Explosives

All chemical explosives, whether solid, liquid, or gas, consist of a fuel, a substance that burns, and an oxidizer, a substance that provides oxygen for the fuel. The burning and the resulting release and expansion of gases during explosions can occur in a few thousandths or a few millionths of a second. The rapid expansion of gases produces a destructive shockwave. The greater the pressure of the shockwave, the more powerful the blast.

Fire or combustion results when a substance combines with oxygen gas. Many substances that are not explosive by themselves can explode if oxygen is nearby. Turpentine, gasoline, hydrogen, and alcohol are not explosives. In the presence of oxygen in the air, however, they can explode if ignited by a flame or spark. This is why drivers are asked to turn off their automobile engines, and not smoke, when filling fuel tanks with gasoline. In the automobile engine, the gasoline fuel is mixed with oxygen in the cylinders and ignited by spark plugs. The result is a controlled explosion. The force of the expanding gases drives the piston down and provides power to the wheels.

This type of explosion is not useful for most military and industrial purposes. The amount of oxygen in the air deep in a cannon barrel or a mine shaft may not be enough ensure a dependably powerful blast. For this reason, demolition experts prefer to use explosive chemicals that contain their own supply of concentrated oxygen to sustain the explosion. Potassium nitrate, for example, provides oxygen. Still, if the heat generated by a compound that breaks apart is great enough, the compound can still be an explosive even if it does not contain oxygen. Nitrogen iodide is one of the few examples.

Many chemical explosives contain nitrogen because it does not bind strongly to other atoms. It readily separates from them if heated or shocked. Nitrogen is usually introduced through the action of nitric acid, which is often mixed with sulfuric acid. Nitrogen is an important component of common chemical explosives like TNT, nitroglycerin, gunpowder, guncotton, nitrocellulose, picric acid, and ammonium nitrate.

Another type of explosion can happen when very fine powders or dust mixes with air in an enclosed space. Anytime a room or building is filled with dust of flammable substances such as wood, coal, or even flour, a spark can start a fire that will spread so fast through the dust cloud that an explosion will result. Dust explosions such as these have occurred in silos where grain is stored.