Thermodynamics - Explaining the Critical Point
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers point people to it as a concrete demonstration of what happens when the liquid-gas interface, and with it surface tension, disappears at the critical point.
Recommended by 0 people across 0 different months, 2017 to 2025.
What it is
A short experiment video showing carbon dioxide heated and cooled through its critical point, where the boundary between liquid and vapour vanishes and later reappears. It ends with a brief explanation on a pressure-temperature phase diagram. Made by mechanical engineering students and a media centre at Bergen University College, Norway.
What people said
Cites Farcher · 2017-01 · Physics Stack Exchange · on “Why does water turn into water vapor?”
Water molecules have attractive forces between them and form the liquid state. First of all start with a container with water liquid and a vacuum above the water liquid. This container and the water within it is kept at a constant temperature. Some the water liquid molecules will have enough kinetic energy to overcome the attraction of their neighbouring water liquid molecules and escape from water liquid surface and become water vapour. There is a net migration with water liquid molecules becoming water vapour molecules. As time goes on and the number of water vapour molecules increases but some of those water vapour molecules will hit the water surface and become part of water liquid. Even
Cites user249451 · 2019-12 · Physics Stack Exchange · on “Why does pressure vary with depth in a fluid at microscopic level?”
Before asking this question I searched for the answer on the web (in particular on Physics.SE) and here are some that I found: In an incompressible Ideal fluid, can the pressure increase with depth? [LINK] Why do water molecule apply more pressure (i.e., more force) at depth? [LINK] ..., etc. But the answer there seem to not answer the question properly. In particular everyone tried to explain it with the fact that for a fluid to be in static equilibrium the pressure at a depth must counter the weight of the water above it and hence is equal to $\rho g h$ . Well that's simple and anyone can get the macroscopic picture but what about the microscopic one? In particular the answer here [LINK] u
Cites Whit3rd · 2025-11 · Physics Stack Exchange · on “What would happen to a soap bubble if there were no surface tension?”
A surface tension disappearance IS experimentally achievable, and we know, more or less, what happens. The critical point behavior of a gas/liquid interface, is an amusing disappearing act [this video] : one moment there's a bit of liquid carbon dioxide, then it all turns cloudy... But that is all one material, the pool of liquid under a pocket of its own vapor. In the case of a bubble, that's a bit of (lightly) compressed gas of composition A inside a film of composition B, turning into a gas mixture of parts A and B. The compression goes away when the bubble does, so that'll generate a gentle 'pop'sound.