General physical properties of oxygen and hydrogen.

Water is a compound made up of two elements – hydrogen and oxygen. The characteristic properties of these elements differ from those of water. However, hydrogen and oxygen have some properties in common. They are colorless, odorless gases, and they both easily react with other elements to make them reactive, but in many ways they are very different from each other.

Hydrogen has the lowest density of all elements. It is highly reactive, which is one of the reasons it is present in very small amounts in the air. It reacts with oxygen. Oxygen also reacts slowly with many substances. Many oxygen-containing compounds are called oxides.