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The essentials

Description 

Here is a brief description of oxygen.

• Standard state: gas at 298 K
• Colour: colourless as a gas, liquid is pale blue
• Classification: Non-metallic
• Availability:

Liquid oxygen. Image adapted with permission from Prof James Marshall's (U. North Texas, USA) Walking Tour of the elements CD.

Oxygen is a Group 16 element. While about one fifth of the atmosphere is oxygen gas, the atmosphere of Mars contains only about 0.15% oxygen. Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, one process responsible for stellar energy production. Oxygen in excited states is responsible for the bright red and yellow-green colours of the aurora. About two thirds of the human body, and nine tenths of water, is oxygen. The gas is colourless, odourless, and tasteless. Liquid and solid oxygen are pale blue (see picture above) and strongly paramagnetic (contains unpaired electrons).

Ozone (O₃) is another allotrope of oxygen. It is formed from electrical discharges or ultraviolet light acting on O₂. It is an important component of the atmosphere (in total amounting to the equivalent of a layer about 3 mm thick at ordinary pressures and temperatures) which is vital in preventing harmful ultraviolet rays of the sun from reaching the earth's surface. Aerosols in the atmosphere have a detrimental effect on the ozone layer. Large holes in the ozone layer are forming over the polar regions and these are increasing in size annually. Paradoxically, ozone is toxic! Undiluted ozone is bluish in colour. Liquid ozone is bluish-black, and solid ozone is violet-black.

Oxygen is very reactive and oxides of most elements are known. It is essential for respiration of all plants and animals and for most types of combustion.

Nearing Zero cartoon included by kind permission of Nick Kim.

Isolation

There is not normally any need ot make oxygen in the laboratory as it is readily available commercially or through in-house air liquefaction plants. However the decomposition of sodium azide is one route to N₂ and decomposition is ammonium dichromate is another. Both reactions must only be carried out under controlled conditions by a professional.

2KClO₃ (400°C) → 2KCl + 3O₂

2KMnO₄ (214°C) → K₂MnO₄ + MnO₂ + O₂

In addition, electrolysis of KOH using nickel electrodes gives clean oxygen.

Nitrogen is made on massive scale by liquefaction of air and fractional distillation of the resulting liquid air at about -183°C) to separate out nitrogen (the main product) and other gases.

Ozone (O₃), the other allotrope of oxygen, is made by silent electric discharge through oxygen flowing through a cooled system. This can give up to a10% proportion of ozone and the ozone is purified by fractional liquefaction (with care!).

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