1. Composition: A mixture of hydrogen and oxygen contains both hydrogen (H2) and oxygen (O2) molecules, while pure oxygen consists solely of O2 molecules.

2. Flammability: A mixture of hydrogen and oxygen is highly flammable and can react explosively when ignited. This is due to the fact that hydrogen and oxygen are both fuel and oxidizer, respectively. In contrast, pure oxygen is not flammable on its own.

3. Color and odor: A mixture of hydrogen and oxygen is colorless and odorless, similar to pure oxygen.

4. Reactivity: A mixture of hydrogen and oxygen is more reactive than pure oxygen. It can react with various substances, including metals, to form compounds. Pure oxygen, on the other hand, is less reactive and primarily supports combustion in other substances.

5. Applications: A mixture of hydrogen and oxygen is used in various applications, including as fuel for rockets and in oxy-fuel welding and cutting. Pure oxygen finds its use in medical applications, such as oxygen therapy, and in industrial processes like steelmaking.