ธาตุ O
Image explanation
The image represents the fundamental importance of the element in air and, when bonded to hydrogen, in water.
Appearance
A colourless, odourless gas.
Uses
The greatest commercial use of oxygen gas is in the steel industry. Large quantities are also used in the manufacture of a wide range of chemicals including nitric acid and hydrogen peroxide. It is also used to make epoxyethane (ethylene oxide), used as antifreeze and to make polyester, and chloroethene, the precursor to PVC.
Oxygen gas is used for oxy-acetylene welding and cutting of metals. A growing use is in the treatment of sewage and of effluent from industry.
Biological role
Oxygen first appeared in the Earth’s atmosphere around 2 billion years ago, accumulating from the photosynthesis of blue-green algae. Photosynthesis uses energy from the sun to split water into oxygen and hydrogen. The oxygen passes into the atmosphere and the hydrogen joins with carbon dioxide to produce biomass.
When living things need energy they take in oxygen for respiration. The oxygen returns to the atmosphere in the form of carbon dioxide.
Oxygen gas is fairly soluble in water, which makes aerobic life in rivers, lakes and oceans possible.
Natural abundance
Oxygen makes up 21% of the atmosphere by volume. This is halfway between 17% (below which breathing for unacclimatised people becomes difficult) and 25% (above which many organic compounds are highly flammable). The element and its compounds make up 49.2% by mass of the Earth’s crust, and about two-thirds of the human body.
There are two key methods used to obtain oxygen gas. The first is by the distillation of liquid air. The second is to pass clean, dry air through a zeolite that absorbs nitrogen and leaves oxygen. A newer method, which gives oxygen of a higher purity, is to pass air over a partially permeable ceramic membrane.
In the laboratory it can be prepared by the electrolysis of water or by adding a manganese(IV) oxide catalyst to aqueous hydrogen peroxide.