Fractional Distillation

Small amounts of nitrogen, usually for student experiments, can be prepared by combining and gently warming a mixture of ammonium and nitrate salts. Using ammonium chloride (sal ammoniac) and sodium nitrite, for example, the reaction is:

NH₄Cl + NaN0₂ --> N₂ + NaCl+ H₂0
(ammonium chloride + sodium nitrite --> nitrogen gas + sodium chloride + water)

Gasses such as nitrogen are recovered from liquified air by means of a process known as fractional distillation. Gasses are liquified by combining high pressure and low temperature.

What is the definition of the critical temperatures of the atmospheric gasses?

The critical temperature is the highest temperature at which the gas will liquify, regardless of the pressure. Each gas has a different temperature at which it changes state between a liquid and a gas.

In the process of fractional distillation, the mixture is first liquified, then it is allowed to warm gradually. As this warming takes place, the gasses boil off separately and in a sequence represented by the upward direction on the critical temperature scale.

Nitrogen has low critical tempareture

Nitrogen has relatively low critical temperature. This makes Nitrogen gas the coldest element in commercially produced liquid air.