Oxygen unites readily with other elements, particularly metals, and forms compounds called oxides. For example: When iron is exposed to moisture and to the air which contains oxygen, it oxidizes and rusts. When the oxygen combines with a carbon, as coal, it gives off considerable heat and light. This process is called combustion. A substance or element which burns is called a combustible, and a substance or element that does not burn is called a non-combustible. Oxygen is called a supporter of combustion. Elements will not combine until a definite temperature, called a kindling point, is reached.
Ozone is a form of oxygen, and is a powerful oxidizing agent. When an electric spark passes through the air, it changes oxygen into ozone. Three atoms of oxygen form a molecule of ozone.