含义Meaning
平衡体系中可独立变化而不改变相数的强度变量数（自由度）。单组分体系：单相区 F = 2，两相线 F = 1，三相点 F = 0。
The number of intensive variables that can vary independently without changing the number of phases. One component: F = 2 in a single-phase region, 1 on a two-phase line, 0 at the triple point.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(F\) | 自由度degrees of freedom | 1 |
| \(C\) | 独立组分数number of components | 1 |
| \(P\) | 相数number of phases | 1 |
推导Derivation
每相的组成需 C − 1 个摩尔分数，P 相共 P(C − 1) 个，加上 T、p 两个，总变量 \(P(C-1) + 2\)。
Each phase needs C − 1 mole fractions; with T and p there are P(C − 1) + 2 variables.
每个组分在 P 相中化学势相等，给出 C(P − 1) 个约束。
Equal chemical potentials give C(P − 1) constraints.
F = 变量 − 约束 = C − P + 2。
F = variables − constraints.
适用条件与局限Assumptions & limitations
C 为独立组分数 = 物种数 − 独立反应数 − 其他约束；压强固定（凝聚体系）时用 F = C − P + 1。
C counts independent components; at fixed pressure use F = C − P + 1.
例题Worked example
水的三相点（273.16 K、611.657 Pa）F = 1 − 3 + 2 = 0：温度和压强都固定，曾用于定义开尔文。
Water’s triple point has F = 0 — once used to define the kelvin.