Oxygen

Oxygen is the most abundant element on the Earth's surface. It makes up 20.948% of the atmosphere by volume and 45.5% of the lithosphere by weight. It occurs both as the free element (in the atmosphere) and in the combined form (in the lithosphere and hydrosphere). Its most common and best known compound is probably water. Water contains 88.9% oxygen, by weight.

In the lithosphere, oxygen occurs in a wide variety of compounds, such as the oxides, silicates, carbonates, phosphates, sulfates, and a variety of more complex compounds.

Nearly all of the oxygen found on Earth today is produced by biological activity. During the process of photosynthesis, carbon dioxide and water react in the presence of chlorophyll to produce carbohydrates and oxygen. Scientists believe that oxygen was essentially absent from the earth's atmosphere when the planet was first created. As life developed on Earth and photosynthesis became more common, the rate of production increased until the present concentration of oxygen in the atmosphere, the oceans, and the crustal rocks was reached about 580 million years ago.

Oxygen can be prepared on a small scale by the decomposition of oxygen-containing compounds. In the laboratory, for example, it can be produced by heating potassium chlorate with manganese dioxide as a catalyst or by the gentle warming of mercury(II) oxide. The element can also be produced by the electrolysis of water. When an electrical current is passed through water to which a small amount of inorganic acid has been added, water molecules break apart to form hydrogen gas and oxygen gas. Although this method is relatively simple to employ, the cost of energy needed to carry out the reaction is usually prohibitively high for commercial applications.

By far the most important method of producing oxygen commercially is by the fractional distillation of liquid air. A sample of air is first cooled below the boiling point of most gases that make up air, a temperature of less than -328°F (-200°C). The liquid air is then allowed to evaporate. At a temperature of -320.44°F (-195.8°C), nitrogen begins to boil off. When most of the nitrogen is gone, argon and neon also boil off, leaving an impure form of oxygen behind. The oxygen is impure because small amounts of krypton, xenon, and other gases may remain in the liquid form. In order to further purify the oxygen, the process of cooling, liquefying, and evaporation may be repeated.

Oxygen is commonly stored and transported in its liquid form, a form also known as LOX. LOX containers have the general appearance of very large vacuum bottles consisting of a double-walled container with a vacuum between the two walls. The element can also be stored and transported less easily in gaseous form in steel-walled containers 4 ft (1.2 m) high and 9 in (23 cm) in diameter. In many instances, oxygen is manufactured at the location where it will be used. The process of fractional distillation described above is sufficiently simple and inexpensive that many industries can provide their own oxygen-production facilities.