Oxygen is one of the commonest chemical elements on Earth. It is found in most minerals, chemicals that form rocks. Oxygen makes up one fifth of the air. Joined with hydrogen, it forms water. Oxygen joins up easily with many other elements in chemical reactions. This kind of reaction is called oxidation. Sometimes this is a slow process, as when iron turns to rust in water or damp air. But if oxygen reacts very quickly, a process called combustion, or burning, takes place. Flames, light and heat are the result.

Respiration

We cannot see, smell or taste oxygen, but it is essential for life. All animals, including humans, must breathe oxygen to stay alive. It is used as fuel to turn the food we eat into energy. This is called respiration.