Chemistry - What does the phase diagram mean by "liquid" at NTP? (The liquid and vapor coexist even when the situation is not on the curve)

Solution 1:

The phase equilibrium diagram is for a system consisting of just water.

The diagram pressure is the total system pressure.

It may be equal to the saturated vapor pressure, if no external pressure is imposed on the system.

Or, it can be ( much ) higher, if some external pressure higher than the saturated vapour pressure exists. In such a case all vapor is at equilibrium transformed to liquid or solid phase.

Really high pressures can be seen in the water phase diagram for very high pressures, displaying rare ice phases.

Solution 2:

Imagine filling the e.g. water into a cylinder with a piston on top. No air inside.

Now the experiment is to control the temperature inside the cylinder, and move the piston to get the wanted pressure inside. If the pressure is above the vapour pressure, your cylinder is completely filled with liquid water, if it is below, all the water will be evaporated.

Of course you have to pull out the piston very far to make room for the complete phase transition. While you still have two phases in the cylinder, the equillibrium T/p value pair is always on one of the lines in your phase diagram.