Gibbs Phase Rule
In discussing phase equilbrium, you need only to consider intensive
variables (T, P, and c)
– Only certain of these can be varied independently
– Some are fixed by the values chosen for the independent
variables and by the requirements of thermodynamic equilibrium
e.g. if you chose r and T for a system P is fixed
– Number of variables which can be varied independently without
changing the number of phases is called the degrees of freedom
of the system
– The number of degrees of freedom or variance of a
system, F, is related to the number of components(C)
and number of phases(P)
F = C - P + 2
• Phase(P) -“State matter which is uniform throughout not only
in chemical composition but also in physical state”
• Solid
Various phases [e.g. diamond; graphite) or compositions
NaCl, NaCl.2H2O
Alloys (sometimes its difficult to tell this - microscopic
examination may be necessary {dispersions uniform on
macroscopic scale})
Miscible one phase (P=1)
Immisible multiple phases (P>1)
– Liquid
Miscible liquids (solutions) are one phase
Immiscible liquids are multiple phases (P>1)
– Gas
Systems consisting of gases can have only one phase
• Heterogeneous and homogeneous systems
– Systems with one phase are homogeneous
– Systems with more than one phase are heterogeneous
• Constituent- a chemical species (ion or molecule which is
present
• Component (C) - chemically independent constituents of a
system
Examples:
H2O, C – one component system
NaCl-H2O – two component system
Benzene-toluene – two component system (miscible liquids)
