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Substances capable of very rapid combustion (or other exothermic reaction) to produce hot gases whose rapid expansion is accompanied by a high velocity shock wave, shattering nearby objects. The detonation travels 1,000 times faster than a flame.

The earliest known explosive was gunpowder, invented in China in the 10th century AD, and in the West by Roger Bacon (1242).

Explosives are classified as primary explosives, which explode at once on ignition, and are used as detonators; and high explosives, which if ignited at first merely burn, but explode if detonated by primary explosion. The division is not rigid. Military high explosives are usually mixtures of organic nitrates, TNT, RDX, picric acid, and PETN, which are self-oxidizing. Commercial blasting explosives are less-powerful mixtures of combustible and explosive substances; they include dynamite (containing nitroglycerin, ammonium nitrate, and sometimes nitrocellulose), ammonals (ammonium nitrate + aluminum), and Sprengel explosives (an oxidizing agent mixed with a liquid fuel such as nitrobenzene just before use). Obsolete explosives include the dangerous chlorates and perchlorates, and the uneconomical liquid oxygen explosives (LOX). Explosives which do ignite firedamp are termed "permissible," and may be used in coal mines. Propellants for guns and rockets are like explosives, but burn fast rather than detonating. See propellant.

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