An explosion is defined as which of the following?

• A. A rapid combustion without pressure change
• B. A chemical reaction producing a loud sound
• C. A phase transition causing pressure drop
• D. A rapid expansion of gases resulting in a rapidly moving pressure or shock wave

Answer: (D)

Explosions come from a rapid release of energy that drives a sudden, large rise in pressure and quick expansion of the hot gas products, creating a shock wave that travels through the surroundings. The defining feature is this overpressure front moving outward, often at or above the speed of sound. The option describing a rapid expansion of gases that generates a rapidly moving pressure or shock wave captures this essential behavior, making it the best description. Other scenarios don’t fit because an explosion isn’t just a loud noise or a rapid event without overpressure, and it isn’t a phase transition that causes a pressure drop—the crucial element is the strong, propagating pressure wave produced by the rapid energy release.