CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 180
Solution
The number of intensive independent variables is 4: pressure, temperature, the molar fraction of
nitrogen and the molar fraction of oxygen. The number of intensive independent variables in an
isobaric system is 3. Pressure is constant and it is not understood as an intensive variable (the
system is studied under a fixed condition [p]).Example
Give the number of intensive independent variables in gaseous air while assuming that air is a
mixture of nitrogen, oxygen and argon of the compositionxN 2 = 0. 78 , xO 2 = 0. 21.Solution
The composition of air is invariable (the system is studied under two fixed conditions). The
number of intensive independent variables is 2: pressure and temperature.7.3.3 Degrees of freedom
Every intensive independent variable in ak−component system formed byfcoexisting phases
for which there exists an interval in which it may be changed without changing the number
and type of components and phases is referred to as thedegree of freedom.
Thenumber of degrees of freedom is the number of intensive independent variables
which unambiguously specify a system in the state of thermodynamic equilibrium. The number
of degrees of freedom is always lower than or equal to the number of intensive independent
variables. Equality occurs in homogeneous systems.
7.3.4 Gibbs phase rule
In ak−component,f−phase system, the number of degrees of freedomvis
v=k−f+ 2−C , (7.7)whereC is the number of the other fixed conditions.