What is the phase equilibrium in thermodynamics

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Phase equilibria

The state functions can be used to describe states of equilibrium, which are clearly defined by the external, arbitrarily selectable state variables (state variables). Pressure and temperature (thermal state variables), strictly speaking also the chemical composition of a system, are completely determined. This means that thermal equilibrium usually also requires chemical equilibrium.

The thermal equilibrium is characterized by the fact that it is permanently maintained in the event of reversible changes in state and that the total entropy serving as a measure of stability of a closed system has a maximum value. This results in the equilibrium conditions for non-closed systems, as well as for adiabatic or isochoric infinitesimal changes of state and for further combinations of state variables.

In any case, however, equilibrium conditions are practically important in which either isothermal and isochoric or isothermal and isobaric infinitesimal changes of state produce the equilibrium conditions.

In the simplest case of a closed one-component system consisting of two phases, the phases of which are distinguished by the indices and, the following results for the phase equilibrium:

Since the following applies to the transition of a substance from one phase to the other, this results in

In equilibrium between two coexisting phases of a pure substance, the following applies