Entry by Andrew Capulli
Definition: Phase Rule (Gibbs' Phase Rule)
The phase rule relates:
- F: The degrees of freedom of the system; see below.
- P: The number of phases that can coexist; any separable material in the system. A phase can be a pure compound (say water for example) or a mixture (solid or aqueous), but the phase must "behave" as a consisten substance.
- C: The number of components (that make up the phases)
The Phase Rule States: the degrees of freedom of a system is equal to the number of components minus the number of phases plus two
The 'degrees of freedom' of the system (at chemical equilibrium) refer to the number of conditions or variables that can be altered, independent of each other, without effecting the number of phases in the system. Essentially, the degrees of freedom of a system describe the dependency of parameters such as temperature and pressure on each other.
The Phase Rule describes the number of variables (and equations) that can be used to describe a system (at chemical equilibrium). The number of chemical components (C in the equation above) in addition to the "extensive variables" (temperature and pressure) comprise the 'variables' of a system. The degrees of freedom of a system dictate the number of phases (as described above in the bullet list) that can occur in the system.
The critical point (on a phase diagram) can only exist at one temperature and pressure for a substance or system and thus the degrees of freedom at any critical point is zero.
The Wikipedia article has a number of examples using the phase rule on a phase diagram to determine the degrees of freedom of a system at a given point; it can be found at: http://en.wikipedia.org/wiki/Gibbs%27_phase_rule.