The vast majority of explosives are chemical explosives. Explosives usually have less potential energy than fuels, but their high rate of energy release produces a great blast pressure. TNT has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the detonation of a pentane air mixture, and the 0.34-m/s stoichiometric flame speed of gasoline combustion in air.
The properties of the explosive indicate the class into which it falls. In some cases explosives can be made to fall into either class by the conditions under which they are initiated. In sufficiently large quantities, almost all low explosives can undergo a Deflagration to Detonation Transition (DDT). For convenience, low and high explosives may be differentiated by the shipping and storage classes.
A chemical explosive is a compound or mixture which, upon the application of heat or shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat. Many substances not ordinarily classed as explosives may do one, or even two, of these things. For example, at high temperatures (> 2000 °C) a mixture of nitrogen and oxygen can be made to react with great rapidity and yield the gaseous product nitric oxide; yet the mixture is not an explosive since it does not evolve heat, but rather absorbs heat.
- N2 + O2 → 2 NO − 43,200 calories (or 180 kJ) per mole of N2
For a chemical to be an explosive, it must exhibit all of the following:
- Rapid expansion (i.e., rapid production of gases or rapid heating of surroundings)
- Evolution of heat
- Rapidity of reaction
- Initiation of reaction
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