Explain the terms involved in Gibbs phase rule with an example of a one-component system. (Phase, Component and Degree of Freedom)

Gibbs phase rule: F = C − P + 2

Phase (P): a homogeneous, physically distinct and mechanically separable part of a heterogeneous system, separated from the other parts by definite boundaries. Example: in the water system, ice, liquid water and water vapour are three phases. A gas mixture is always one phase.

Component (C): the smallest number of independently variable chemical constituents needed to express the composition of every phase. Example: in the water system all three phases (ice, water, vapour) are expressed as 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 at equilibrium.

Water system (C = 1):

• In the single-phase areas (ice, water or vapour), P = 1, so F = 1 − 1 + 2 = 2 (bivariant: both T and P must be stated).
• On a curve where two phases coexist (e.g. water ⇌ vapour), P = 2, so F = 1 (univariant: fixing T fixes P).
• At the triple point (ice ⇌ water ⇌ vapour, 0.0098 °C and 4.58 mm Hg), P = 3, so F = 0 (invariant).