Oxygen, or O2, which comprises 21 percent of the earth’s atmosphere, supports life and makes combustion possible. The most abundant of all elements on earth, oxygen comprises 85 percent of its oceans and, as a component of most rocks and minerals, 46 percent of its solid crust. In addition, it constitutes 60 percent of the human body.
Colorless, odorless and tasteless, oxygen has poor solubility in water. A specific gravity of 1.105 makes it slightly heavier than air. When cooled to its boiling point of -297o F (-183o C), oxygen becomes a transparent, pale blue liquid that is slightly heavier than water.
Oxygen reacts with all elements, except inert gases, to form compounds called oxides. The rate of reaction – known as oxidation – varies. For example, magnesium oxidizes very rapidly, igniting spontaneously in air. However, noble metals, such as gold and platinum, oxidize only at very high temperatures.
Although oxygen itself is nonflammable, it enhances combustion and enables all materials that are flammable in air to burn much more vigorously. These combustion-supporting properties account for its use in many industrial applications.