We were discussing super-critical fluids with a colleague and as were thinking about different thought-experiments, it occurred to me that you cannot really have only liquid in a piston in a real situation. I want to make sure I'm not missing something. Are there liquids which have zero vapor pressure under certain conditions? If not, am I right to think even if you would perfectly enclose a volume of liquid in an hermetic container, you would always have a tiny fraction of vapor due to evaporation and vapor-pressure equilibrium? If that is the case, ato turn this liquid into a supercritical fluid, I will only need to reduce the volume of the container by a very small amount and this can be explained in two ways:
- from the liquid point of view, since it is almost incompressible, I need a very large force to change that volume even by a small amount. Thus the pressure in my piston is large and likely over the critical pressure of the liquid, turning it into a super-critical fluid
- from the gas point of view, since the volume occupied by the gas is very small and since it will "feel" the compression first, it is easy to see the gas pressure increasing very rapidly for small changes in volume, once again leading to a supercritical fluid.
Thanks in advance for your answers !