gunpowder (or gun powder) is a mix of chemical substances (75% saltpeter, 15% charcoal and 10% sulfur). it is used primarily in firearms, burns very quickly, and creates gases. those gases use up more space than the gunpowder they come from, so they push outward. if the gunpowder is in a small space, the gases will push on the walls of the space, building up pressure. in a gun, the pressure pushes against a bullet, causing it to fly out at high speeds. if the pressure became too high, it could destroy the gun barrel.