The atmosphere

Gases in air

The air is a mixture of gases. The amount of water vapour in the air varies from place to place, and day to day. For this reason, the proportions of the gases in the air are usually given for dry air.

Some of the gases in the air are elements:

• nitrogen, N2
• oxygen, O2
• argon, Ar

Some are compounds, including:

• carbon dioxide, CO2
• water vapour, H2O

Fractional distillation of air

About 78 per cent of the air is nitrogen and 21 per cent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. The air has been liquefied.

Here's what happens as the air liquefies:

• water vapour condenses, and is removed using absorbent filters
• carbon dioxide freezes at -79°C, and is removed
• oxygen liquefies at -183°C
• nitrogen liquefies at -196°C

The liquid nitrogen and oxygen are then separated by fractional distillation.

The liquefied air is passed into the bottom of a fractionating column. Just as in the columns used to separate oil fractions, the column is warmer at the bottom than it is at the top.

Oxygen in the air

The percentage of oxygen in the air can be measured by passing a known volume of air over hot copper, and measuring the decrease in volume as the oxygen reacts with it. Here are the…