What is Fire?

We have all seen a fire. But what is it really? In simplistic terms, fire is a rapid reaction between a material, i.e. the fuel, and any oxygen in the environment surrounding that fuel, i.e. the oxidizer. This is the same reaction which creates rust on iron and steel surfaces, although the rate of the “rusting” reaction is many orders of magnitude slower than the “burning” reaction. While I suppose it would be technically correct to say “We melted marshmallows over a camp-rust” or “There was a forest-rust that destroyed 4,000 acres” it just doesn’t sound correct. We understand the word “fire” to mean an energetic and rapid release of hot gases and visible flames and the word “rust” to mean a relatively slow process. Although in wet, humid environments the rusting can take place in a very short time.

The flames we see during a fire are the visible release of energy from the oxidation reaction that is occurring on the surface of the fuel. In most cases the flames are visible but there are some fuels that emit invisible flames. I can remember seeing video footage of the pit area of a car race where all the pit members started to act excitedly: they were jumping up and down, frantically waving their arms, running around. It was just general chaos. What I didn’t realize was that the fuel they were using did not have visible flames. Those pit crew members were on fire! (This occurred in the 1981 Indianapolis 500. Rick Mears was the driver and he received 1^st and 2^nd degree burns. His crew chief received much worse burns.) There were no visible signs of the rapid oxidation reaction that was taking place. This incident led to a change in fuel so that visible flames could be seen if the fuel was burning.

As a first step look at the basic oxidation reaction for methane (CH₄):

Two sides of the fire triangle are represented in this equation: fuel and oxidizer. The left side of this equation is before the reaction takes place and there is a fuel (CH₄) and an oxidizer (O₂) present. If you have a methane and oxygen mixture and an ignition source is present, the reaction occurs. After the reaction (the arrow), there are two different substances present: carbon dioxide and water. The reaction also releases a lot of heat.

The oxidization reaction for rusting iron (Fe) is:

Like the reaction equation for methane, the left side of the equation contains a fuel (Fe) and an oxidizer (O₂). The right side contains the products of combustion. Besides the fact that one fuel is a gas and one is a solid, the primary differences between these two reactions are:

1. The activation energy for the iron oxidation (rusting) is much lower than that required to start the burning process for methane.
2. The amount of heat released by the rusting iron is less than heat released by the burning methane.
3. The rate of the methane oxidation is much faster than the iron oxidation.

So fire is basically the same reaction as the rusting of those old garden tools stored in the shed. It occurs many times faster than the rusting process but it is the same reaction: the oxidation of a material.