| (1) | F = C - P +2 |
where
F | number of degrees of freedom |
C | number of mixture components |
P | number of phases |
For Pure substance : C=1 and F = 3 - P.
It means that 3-phase equilibrium (solid, liquid an gas) has zero degrees of freedom : F = 0 and can only exist at fixed Temperature and Pressure, called Triple Point.
The 2-phase equilibrium has one degree of freedom: F=1 and form a smooth line
See also
Natural Science / Physics / Thermodynamics / Thermodynamic system / Phase / Phase Equilibrium
[ State of matter ][ Pure substance ] [ Mixtures ][ Fluid Mixtures ]
[ Thermodynamic equilibrium ][ Vapour Liquid Equilibrium (VLE) ]