Module 3 - Arson and Explosives

A fire and an explosion are often mistakenly considered to be the same thing. If explosive compounds are present where a fire is occurring, an explosion can occur. Heat and gas generated by an explosion often lead to a fire.

Differences between a Fire and an Explosion:

• An explosion is detonated. A fire cannot be detonated. An explosion occurs after compounds are exposed to heat or shock. A fire is initiated after being exposed to a heat source only.
• Because of how quickly the reaction occurs in an explosion, shock waves are produced. A fire does not produce shock waves.
• Explosives usually have less potential energy than combustible hydrocarbons, but explosives release energy at a higher rate, which produces a greater blast pressure.

Similarities between Fire and Explosion:

• Both require oxygen.
• Both require a fuel source.
• Both create heat and light.
• Both usually will damage the environment in which they occur

Categories of Explosives

For a compound to be considered explosive, it must react rapidly when exposed to heat or shock and it must produce gas and heat rapidly. An explosion is the oxidation and combustion of at least two unstable substances that produces a violent reaction.

Two general types of explosives are labelled high and low. Each is categorized by how quickly the explosive compound ignites and how fast the chemical reaction occurs. The speed of the chemical reaction that generates each type of explosion influences various other aspects of the blast.

Low explosives are sensitive to heat, friction, and temperature. The speed of the shock waves generated by a low explosive blast is approximately 2300 meters per second. Low explosive materials are usually lethal only when confined to a sealed container in which a huge increase in pressure occurs. Generally, explosives used in criminal activities are low explosives because they are small and are often created with easy-to-find materials such as fertilizer, gunpowder, or gasoline. Examples of low explosives include pipe bombs, car bombs, gunpowder, flares, and illumination devices.

High explosives tend to be larger, more complex, and more powerful. They also have a much greater speed of reaction than low explosives. Because high explosives react so quickly, the build-up of pressure and gas is almost instantaneous. High explosives tend to be less sensitive to heat, friction, and temperature. They create powerful shock waves that have speeds of up to 6900 metres per second. Examples of high explosives include compounds such as nitroglycerin and TNT. Both of these explosives are used for mining and demolition and are used in military warheads.