An explosive is a reactive substance containing a significant amount of potential energy that undergoes an extremely rapid chemical transformation. This near-instantaneous change results in the sudden production of large volumes of hot, high-pressure gases. The release of this stored energy is accompanied by intense light, heat, and sound, which characterize an explosion. The primary function of these materials is the sudden conversion of a condensed state, like a solid or liquid, into a gaseous state.
Defining the Explosive Reaction
The mechanism of an explosive reaction differs fundamentally from simple, rapid combustion, such as a campfire or a flash fire. In a standard fire, the chemical reaction propagates through the material at a subsonic speed, meaning it is slower than the speed of sound in that medium. An explosion, however, is defined by the rapid, violent release of energy that creates a sudden and intense pressure wave.
The speed of the chemical reaction dictates the type of explosion, which is categorized into two main processes: deflagration and detonation. Deflagration involves a thermal-driven combustion wave that moves through the explosive material at a subsonic velocity. This process relies on heat transfer to ignite adjacent material, and while it is fast, it does not generate a supersonic shockwave.
Detonation, by contrast, is a self-sustaining reaction that propagates through the material at a supersonic speed. This process is characterized by a shockwave, which is a near-instantaneous pressure front that travels faster than the speed of sound in the unreacted material. The extreme pressure and heat generated by this shockwave initiate the chemical decomposition of the remaining explosive.
Primary Classifications
Explosives are fundamentally classified based on the speed at which the chemical reaction propagates through the material. This distinction separates them into two primary categories: low explosives and high explosives.
Low explosives are materials that undergo deflagration, meaning their reaction front moves at a subsonic speed, typically in the range of 171 to 631 meters per second. These materials are generally used as propellants because their slower, more controlled burn generates a push rather than an immediate, shattering blow. Common examples of low explosives include black powder and smokeless powder, which are used to launch projectiles from firearms.
High explosives are compounds that undergo detonation, with the reaction front traveling at a supersonic velocity, often between 1,800 and 8,000 meters per second. This extremely fast reaction generates a powerful shockwave that creates a shattering effect, known as brisance. High explosives are further divided into primary explosives, which are highly sensitive to heat or shock and are used to initiate a reaction, and secondary explosives, which are less sensitive and are used for the main destructive charge, such as TNT or RDX.
Everyday Uses
Explosives are indispensable tools in a wide array of commercial and civilian projects. One of the most widespread uses is in mining and quarrying, where high explosives break apart massive quantities of rock and ore. Materials like Ammonium Nitrate Fuel Oil (ANFO) or various emulsion explosives are selected because their detonation creates the immense shockwave needed to fracture hard rock structures.
In the construction industry, explosives are routinely used for large-scale demolition and for creating tunnels and underground spaces. Specialized, less sensitive high explosives are often employed for these tasks, with electronic detonators used to ensure precise, sequential firing. This precision minimizes ground vibration and directs the energy to achieve the desired structural collapse or rock removal.
Explosives are also used in highly specialized industrial processes, such as explosive welding or bonding. In this technique, a thin layer of explosive accelerates two metal plates toward each other at high velocity. The resulting impact pressure is so great that the metals are permanently bonded together, a technique often used to join dissimilar metals that cannot be welded by conventional means.