Vapour Liquid Equilibrium for a two component mixture.

[dharavsolanki]

I am studying vapor liquid eqilibrium for two component systems. There are some things which confuse me right now.

These are the questions: -

(1) Since there has to be atleast one phase, the degrees of freedom are maximum three. Which means, the degrees of freedom are P, T and composition. Why does composition automatically qualify as the third choice of variable?
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(2) We are plotting a surface in space P-T-Composition. So when we plot the srface what do we exactly draw/plot? The possible states the system an take? Or is it a forth property?<P>
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(3)The under surface and upper surface are saturated vapour and saturated liquid states. What are saturated states in this context?

 

[Almsco]

Hi there! It sounds like you are having some confusion with vapor liquid equilibrium. Let me try to answer your questions:

(1) The composition of the system automatically qualifies as the third choice of variable because it is related to the number of components in the system. In a two component system, there are two variables that can be used to describe the composition (e.g. the molar fraction of each component). These two variables are combined into one composition variable, which is then used as the third choice of variable.

(2) When you plot the surface in space P-T-Composition, you are plotting the possible states that the system can take. This means that you are plotting the possible values of temperature, pressure, and composition that the system can occupy.

(3) Saturated states in this context refer to states where the vapor and liquid phases of the system are in equilibrium. This means that the partial pressures of the components in the vapor and liquid phases are equal.