Understanding Phases and Properties in a Phase Diagram

[mech-eng]

Hi, all. In the picture is a question and its solution. Here I can not understand how is the graph
Ln P versus T established why are there only liquid and vapor parts how can be understand
solid phase should not be there and when the properties under the critical ones why is it written as "you can not say"

Have a good day.

 

[DrClaude]

The graph is a schematic representation of a phase diagram near the critical point. Below the critical point, the boundary between the liquid and the gas is given by the Clausius-Clapeyron relation. The solid phase not represented because it is irrelevant to the question.

Below the critical point, the phase depends on which side of the liquid-gas boundary the substance is (it might even be in the solid region). Since the position of this boundary in the phase diagram depends on the substance, you can't tell exactly where the actual (P,T) point is located. For instance, for water, you can more or less define a region b (represented as a line in the graph) where you expect to find the (2 MPa, 17 °C) point, but you can't tell if its a liquid or a gas.

 

[mech-eng]

But in that graph a point in the left of the boundary is in the liquid phase even if it is up or under critical point.
And it seems strange to me that there is a liquid phase part(which is coloured part) up to the critical point. And
all chosen points which are (a),(b), and (c) are located under the critical point but (a) is right of the boundary but others are left.