (-78 C).
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Freezing carbon dioxide is also more complicated than freezing water. To make dry ice, carbon dioxide gas is first cooled and pressurized to turn it into a liquid, according to Continental Carbonic Products, Inc, a dry ice manufacturer. The liquid is then depressurized and allowed to expand back into a gas. This expansion causes a rapid temperature drop, and some of the carbon dioxide freezes into solid pellets of dry ice. Those pellets are then shaped into chunks which can be used in food storage facilities, biomedical labs and, yes, Halloween haunted houses .
Dry ice's bitter chill keeps perishables cold for longer, but also makes it more dangerous than regular ice. Touching dry ice with bare skin is a recipe for instant frostbite, and breathing in too much of its carbon dioxide fog can suffocate you. So use tongs, keep dry ice in a well-ventilated area, and stick with regular ice for your summertime refreshments.
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