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Is fire a solid, a liquid, or a gas?


Science has come a long way since the early metaphysicists’ attempts to define matter, but the exact nature of fire is not yet completely understood. The unknowns provide atmospheric chemists and engineers research opportunities to isolate the precise chemical processes involved in combustion—and the prospect of impacting the economy and the environment. “Many automobile companies employ scientists who specialize in the study of how vehicle engines burn fuel,” says Chen. “Discovering exactly what happens during combustion could lead to improved efficiency and a cleaner burning processes.”

Simply defined, fire is a chemical reaction in a mixture of incandescent gases, typically luminous with intense heat. But candle flames, wood fires, and propane fires aren’t created equal. “What constitutes fire depends on the fuel being burned,” says Chen. “The chemistry of each type of fire is different.” They’re similar to the extent that all fires release energy stored in fuels, and if supplied with enough oxygen and enough time, eventually produce carbon dioxide and water. “That’s the end game,” says Chen. “You can’t get more energy out of it without putting more energy in. All fires eventually burn themselves out, unlike solids, liquids, and gases, which can exist indefinitely in the same state.”

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So fire is none of the above. It's a chemical reaction. 

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