Gibbs phase rule use for wet vapour

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The discussion centers on the application of Gibbs phase rule to a water and water vapor system, questioning the necessity of knowing the dryness fraction alongside temperature and pressure to fully define the system's state. The Gibbs phase rule indicates that for one component and two phases, only one independent variable is needed, leading to confusion about the role of the dryness fraction. Participants clarify that dryness fraction is not an independent variable in this context, as it does not alter the system's degrees of freedom. The concept of saturation is emphasized, indicating that the vapor must be treated as either saturated or not, without considering intermediate states like "wet steam." The conversation highlights the importance of adhering strictly to the definitions and conditions outlined by the Gibbs phase rule.
Ravi Singh choudhary
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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
 
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Ravi Singh choudhary said:
dryness fraction
Not a variable in the sense you are using it.
 
Bystander said:
Not a variable in the sense you are using it.
How? I can have different specific volumes based on different dryness fraction.
 
Ravi Singh choudhary said:
I can have different specific volumes based on different dryness fraction.
Can you?
 
Bystander said:
Can you?
Yeah in in Pv diagram of water I have series of points on the horizontal line inside the vapor dome.
 
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.
 
Bystander said:
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.
 

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