Physical chemistry
Gibbs phase rule
The Gibbs phase rule counts independent intensive variables available in phase equilibrium; it guides interpretation of one- and multicomponent phase diagrams.
For C components and P phases, with temperature and pressure as external variables, the degrees of freedom are . Each extra constraint lowers F.
Model and quantities
Read the relation together with assumptions about state, experimental conditions, and sign conventions. Keep units consistent and check dimensions before interpreting a result.
Definition: Degrees of freedom
F is the number of independent intensive variables (such as T, p, composition) that can change without changing the number of equilibrium phases. The standard rule assumes a nonreacting system with no extra fields.
Quantities in the relation are defined for the reaction or system at hand. In particular, distinguish standard-state quantities from actual conditions and do not infer a mechanism from a general expression alone.
Example: Worked example
Pure water at its triple point has C=1 and P=3. Calculate F and interpret it.
Solution
F=1−3+2=0. Neither temperature nor pressure can be varied independently while retaining all three phases; the triple point is invariant.
| Concept | Description | Unit / note |
|---|---|---|
| Key relation | Use under stated conditions | Check units and sign convention |
| Measured quantity | Relates a state or process | Compare data with model |
| Scope | Model specific conditions | Check assumptions first |
In the one-component water diagram, a two-phase boundary has F=1, so T and p can vary together along the line; a single-phase region has F=2. A binary solution adds composition as a variable.
In the worked example, which result follows from the given data?
Which statement is consistent with this lesson?
References
- Peter Atkins, Julio de Paula (2014). Physical Chemistry