Explosives are reactive substances that undergo a rapid chemical change (with the production of gas) on being heated or struck. Potentially violent, dangerous and deadly, Explosives decompose rapidly under certain conditions with the production of gases, which expand by the heat of the reaction. The energy released is used in firearms, blasting, and rocket propulsion. They are most commonly used for military purposes and are categorized by the speed at which they expand. Among a few are bombs, detonators, gunpowder and propellants.