Explosive is a material that produces a rapid, violent reaction when heated or struck. The reaction gives off large amounts of gases. The expanding gases produce extreme pressure, giving explosives their destructive power.
Explosives have many uses. Construction workers use explosives to blast away rocks and to tunnel through mountains. Miners use explosives to break up rock deep underground to extract minerals and petroleum. In war, explosives are widely used to destroy buildings, bridges, vehicles, ships, and airplanes and to harm enemy troops. Terrorists also use explosives, taking advantage of their dramatic and frightening power.
Explosives may be solids, liquids, or gases. Most explosives consist of a fuel and an _oxidizer—_a substance that supplies the oxygen needed to burn the fuel.
Explosive power
varies with the rate of the chemical reaction. Conventional explosives are often grouped into two categories: high explosives and low explosives. The most powerful explosives involve nuclear reactions rather than chemical reactions. For a detailed discussion of nuclear explosions, see Nuclear weapon .
High explosives
explode in reactions called detonations. In detonation, the chemical reaction takes place at an extremely high rate. The reaction spreads through the explosive material faster than sound can travel through it. The supersonic reaction creates extremely powerful shock waves that can shatter metal and stone.
The most common modern high explosive is emulsion explosive. It is often used in mining and construction blasting. Other common types of high explosives include nitroglycerin, RDX, TNT, PETN, and pentolite, a combination of TNT and PETN. Bombs, artillery shells, grenades, and guided missiles contain high explosives.
Plastic explosives combine high explosives with a moldable material, such as oil or wax. They can be formed into various shapes. C4 is a common plastic explosive. Most plastic explosives contain chemical taggants (tracers), making it easy for authorities to detect them.
Low explosives
are those that deflagrate (burn rapidly) rather than detonate. In deflagration, the reaction spreads slower than the speed of sound. But deflagration still produces hot, high-pressure gases. The most common type of low explosive is gunpowder. It serves as a propellant to shoot projectiles from guns and other weapons. Fireworks also make use of low explosives.
Explosive sensitivity.
Explosives also vary in their sensitivity—that is, how easy they are to set off. They are often divided into three categories by sensitivity: primary explosives, secondary explosives, and blasting agents.
Primary explosives are the most sensitive, or the easiest to explode. A small amount of heat—even a spark of static electricity—can set off a primary explosive. Common primary explosives include lead azide, lead styphnate, and mercury fulminate.
Secondary explosives are less sensitive—but generally more powerful—than primary explosives. Secondary explosives are designed so that they can be stored, transported, and handled safely. They usually form the bulk of high explosive devices. Blasting agents are the least sensitive explosives. Secondary explosives and blasting agents are typically triggered by using primary explosives.
