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? <P> (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> <P> (3)The under surface and upper surface are saturated vapour and saturated liquid states. What are saturated states in this context?