Burning question: Did you ever wonder how fire works? We did. Here's what we found out.
The scientific definition of fire is "a rapid, persistent chemical reaction that releases heat and light, especially the exothermic combination of a combustible substance with oxygen." That chemical reaction is called oxidation, which happens when oxygen atoms in the atmosphere combine with atoms "borrowed" from other elements, in this case, from fuel. (Another form of oxidation: rust- it;s just a lot slower.) For fire to occur, three ingredients must be present:
*An oxidizing agent. Can come from a pure oxygen source (like a welding tank) or, more commonly, the air. All that's needed is an atmosphere of at least 16 percent oxygen; normal air is about 21 percent.
*Fuel. Can be anything from a solid (wood, plastic, or wax), to a liquid (gasoline or alcohol), or a gas (propane).
*A heat or ignition source. Could be lightning, friction (as when striking a match), focused light, or a chemical reaction.
For oxidation to take place, the fuel must be heated to a certain temperature, known as the ignition temperature. It's different for different substances: paper's ignition temperature, for example, is 451°F. When a fuel reaches it's ignition temperature, a chemical reaction occurs that begins to decompose it into flammable gases known as volatiles. Some solids, like wood, go directly from solid to gas, while others, like wax, go from solid to liquid and then to gas. This depends on the chemical makeup of the fuel. In either case, the volatiles then violently interact with the oxygen in the atmosphere-that's called combustion.
Using a candle as an example, when you apply a burning match (the ignition source) to the wax on the wick (the fuel), the wax will heat to a certain temperature (the ignition temperature). It will begin to evaporate and release gases (the volatiles), which then react with the oxygen in the air (combustion). Result: fire.
The heat from the fire will then cause the wax to keep melting and moving back down the wick, evaporating, igniting, and burning away. Because the fire then produces its own heat-a necessary ingredient-it's called a persistent chemical reaction.
You already know how to put out a candle-but do you know why it goes out? When you blow out a candle, the wax has cooled below its ignition temperature. If it didn't go out, you didn't lower the temperature enough-or for long enough. Try pressing the wick between your thumb and finger. The fire will go out because you removed the fuel source by stopping the wax from climbing the wick. Or put a glass over the candle, taking away the oxygen.
With larger fires, it's usually difficult to take away the fuel, so fire extinguishers work by eliminating either the oxygen, the heat, or both. Water extinguishers work by cooling the fuel; dry powder extinguishers work by smothering the fire, thereby taking away the oxygen; foam extinguishers both smother and cool the fuel; and carbon dioxide extinguishers displace the oxygen in the air while simultaneously cooling it.
*Spontaneous combustion occurs when a fuel reaches its ignition temperature without the aid of an outside ignition source.This can happen because some substances naturally react with oxygen in the air, but most often it's from spontaneous heating, a slow buildup of heat. A cause of many house fires is the spontaneous heating of oily rags. If there is insufficient ventilation-like in the back of a garage-the heat can build up enough for fire to occur.
*Hot fact: You can't have fire without oxygen, right? Wrong. All that's necessary is an oxidizing agent, meaning an element that easily takes electrons from other atoms. Oxygen is the most common agent, which is why the reaction is called "oxidation". Fluorine, however, is the strongest known oxidizer-much stronger than oxygen. Used in the production of atomic bombs and rocket fuel, fluorine can cause substances like steel or glass to instantly burst into flame. And those flames are virtually impossible to put out.
(Image credits: Flicker users Kuzeytac and ViaMoi)
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