Why does Hydrogen explode?
What makes hydrogen flammable is one of the three things that will keep a fire going–oxygen. Neither pure hydrogen nor pure oxygen by themselves will burn. However, if hydrogen is given an oxidizer, and oxygen is given a reducer (i.e., they two are mixed together), they will form an unstable compound that becomes quite flammable and under the right circumstances, explosive. Water on the other hand is a stable, non-reactive mixture, which is why water is not flammable. In short, what makes hydrogen flammable is that it is often mixed with oxygen (or another oxidizer), and when that happens it becomes more unstable and more open to chemical reactions like fire.
