For a system at equilibrium the phase rule relates:
The degrees of freedom represent the environmental conditions which can be independantly varied without changing the number of phases in the system. Conditions include: Applying the phase rule to the simplified one component H2O system.
Phase rule becomes P + F = 3, because we are dealing with a one component system (C = 1).
  1. At A
  2. At B
    Point B lies on the boundary curve separating Ice from steam field,
  3. At T
Therefore the maximum number of phase which can stably coexist in a one component system is three, and they do so only if there are no degrees of freedom.