Properties of ultra-high-pressure fluids
Effects of high and very high pressure

Several quantities can be defined to characterize gas compression. Here are a few of them.

Compressibility pV is the product of pressure p and volume V, and the virial coefficients are the coefficients in the limited expansion of pV as a function of the inverse of volume or pressure. The development as a function of 1/V is the more fundamental of the two:

For low-density gases (whose molar volume is greater than the critical molar volume), successive coefficients can be related to interactions between pairs, triplets, etc. of molecules. The compressibility factor Z is the ratio :