File:M112 Demolition Charge.jpg|thumb|300px|A number of 1.25lb M112 Demolition Charges, consisting of a C-4 compound, sit atop degraded weaponry scheduled for destruction An explosive material is a material that either is chemically or otherwise energetically unstable or produces a sudden expansion of the material usually accompanied by the production of heat and large changes in pressure (and typically also a flash and/or loud noise) upon initiation; this is called the explosion. An explosive charge is a measured quantity of explosive material.

Chemical explosives

Explosives are classified as low or high explosives according to their rates of burn: low explosives burn rapidly (or deflagrate), while high explosives detonate. While these definitions are distinct, the problem of precisely measuring rapid decomposition makes practical classification of explosives difficult.

The chemical decomposition of an explosive may take years, days, hours, or a fraction of a second. The slower processes of decomposition take place in storage and are of interest only from a stability standpoint. Of more interest are the two rapid forms of decomposition, deflagration and detonation.

The latter term is used to describe an explosive phenomenon whereby the decomposition is propagated by the explosive shockwave traversing the explosive material. The shockwave front is capable of passing through the high explosive material at great speeds, typically thousands of meters per second.

Explosives usually have less potential energy than petroleum fuels, but their high rate of energy release produces the 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.

Explosive force is released in a direction perpendicular to the surface of the explosive. If the surface is cut or shaped, the explosive forces can be focused to produce a greater local effect; this is known as a shaped charge.

In a low explosive (which deflagrates), the decomposition is propagated by a flame front which travels much more slowly through the explosive material.

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.

Explosive compatibility groupings

Shipping labels and tags will include UN and national, e.g. USDOT, hazardous material Class with Compatibility Letter, as follows:

• 1.1 Mass Explosion Hazard
• 1.2 Non-mass explosion, fragment-producing
• 1.3 Mass fire, minor blast or fragment hazard
• 1.4 Moderate fire, no blast or fragment: a consumer firework is 1.4G or 1.4S
• 1.5 Explosive substance, very insensitive (with a mass explosion hazard)
• 1.6 Explosive article, extremely insensitive