Boyle’s law is applied for (almost) vacuum or not Seems like a strange question, I want to know if Boyle’s law is applied for (almost)vacuum or not. Imagine you have a very very small sac which is filled completely with a fluid, and there is no air in it. This sac is elastic so you can expand it, you put this sac in the air and you want to find its pressure when you expand it.First it is said that the pressure is below than atmosphere and this pressure drop occurs because the sac is filled with fluid, which cannot expand to fill the slightly larger volume. Therefore, a vacuum exists in the infinitesimal space in the slightly expanded sac which is not occupied by fluid, producing a small drop in pressure below atmospheric pressure(It was written in a book but I can’t understand it very well).My question:if now I expand this sac more, if its pressure drops more is that “because of Boyle’s law”?
If it is not because of Boyle’s law so how can I justify the more drop in pressure? Thank you for your attention.
 A: (a) In Physics "fluid" means gas or liquid. I'll assume that you mean liquid.
(b) When you expand the sac, there will be some evaporation from the liquid, so the 'extra' space in the sac will contain vapour from the liquid. A vapour is gas-like in that it consists of molecules moving randomly in (otherwise) empty space, continually colliding with each other and the container walls, so exerting a pressure.
(c) The vapour won't obey Boyle's law because its mass isn't fixed: if you expand the sac further, more of the liquid will evaporate (become vapour). If time is allowed after every expansion for equilibrium to occur between liquid and vapour, we will have a so-called saturated vapour whose pressure depends only on the temperature and the particular liquid.
(d)  As we contract the space, the vapour will condense back to a liquid. But if we raise the temperature above the substance's so-called critical temperature, it won't condense, but will remain in the gas state, and we call it a gas, not a vapour.
