When you learn about phase diagrams of pure substances, you learn that the liquid and gas phases are in equilibrium only along the line separating the pure liquid and pure gas regions. But if you have a sample of liquid in a closed container with some empty space in it, that empty space eventually fills with vapor, which reaches equilibrium with the liquid phase. And this will happen at any p and T that are within the pure-liquid region of the phase diagram. So what is the correct way to interpret this observation using a phase diagram? They seem contradictory.