What Is Fire?

From a young age, children are taught not to play with fire. Fire is hot and it hurts, so stay away. It’s always a vague warning with no real explanation as to what fire is. Let’s understand how fire works. What is fire? What gives fuels it and what is it composed of? Why does it spread the way it does?

Fire comes about from a chemical process. You need 3 things to create and maintain fire: heat, fuel, and oxygen. The removal of any of these 3 will extinguish the fire. If there is not enough heat, if the fuel is exhausted, or if the oxygen supply is limited, then the fire will go out.


There are 3 mechanisms of heat transfer. They are convection, radiation, and conduction. Convection is when heat transfers through liquids or gases, such as hot air rising through a chimney. Radiation is heat transmitted by rays, like the sun. Conduction is when heat transfers between two objects, like when you’re heating up dinner, and the heat transfers from the stovetop, to the frying pan, to the food.


Fuel is anything combustible. Basically anything the fire can “eat” through is fuel. Twigs, leaves, organic substance material, human built structures, etc. are all fuel for the flames. Once there is nothing left to burn, the fire dies down. Moisture slows the burning process, while dryness expediates it. Grass and leaves quickly lose moisture, and therefore burn more quickly. Branches and logs take longer to burn.


Generally, fire requires air with 16% oxygen. On average, air contains 21% oxygen. While fuel burns, the oxygen reacts with it, generating combustion byproducts, while releasing heat.



Combustion is the chemical reaction, of which the end result is fire. It occurs between a fuel and oxidizing agent which produces energy, generally in the form of light and heat. The chemical process in a fire is called oxidation. This is due to the oxygen atoms combining with the hydrogen and carbon to for water and carbon dioxide. Oxidation can occur rapidly with things such as paper or wood. Smoke comes about due to the evaporated water, carbon dioxide, and particles of fuel which were unburnt.

How Fast Does Fire Spread?

The three ways a fire can spread are conduction, convection, and radiation. With the right fuel and enough of it, fire can spread rapidly. In thirty seconds a small flame can erupt into an enormous fire. Flashover is when nearly all combustible material in a given enclosed area ignites. The following videos were developed by the National Institute of Standards and Technology.They were made to show how quickly a fire can spread: