Gas molecules such as oxygen and nitrogen in the air are in a chaotic state of motion, and attract and influence each other. Although the liquefaction temperature of oxygen is higher than that of nitrogen, when the temperature drops to a part of air and begins to condense into liquid, the initial condensation is not only the oxygen molecules, moreover, some nitrogen molecules begin to liquefaction at the same time under the attraction of oxygen molecules, which makes nitrogen component in the liquid, thus reducing the purity of oxygen in the liquid and not forming pure liquid oxygen. Similarly, nitrogen molecules in the vapor on the liquid layer also attract oxygen molecules, so that part of oxygen molecules can not condense into liquid first, so the vapor can not be pure nitrogen. Assuming that under 1 standard atmospheric pressure (0.1013MPa), if the air is cooled to 81.3 K, the Moore composition of oxygen in the liquid phase is only 53% O2.
Due to the above reasons, air cannot be separated into oxygen and nitrogen by simply cooling. Moreover, the liquefaction temperature of air is not constant, it is between the liquefaction temperature of pure oxygen and pure nitrogen. For example, under the condition of 0.1013MPa, the liquefaction temperature of pure oxygen is 90.19 K, the liquefaction temperature of pure nitrogen is 77.347 K, and the temperature of air condensing is 81.81 K, the temperature of all condensation is 78.8 K.
