Violation of Gibbs Phase Rule? According to Gibbs phase rule, only one intensive property is required to completely specify a simple compressible system which is in a liquid/vapor equilibrium.
F = 2 + C - P
For a pure substance, C = 1
For a liquid/vapor equilibrium, P = 2
This gives F = 1
But is one property enough?
If we specify the temperature, don't we still need the quality to specify the state completely? Or am I missing some conceptual understanding of the topic?
Thanks!
 A: Yes the one property is enough if it automatically determines a second property. For example, apply Gibbs phase rule to saturated steam. If you know T you know P and vice versa.
What's more you also know the intensive properties of specific volume v ($m^3/kg$), internal energy u ($kJ/kg$), and enthalpy h ($kJ/kg$) for the liquid and vapor phases at a given T (or related P). You can find them in the saturated steam tables.
You only need the quality of the steam to determine the related extensive properties, volume (V), internal energy (U), and enthalpy (H) of the liquid and vapor phases.
Hope this helps.
A: By definition, the degrees of freedom, F, in  Gibbs phase rule designates the number of intensive independent properties that must be specified to fix the state of a system for each phase not the state of a system itself.
Shah M Hasan, I think you are right, the state of the mixture as system is not fixed completely only by determining its pressure or temperature.
Indeed, we have two kinds of intensive properties: phase-intensive properties such as: specific volume of each phase i.e. vf and vg, and system-intensive properties such as quality. Pressure and temperature are both phase-intensive and system-intensive properties.Gibbs phase rule is about the number of phase-intensive properties rather than system-intensive properties.
see:
https://www.sciencedirect.com/science/article/pii/037838129087009E
