Explosions consist of a chemical reaction or a series of reactions in which a substance burns or decomposes very quickly, almost instantaneously, producing intense heat and gases that expand very rapidly. If the gas expansion is faster than the speed of sound in the air, the explosion produces a shock wave. Due to the rapid expansion, explosions apply force to surrounding objects and have the potential to cause damage.
Depending on the reaction speed and the amount of impact they cause, explosive substances are classified into low explosives, such as gunpowder or the propellant in bullet cartridges, and high explosives, such as TNT and C-4. The explosive reaction is initiated by a detonator, which typically consists of a physical shock or heat in the case of low explosives and a smaller explosion in the case of high explosives.
Explosions are used in the controlled demolition of buildings and bridges or to break large rocks into manageable pieces for use in construction. High explosives are also used by military forces during wars, due to their large destructive potential. Although they are not classified as explosives, certain mixtures of flammable materials or pressurized containers may cause unintentional explosions. Several countries have imposed safety measures and regulations to mitigate these risks.