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How Does Fire Work?


picture of fire

What is fire? How does it work? According to the American Heritage Dictionary, 2nd College Edition, fire is, "A rapid, persistent chemical reaction that releases heat and light, especially the exothermic combination of a combustible substance with oxygen."

That's a mouthful. Before we explain that definition, let's look way back in history.

Right after the sun, fire is the oldest form of energy used by man. Fire can be extremely dangerous and destructive, like shown in the photo on the right. But it can also be beneficial when used properly.

In L'Escale Cave in southeastern France, a layer of ash was found on the roof of the cave that dates from 700,000 to 400,000 BCE (Before the Common Era). This layer is viewed by some scientists as the earliest known evidence of fire used by hominids in Europe, although it cannot be established that the ash is not the result of naturally caused fires.

Middle Pleistocene humans controlled fire (burning wood) as early as 500,000 BCE. Direct evidence was found outside a cave at Chou k'ou-tien, China. Here charcoal was found along with traces of a stone toolmaking industry in an open gully deposit. This deposit appears to be slightly older than the cave deposit itself, which contained the bones of Homo erectus, early man.

Fire was treated as a gift from the gods. Usually fire was found naturally from a lightning strike or near volcanic areas. Fire helped early man cook food, provided warmth and kept wild animals away. Today, fire is used in homes to do much the same...except for maybe keeping the animals away.

Let's explain what fire is...

Fire is a chemical process. Three things are needed for this process: oxygen, heat and fuel. Without one of these elements a fire cannot start or continue.

In a chemical process, the molecules rearrange themselves. Energy is either released or absorbed. The process in a fire is called oxidation, where oxygen atoms combine with hydrogen and carbon to form water and carbon dioxide. Oxidation is the same chemical process that turns iron into rust. But with iron, the reaction is VERY slow. So, the heat energy that is released is VERY low.

With certain things, like paper or wood, the oxidation rate of the molecules can be very fast. If the heat cannot be released faster than it is created, then combustion happens.

Besides heat, there must also be flames or smoldering present during the chemical process for it to be called fire. Exhaust gases also are produced. If the burning process is very clean, you don't see the exhaust gases. If some of the particles of the fuel are not completely burned, you see smoke. Smoke is made up of evaporated water, carbon dioxide and unburnt particles of the fuel.

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Page updated: April 22, 2002
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