Potassium nitrate is used in explosives. Oxygen from the air is the most common chemical that is used for oxidation, which is how most fires and explosions happen. Some explosions will use up oxygen so quickly, though, that there is not enough to finish the reaction, and the explosion is not very strong. Potassium nitrate replaces the need for oxygen and takes up much less space, which lets the explosion be stronger and faster. It is a main ingredient of gunpowder.