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Oxygen
Oxygen is one of the most abundant elements. It forms 21 per cent of the atmosphere, 89 per ccnt of the water, and about 50 per ccnt of the earth's crust. Without oxygen, life cannot exist, as well as fire. Oxygen is essential in supporting respiration and combustion, it is used in many modern industrial processes. The element consists of diatomic molecules.
It is a colourless, odourless gas, which is slightly soluble in water: 1 litre of water at 0° dissolves 48.9 ml of oxygen gas at 1 -atm pressure. Its density at 0°C and 1 atm is 1.429 g litre'1. Oxygen condenses to a pale blue liquid at its boiling point, -183.0°C, and on further cooling freezes at -218.4°C to a pale blue crystalline solid.
Oxygen is easily prepared in the laboratory by heating potassium chlorate,
KCIO,: 2KC10, -> 2KC1 + 302 (g).
The reaction proceeds at a temperature just above the melting point of potassium chlorate if a small amount of manganese dioxide, MnO:, is mixed with it. Although the manganese dioxide accelerates the rate of evolution of oxygen from the potassium chlorate, it itself is not changed.
Oxygen is made commercially mainly by die distillation of liquid air. Nitrogen is more volatile than oxygen, and tends to evaporate first from liquid air. Nearly pure oxygen is obtained by properly controlling the conditions of the evaporation. Oxygen is stored and shipped in steel cylinders, at pressures of 100 atm or more. Oxygen is also made commercially, together with hydrogen, by the electrolysis of water.
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