Gibbs phase rule use for wet vapour

[Ravi Singh choudhary]

I have a doubt regarding gibbs phase rule in thermodynamics.. It says the number of independent intensive properties required to specify the state of a system is F=C-P+2 where C is number of components and P is the number of phase.. So for a water and water vapour system, C=1, P=2 . So F=1. If we specify the temperature,pressure is fixed. But inorder to completely fix the state of the system, the dryness fraction should also be known. As the dryness fraction determines specific volume of the system. So 2 variables are required, which is not correct according to Gibbs phase rule. Am i making any mistake in this? Please help

 

[Bystander]

dryness fraction

Not a variable in the sense you are using it.

 

[Ravi Singh choudhary]

Not a variable in the sense you are using it.

How? I can have different specific volumes based on different dryness fraction.

 

[Bystander]

I can have different specific volumes based on different dryness fraction.

Can you?

 

[Ravi Singh choudhary]

Can you?

Yeah in in Pv diagram of water I have series of points on the horizontal line inside the vapor dome.

 

[Bystander]

For application of the phase rule vapor is saturated or it is not; you don't have the option of looking at the average density of "wet steam." That is not a variable you can arbitrarily include in the degrees of freedom.

 

[Ravi Singh choudhary]

For application of the phase rule vapor is saturated or it is not; you don't have the option of looking at the average density of "wet steam." That is not a variable you can arbitrarily include in the degrees of freedom.

I am getting your point but somehow I am not satisfied, can you please elaborate espicially your density logic.