supercritical fluids
Why Chemistry Stack Exchange recommends it
Chemistry Stack Exchange answerers point people to it as a visual of the liquid-gas boundary disappearing at the critical point, to show what a supercritical fluid actually looks like.
Recommended by 2 people across 2 different months, 2014 to 2019.
What it is
A short 2011 demonstration video from the Flachzange1337 channel in which liquid CO2 is heated in a sealed pressure cell. The viewer watches the liquid-gas interface fade as the critical point is reached and the CO2 becomes a supercritical fluid.
What people said
Recommends Gimelist · 2014-12 · Chemistry Stack Exchange · on “Does the density of a liquid and gas phase of a substance(nitrogen or water) converge when reaching critical point”
Does the density of a liquid and gas phase of a substance converge when reaching critical point? Yes. This is exactly what happens. Let's take a look at a qualitative $P-T$ (pressure–temperature) phase diagram of an arbitrary substance: source: Cansell, F. et al. Supercritical fluid processing: a new route for materials synthesis. J. Mater. Chem . 9, 67–75 (1999). [LINK] As you can see, moving directly from gas to liquid involves crossing the phase boundary. However, if you're doing it in the supercritical, the change in density is continuous. Maybe you can get a better, more quantitative view with this example of $\ce{CO2}$: source: [LINK] This one is also a a $P-T$ phase diagram, with equa
Recommends Karsten · 2019-09 · Chemistry Stack Exchange · on “What happens when a super-critical fluid is compressed?”
When you heat up a liquid at constant volume (leaving sufficient space for the gas phase), the density of the liquid will decrease and the intermolecular interactions will weaken. Some of the liquid will transition to vapor, so the vapor above the liquid will get denser and the frequency of intermolecular interactions will increase (it behaves less and less as an ideal gas as the number of collisions increases with increasing density). At the point where the density of the liquid and the gas are the same, there is no more phase boundary (with gravity, we picture liquid on the bottom and gas on the top, but if the density is identical, that is no longer the case). This is all pretty outlandis
Cites ACR · 2019-02 · Chemistry Stack Exchange · on “what is actually supercritical fluid?”
The name supercritical fluid is fancy but don't be confused by thinking that it this would a very odd looking state of material. Supercritical CO2 is just a state of CO2 at a certain T and P , so it looks very much like gaseous CO2 i.e., it is a transparent clear "fluid" whose density and viscosity is higher than a gas but lesser than that of the liquid. If you are able to see an interface in those transparent cells (e.g. here [this video] ), it means that the liquid phase and the gas phases exist in the system. We are able to see the interface (boundary) when the refractive index is different from the container walls, and the surroundings. In short do not expect a new form of matter in supercrit