What is the State Postulate and How Does it Relate to Thermodynamic Properties?

[aiklone1314]

I was refer to the textbook thermodynamics an engineering approach 6th edition, at page 14&15, the definition is that a state is specified by two independent, intensive properties for a simple compressive system.
The book says that temp. & pressure is dependent on each other at multiphase and independent to each other on single phase.

I was wondering if the pressure is held constant , let's say 1atm, and the water is boils until it reaches 100 degree celsius the water starts to change phase to gas. That means the pressure is independent of the temp. right?because when the temp. increasing, the pressure remains constant. But why the book says it is dependent on multiphase?

 

[aiklone1314]

Is state postulate means we can know the state of a compressible system if we know two properties of it?State in the book means phase right?

I found that whatever if we use two completely independent properties or two dependent properties at multiphase we can also know it's phase.

E.g: two completely independent properties: Temp. & Specific Volume.
If we know the temp. & specific volume of the compressive system, then we can know it's phase/state.

two dependent properties at multiphase : Temp. & Pressure.
If we know the Temp. & Pressure. of the compressive system, we can also know it's phase/state.

Given: Temp:100degree celsius, P:1atm
we know it is gas.
Or at a specific height mountain : 95degree celsius, P:0.95atm
we know it is also a gas.