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why does fire need oxygen to burn

Fire is a reduction-oxidation reaction, so technically any so-called redox pair is similar to fire, and their are thousands that we know of. What makes oxygen particularly special is three things. First, it is very oxidative, with a reduction potential of, which is pretty high on the list of oxidants. Second, there is a ton of it in the atmosphere, so we have a lot of experience with it. Third is the somewhat peculiar electron arrangement in oxygen, where it is most commonly found in a
state, with two unpaired electrons in its highest molecular orbital (the one that typically reacts). The interesting thing about this and fire is that this state for quantum mechanical reasons prevents oxygen from interacting directly with its most common fuel carbon, which is found in a singlet state. So unless catalyzed by temperature, some inorganic catalyst, or an enzyme, oxygen will react with its reductant very, very slowly. Basically you have a highly energetic reaction that stays somewhat stable, which is why you can have a lot of oxygen around without it all burning away in an instant, primed to build up a lot of carbon (trees, brush, homes, whatever) and ignite when conditions are right.

According to Greek mythology, the gods took fire away from people. Then a hero named Prometheus stole it back. As punishment, the gods chained the thief to a rock, where an eagle fed on his liver. Every night, his liver grew back. And each day, the eagle returned. Like other myths, the Prometheus story offered one explanation for the origins of fire. It doesnБt, however, offer clues to why things burn. ThatБs what science is for. Some ancient Greeks believed that fire was a basic element of the universe Б one that gave rise to other elements, like earth, water and air. (Aether, that stuff the ancients thought stars were made of, was later added to the list of elements by the philosopher Aristotle. ) Now scientists use the word БelementБ to describe the most basic types of matter. Fire does not qualify. A fireБs colorful flame results from a chemical reaction known as combustion. During combustion, atoms rearrange themselves irreversibly. In other words, when something burns, thereБs no un-burning it. Fire also is a glowing reminder of the oxygen that pervades our world. Any flame requires three ingredients: oxygen, fuel and heat.

Lacking even one, a fire wonБt burn. As an ingredient of air, oxygen is usually the easiest to find. (On planets such as Venus and Mars, with atmospheres containing far less oxygen, fires would be hard to start. ) OxygenБs role is to combine with the fuel. Any number of sources may supply heat. When lighting a match, friction between the matchБs head and the surface against which itБs struck releases enough heat to ignite the coated head. In the Avalanche Fire, lightning delivered the heat. Fuel is what burns. Almost anything can burn, but some fuels have a far higher flash point Б the temperature at which theyБll ignite Б than others. б People feel heat as warmth on the skin. Not atoms. The building blocks of all materials, atoms just get antsy as they warm. They initially vibrate. Then, as they warm even more, they start dancing, faster and faster. Apply enough heat, and atoms will break the bonds linking them together. Wood, for example, contains molecules made from bound atoms of carbon, hydrogen and oxygen (and smaller amounts of other elements). When wood gets hot enough Б such as when lightning hits or a log is tossed on an already burning fire Б those bonds break.

The process, called pyrolysis, releases atoms and energy. Unbound atoms form a hot gas, mingling with oxygen atoms in the air. This glowing gas Б and not the fuel itself Б produces the spooky blue light that appears at the base of a flame. But the atoms donБt stay single long: They quickly bond with oxygen in the air in a process called oxidation. When carbon bonds with oxygen, it produces carbon dioxide Б a colorless gas. When hydrogen bonds with oxygen, it produces water vapor Б even as the wood burns. Fires burn only when all that atomic shuffling releases enough energy to keep the oxidation going in a sustained chain reaction. More atoms released from the fuel combine with nearby oxygen. That releases more energy, which releases more atoms. This heats the oxygen Б and so on. The orange and yellow colors in a flame appear when extra, free-floating carbon atoms get hot and begin to glow. (These carbon atoms also make up the thick black soot that forms on grilled burgers or the bottom of a pot heated over a fire. )

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