This question has been bugging me for long. Specifically I'm wondering about it in the context of Pressure-Tempreature phase diagrams. With these diagrams you can deduce stuff like the Clausius-Clapeyron equation and stuff like that. But certain things confuse me, e.g.: There is this mental picture that after placing a liquid inside a container and sealing it (the liquid has a vacuum on top), after a long time a vapor pressure will form because of dynamic equilibrium and its value is given by the Clausius-Clapeyron equation as a function of temperature. But I don't get this. Suppose I have a GIGANTIC container, and I dropped only a litre of water. It seems unlikely to me that a vapor phase with a pressure will form on top of it because of dynamic equilibrium. Namely because it implies that on top of the liquid a vapor phase micro-atmosphere will form. Why isn't this micro-atmosphere dissipating into the surrounding vacuum? What exactly is supposed when you say that there is some substance in a container and that its phases will be given by the P-T diagram? Also: I interpret P-T diagrams as if I am the one exerting the pressure. Why is that the L-G line indicates the pressure of the vapor pressure alone and not the pressure I am exerting to the system? Or are both implied?