Compressibility Factor
The ratio of the actual molar volume of a gas to that predicted by the ideal gas law.
Formula / equation
Z = pV / nRT
Unit
Dimensionless
In depth
Z equals one for an ideal gas. Values below one indicate that attractive forces dominate and the gas is more compressible than ideal; values above one show that finite molecular volume dominates. Plotting Z against pressure gives a compact summary of a real gas's non-ideality.
Examples in the real world
Nitrogen dips to Z ≈ 0.97 near 100 atm before rising above 1 at higher pressures; hydrogen exceeds 1 from the outset.