The Phase Rule describes the relationships between components, phases, and degrees of freedom in a chemical system. It states that the number of intensive variables that can be independently varied, like temperature and pressure, is equal to the total number of components (C) minus the number of phases (P) plus 2. The Phase Rule is derived from equating the number of equations, which is the number of components times the number of phases minus 1, to the number of variables in the system. According to the Phase Rule, each area of a phase diagram represents a bivariant system where both temperature and pressure can be varied without changing the equilibrium state.

1. The Phase Rule
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2. The Phase Rule:
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12. Derivation of Phase rule:
To be more specific, the composition of each phase is determined
by C − 1 intensive variables (such as mole fractions) in each phase. The total
number of variables is (C − 1)P + 2, where the extra two are temperature and
pressure. The number of equations is C(P − 1), since the chemical potential of
each component must be equal in all phases.
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14. Thus each area of phase diagram represents a
bivariant system. At conditions wholly within
one of the three areas, both the variables
(pressure and temperature) can be varied
independently without causing a change in the
state of equilibrium. Therefore in order to define
the condition of the phase both pressure and
temperature must be stated.
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