Answer: \(F=C-P+2\). For the water system at the triple point, \(C=1\), \(P=3\) (ice, water, vapour), so \(F=0\).
Gibbs phase rule: for a heterogeneous system in equilibrium, influenced only by temperature, pressure and concentration, \(F=C-P+2\).
- Phase (P): a homogeneous, physically distinct and mechanically separable part of the system, separated from other parts by a definite boundary. In the water system, ice, liquid water and water vapour are three phases.
- Component (C): the smallest number of independent chemical constituents by which the composition of every phase can be expressed. In the water system every phase is H₂O, so \(C=1\).
- Degree of freedom (F): the minimum number of independent variables (temperature, pressure, concentration) that must be fixed to define the system completely.
Water system examples:
- Only liquid water (one phase): \(F=1-1+2=2\), bivariant (both T and P must be stated).
- Water ⇌ vapour along the vaporisation curve: \(F=1-2+2=1\), univariant.
- Triple point (ice ⇌ water ⇌ vapour, at 0.0098 °C and 4.58 mm Hg): \(F=1-3+2=0\), invariant.
