Compressibility Factor


Quick
The compressibility factor is used to determine whether the ideal-gas equation is a valid approximation for a pure substance in question.


Equation
(Eq1)    
Z =
Pv
RT
compressibility factor


Nomenclature
Ppressure
vspecific volume
Rgas constant (NOT the universal gas constant)
Ttemperature


Details

A more quantitative study of the question of the ideal-gas approximation can be conducted by introducing the compressibility factor Z, defined as:

(Eq1)    
Z =
Pv
RT

or:

Pv = ZRT

Note that for an ideal gas Z = 1, and the deviation of Z from unity is a measure of the deviation of the actual relation from the ideal-gas equation of state.

Certain gases may only be compatible within certain ranges of temperatures and pressures in order for the ideal-gas equation to be valid.

At lower temperatures or at very high pressure, the compressibility factor may deviate from the ideal-gas value. Moderate-density forces of attraction may tend to pull molecules together, resulting in a value of Z < 1, whereas very high density forces of repulsion may tend to have the opposite effect. These characteristics may be similar for pure substances, at least in a qualitative sense. Quantitatively, compressibility vs. pressure diagrams for different pure substances are all different, since the critical temperatures and pressures of different substances vary over wide ranges.

These substances can be put on a comman basis by "reducing" the properties with respect to the values at the critical point.





Related
▪ L - Ideal Gas Equation