Gibbs phase rule provides a relationship among the number of degree of freedom (F) of the system, the number of phases (P) in the system and the number of chemical components (C).
The Gibbs Phase rule is
The number ‘two’ is specified in the above equation because Gibbs phase rule formulation assumes that both temperature (T) and ‘P’ can be varied.
The degree of freedom represent the number of intensive parameters that can be varied independently.
Pure substances are always considered as single component substance.
Ex: For water at triple point, P = 3, C = 1
F + P = C + 2
Therefore F = 1+2-3 = 0
For water at liquid vapour separation line F + 2 = 1+ 2
Therefore F =1
Simple compressible substance is a substance which has only P.dV work mode. According to state postulate, a simple compressible substance has only two variables which can be varied independently.