How Does a Fire Work?


What is Fire?

  • Fire is a combustive oxidation reaction--a chemical reaction between oxygen molecules and any molecules that have the potential to take on an oxygen molecule. Energy, in the form of heat or light, is released from the molecules as the reaction takes place.


  • Any substance that has the potential to oxidize eventually will if exposed to oxygen. And an oxidation reaction can occur without fire--iron rusting, for instance--and still be described as "burning," in a scientific sense. Without fire however, oxidation can take a long time. Fire speeds up the reaction dramatically, but fire cannot occur spontaneously. Certain conditions must exist first.

Oxygen Must be Present

  • Oxygen from the air is sufficient for many combustive reactions. The more oxygen that is available, the greater the potential for combustion. Pure oxygen poses such a great fire hazard that even a small flame or heat source may cause a flammable substance to ignite.

Fuel Must be Present

  • Fuel is any substance that is consumed, or oxidized, by the fire. Fuel may be a solid material, like wood or paper. Or it may be a liquid, like gasoline or kerosene. Combustible gases, like natural gas and propane, are familiar types of fuel. Flammable dust, like grain dust or flour, suspended in the air, is also combustible. Any substance that will oxidize will also burn, if conditions are just right. For instance, iron will burn if it is in the form of fine dust. Substances like glass, that are already oxidized, cannot oxidize further and will not burn.

Heat Must be Present for Ignition

  • Sunlight, friction and electrical current are all common heat sources that can cause a fire to ignite. Heat is what causes the oxidation reaction to speed up. In scientific terms, heat catalyzes the reaction.

The Fire Ignites and Emits Heat Energy as it Burns

  • When enough oxygen and heat combine with the fuel source, ignition occurs. Once the initial heat source has ignited the fire, the flame generates its own heat for combustion, and oxidation continues in a chain reaction. The fire will burn until one or more of the necessary components is exhausted or removed.

The Fire Emits Light Energy as it Burns

  • The light emitted by the flame may or may not be visible to the human eye. An alcohol fire for instance, produces an extremely hot flame that is invisible. In flames that are visible, the color of the flame is an indicator of heat and the efficiency of combustion. White areas of the flame are hotter than yellow, yellow hotter than red. The hottest flame is blue. A blue flame is the most efficiently burning flame, meaning that the least amount of fuel escapes combustion in the form of smoke or soot.

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  • Photo Credit M. J. Doran
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