Hi I was studding Van-der-waals equation for real gases when I got in my mind two questions that i couldn't get a sufficient answer for from my professors, hope you have a better explanations : 1- Why there is critical point? It was shocking for me to know that there is a particular temperature above which we can't convert gases to liquid, I always thought that what ever the gas temperature is, we always can apply a sufficient pressure on the gas to get liquid, but according to van-der-waals that wrong, and I can't get the physical reason beyond that, why at that point we can't force the connections between molecules get stronger to get liquid, anyway i get answer such as "because the density of the gas becomes as much as the liquid but it's will remains gas" of course it's not answering me, I want to know physically why it remains gas? maybe I should dig here into quantum physics to get answer? or I'm getting something wrong? 2- when we converting gas from gas to liquid, why we should reduce gas volume more when gas temperature is lower? I mean if we have to isotherms, to convert the gas to liquid, we need to reduce ([tex]\Delta V[/tex]) the volume of the gas to accomplish the conversation quite more if gas temperature is lower! somehow it's controversy with my intuition because at lower temperature, molecules energy is less, so it should be easier to push molecules to new state of mater! For this one I got the answer: because cool liquid has a smaller volume! somehow it's logical but I think that it's volume will reduce very little comparing to the needed decrease that shows van-der-waals eq. to change the state of matter. Sorry for the lengthen and thx in advance for everyone.