Preparation of Oxygen - From Air

On a laboratory scale

Air, free from moisture and carbon dioxide, is bubbled through boiling mercury. The oxygen of the air combines with the mercury to form mercuric oxide, which is a red solid (Fig.13.1).

This oxide is then separated from the uncombined liquid mercury, and heated strongly. The mercuric oxide decomposes to form mercury and oxygen.

On industrial scale

Industrially, oxygen is manufactured from the air. The industrial method is based on a very simple principle known as Joule-Thomson effect. Air is first subjected to liquefaction (made liquid) and then to fractional distillation, to separate out oxygen from it.

When a gas under pressure is released its temperature falls, and causes cooling. This effect is known as Joule-Thomson effect.

To understand it, do the following experiment.

Blow up a balloon. In the process, you have compressed a lot of air into the balloon. The pressure of the compressed air inside the balloon is much more than that of the outside air. Now release the air against your face. You will experience a cooling effect.

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Liquefaction

Air, free from dust, water vapour and carbon dioxide is compressed by increasing the pressure to about 200 atmospheres. When the pressurised air is released into a chamber through a small orifice intense cooling of the air takes place. The cooled air is subjected to the same process of compression, sudden expansion and cooling repeatedly, till it ultimately condenses into a liquid. The temperature of liquid air is around -200^oC.

Fractional distillation

This liquid air contains two main substances, nitrogen and oxygen in liquid forms whose boiling points are -196^oC and -183^oC respectively. So when this liquid air is allowed to warm up, nitrogen with low boiling point boils over first and is separated. What remains behind is liquid oxygen, which is then packed in steel cylinders under high pressure.

This process of separating nitrogen and oxygen from liquid air is commonly known as "fractional distillation of liquid air".