Gibbs' Phase Rule Calculator
Evaluate the number of independent intensive variables for multicomponent phase equilibria using the phase rule.
Count of chemically independent species.
Examples: liquid, vapor, solid.
Use 1 if temperature or pressure is fixed, 2 if both are fixed.
Degrees of freedom F
3
Multivariant system: multiple intensive variables may vary independently.
How to Use This Calculator
Count independent components
Identify the number of chemically independent species, excluding those linked by reactions or conserved ratios.
Identify phases present
Determine how many distinct phases coexist (solid, liquid, vapor, and so on).
Select constraints
Choose 0, 1, or 2 to represent how many intensive variables (temperature, pressure) are fixed externally.
Interpret degrees of freedom
The resulting F value shows how many intensive variables can vary independently.
Formula
F = C - P + 2 - constraints
Constraints account for externally fixed intensive variables such as temperature or pressure. When neither is fixed, constraints equals 0.
Example
For water (C = 1) with liquid and vapor present (P = 2) at fixed pressure (constraints = 1): F = 1 - 2 + 2 - 1 = 0 (invariant).
Full Description
Gibbs' phase rule derives from mass balance and intensive variable relationships in equilibrium thermodynamics, constraining phase diagrams.
By accounting for independent components, phases, and external constraints, the rule reveals how many variables must be specified to define the state of a system.
Frequently Asked Questions
What counts as a component?
Components are the minimum number of species needed to describe all compositions subject to reaction constraints.
How do reactions affect components?
Independent reactions reduce the effective component count because they impose composition relationships.
Why subtract constraints?
Each externally fixed intensive variable removes one degree of freedom from the system.
Can F be negative?
Negative F indicates the chosen values conflict with equilibrium requirements or double count constraints.
Does phase rule apply to non-equilibrium systems?
No. The phase rule assumes thermodynamic equilibrium. Non-equilibrium systems require kinetic modeling.