How can I estimate the density of a liquid mixture (e. g. liquid air?)

What is the density of liquid air at its highest temperature?
Sue Kamal

I can give you a quick back-of-the-envelope estimate, but I don't have that data.
Cryogenic properties of gases, taken from the CRC Handbook of Chemistry and Physics
GasDensity, kg/m3Boiling point at 1 atm, K
O2114290.18
N280477.35
Ar139087.45
The highest temperature for liquid air is the boiling point for nitrogen gas. If I assume that liquid air is an "ideal solution" (that is, all the molecules in the mixture interact the same way, whether they're oxygen, nitrogen, or argon), I can write
density of liquid air = 0.78084 × density of nitrogen + 0.20946 × density of oxygen + 0.00934 × density of argon

where the fractions are the volume fractions of each component in pure, dry air. (They don't add up to 1 because I'm ignoring all the trace gases, including CO2, to keep things simple). If I further assume that the densities of liquid oxygen and liquid argon aren't much different at the boiling point of nitrogen then they are at their own boiling points, I can write the density of liquid air at the boiling point of nitrogen as
(0.78084 × 804 kg/m3) + (0.20946 × 1142 kg/m3) + (0.00934 × 1390 kg/m3) = 880 kg/m3
or a bit less dense than water is around room temperature.

Author: Fred Senese senese@antoine.frostburg.edu


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