The fundamental concept behind explosives is very simple. At the most basic level, an explosive is just something that burns or decomposes very quickly, producing a lot of heat and gas in a short amount of time.
A typical explosive consists of some explosive material, some sort of detonation device and, typically, some sort of housing. The explosive material undergoes a rapid chemical reaction, either a combustion or decomposition reaction, when triggered by heat or shock energy from the detonator.
In the chemical reaction, compounds break down to form various gases. The reactants (the original chemical compounds) have a lot of energy stored up as chemical bonds between different atoms. When the compound molecules break apart, the products (the resulting gases) may use some of this energy to form new bonds, but not all of it. Most of the "leftover" energy takes the form of extreme heat.
The concentrated gases are under very high pressure, so they expand rapidly. The heat speeds up the individual gas particles, boosting the pressure even higher. In a high explosive, the gas pressure is strong enough to destroy structures and injure and kill people. If the gas expands faster than the speed of sound, it generates a powerful shock wave. The pressure can also push pieces of solid material outward at great speed, causing them to hit people or structures with a lot of force.
C-4 is a high explosive designed for military use. In the next section, we'll find out what sets it apart from other explosives.