Fire is something that most people believe they know a lot about, when in fact most have little understanding of it. We will try to explain what fire is and how it works along with clearing up some misconceptions. Since the phenomenon of Fire is very complex we will not get into the complexities, but rather provide a general overall explanation that does not require a Masters in Chemistry and Physics to understand.

Simply, Fire is a chemical reaction that changes a combustible material in the presence of heat and oxygen into products of combustion heat. This is referred to as the "Fire Triangle" (which is actually a fire tetrahedron as four items must be present for combustion). The four requirements of combustion are;

• Combustible material
• Air
• Heat
• Chemical reaction of fire.

Now, lets get into this in a bit more detail. One common misnomer is that the material itself (wood, paper, cloth) is Combustible, which is NOT correct. In all fires (except reactive metals) the combustible products are actually "gases" generated when a Combustible product is heated. This is why heat is one of the four requirements of combustion, as without the gases generated by "heating" the material, combustion can not happen. When these gasses are produced and mixed with an "oxidizing agent" (oxygen) in the presence of heat (above the mixtures auto ignition temperature) the chemical process of combustion takes place (fire). Thus, the four requirements of combustion required to produce fire are more correctly;

1. Fuel (Gasses produced by heating a Combustible material)
2. Oxidizing agent (Oxygen from the atmosphere)
3. Heat ( Temperature at or above the auto-ignition point for that fuel)
4. Chemical reaction of fire.

Since the combustion process creates heat as one of its byproducts, which is also required for the combustion process, once started it becomes self-sustaining as long as it has fuel, oxygen and the chemical reaction. Removing any one of these 4 things will cause the combustion process to stop, thus extinguishing the fire.

Reactive Metals:

We mentioned above that reactive metals were an exception to the normal combustion process, and this is because reactive metals (commonly called Combustible metals) are NOT Combustible They can (and do) cause fires, but not in the manner that Combustible materials do. Here is the difference;

1. Heat is NOT required in the reaction process of these metals

Thus, it does not conform to the normal requirements of combustion as removing heat will do nothing to the process. What actually takes place with reactive metals is a process called oxidation. This is the exact same process that occurs to all metals exposed to an oxidizing agent (oxygen and water), which most know by its common name, "rusting". The process of rusting looks like this;

Iron + Oxygen = Iron Oxide (and Hydroxide) + Heat

As shown above, heat is produced, but it is so small due to the length of time the process takes. The main difference with reactive metals is the speed at which the process happens. In reactive metals, the oxidation process is extremely fast, and generates huge amounts of heat. This heat is where the misnomer about the metal being Combustible comes from. Here is what happens;

Potassium + Oxygen (Water vapour from the air) = Potassium Hydroxide + Heat

The metal starts to oxidize extremely fast due to its "reactive" nature using both oxygen and water vapour in the air to form a hydroxide. During the process a large amount of heat is generated in avery short period of time. As an example, a piece of iron placed in oxygen could take years to fully oxidize, while the same amount of Potassium would take seconds. It is the heat generated, usually hundreds of degrees, that can cause other combustible products near the metal to catch fire. This is one of the main reasons that special handling of reactive metals is important, but there is one more extreme hazard with reactive metals when it is involved in fire fighting.

What is the normal method used today to fight building fires? It is Water. The Fire department sends out firefighters to the scene, who then hook up fire hoses and use water to fight the fire. Water is used as it cools a fire and removes heat, which is one of the 4 requirements of combustion. Now, lets add reactive metals to the fire (which may or may not have started the fire) and see what happens. The fire department show up and hose down the fire, which also gets water on the reactive metals. Here is what happens;

Potassium + Oxygen = Potassium Hydroxide + Heat

This does two things to the oxidation process, both considered very bad in terms of fire fighting.

1. It dramatically increases the already fast oxidation process
2. It produces Hydrogen gas as one of the products of the reaction.

Since the reactive metal reacts violently with water, the process speed is dramatically increased. This dramatically increases the heat output of the reaction, which will reduce the overall cooling effect of the Firefighters water on the fire. It also generates Hydrogen gas, the most volatile and combustible of the elemental gasses. This now creates a risk of rapid spread ignition, which is also known as an Explosion. A piece of Potassium dropped on water will actully ignight the hydrogen it produces due the large amount of heat generated.