Compressibility Factor - Definition and Physical Significance

Definition and Physical Significance

The compressibility factor is defined as

where is the molar volume, is the molar volume of the corresponding ideal gas, is the pressure, is the temperature, and is the gas constant. For engineering applications, it is frequently expressed as

where is the density of the gas and is the specific gas constant, being the molar mass.

For an ideal gas the compressibility factor is per definition. In many real world applications requirements for accuracy demand that deviations from ideal gas behaviour, i.e., real gas behaviour, is taken into account. The value of generally increases with pressure and decreases with temperature. At high pressures molecules are colliding more often. This allows repulsive forces between molecules to have a noticeable effect, making the molar volume of the real gas greater than the molar volume of the corresponding ideal gas, which causes to exceed one. When pressures are lower, the molecules are free to move. In this case attractive forces dominate, making . The closer the gas is to its critical point or its boiling point, the more deviates from the ideal case.