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In thermodynamics, the fugacity is a state function of any isothermal system. The fugacity, which has units of pressure, represents the tendency of a fluid to escape or expand isothermally. For gases at low pressures where the ideal gas law holds, fugacity is equal to pressure. The ratio between fugacity and pressure is called the fugacity coefficient; therefore, for an ideal gas.

The fugacity is defined at fixed temperature by the following differential relation:

where is the Gibbs free energy, is the molar gas constant, and is the fluid's volume. For any two isothermal physical states, represented by subscripts 1 and 2, the ratio of the two fugacities is as follows: