Compression Factor
Definition and meaning of Compression Factor in chemistry.
Compression factor (Z), also called the compressibility factor, is the ratio of a real gas's molar volume to the molar volume an ideal gas would occupy under the same temperature and pressure, given by Z = PVm/RT.
In more detail
For an ideal gas, Z equals 1 at all conditions because PVm = RT exactly. Real gases deviate from this because their molecules have finite volume and experience intermolecular forces. At low to moderate pressures, attractive forces pull molecules closer together than ideal behavior predicts, making Z less than 1; at high pressures, the finite volume of molecules dominates and repulsive effects push Z above 1. Plotting Z against reduced pressure at a given reduced temperature (the principle of corresponding states) lets chemists compare how different gases deviate from ideality.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | Z = PVm/RT = PV/nRT |
| Ideal gas value | Z = 1 (exactly, at all P and T) |
| Also known as | Compressibility factor |
Nitrogen gas at 0°C behaves nearly ideally at 1 atm (Z close to 1), but at 100 atm its Z drops to about 0.9942, showing attractive intermolecular forces slightly reducing its volume below the ideal prediction; at much higher pressures Z rises above 1 as repulsive forces take over.
Frequently asked questions
What does Z less than 1 mean?
It means intermolecular attractive forces dominate, pulling the gas molecules closer together so the gas occupies less volume than an ideal gas would.
What does Z greater than 1 mean?
It means repulsive forces and the finite volume of the molecules dominate, typically at high pressure, so the gas occupies more volume than an ideal gas would.