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How does a supercritical fluid look like? Like a gas or like a liquid?

  1. Mar 15, 2012 #1
    Hello Forum,
    I have been studying phase diagrams. If a pure substance is brought to a temperature and pressure above the critical T and P, it becomes a supercritical fluid, i.e. a substance having properties of liquids and of gases...

    But how does it look to us? More like a liquid or like a gas? I have read of different opinions....

    Supercritical CO2 is used quite often commercially,
  2. jcsd
  3. Mar 18, 2012 #2
    You should possibly look at it from a different angle: Consider a fluid with a fixed number of molecules at different temperatures T and pressures p (i.e. the phase diagram you already spoke of). Such fluids can have the remarkable property that at certain regions in the (T,p) plane, a small change in either parameter can result in a drastic change in density. This rapid change is called a "first order phase transition" between a "liquid" (which labels the states with the high density) and a gas (low density). The difference in density between liquid and gas becomes smaller as the critical point is approached. At the critical point, the two densities become equal and the phase transition ends. As "liquid" and "gas" refer to the two sides of the phase transition line, no notion of these two states can exist, then. This region where the notion of liquid and gas vanishes is (losely) referred to as the "supercritical region".

    So let me draw a few conclusions from this small introduction, and hopefully answer your question:
    • If one identifies two points of the phase diagram as "liquid" and "gas", respectively, then a continuous path in the (T,p) plane from liquid to gas exists that does not cross a phase transition. Hence, liquid and gas phase should not be considered as fundamentally distinct, but as effective concepts that help speaking about the jump in density that exists in the phase diagram.
    • There is (to my knowledge) no generally agreed upon definition of "supercritical region". There is no phase transition from a liquid or gas to a supercritical phase. One may define "supercritical phase" as the states with T greater than the critical temperature and/or p larger than the critical pressure. But that is merely a book-keeping definition with no physical significance.
    • So what does a supercritical state look like, then? If you followed my extensive explanation so far, then you hopefully agree that liquid and gas are not that different but merely characterize the high-density and the low-density side of the density jump that exists in the phase diagram. Or more towards your question: A liquid and a gas look identical, and merely differ in density (by a possibly huge factor if the pressure fixed to standard pressure). And a supercritical state ... also look the same, except that the density always is a continuous function of T and p. For actual envisioning it, I would think of a transition from liquid to gas that has been streched out over a range of temperatures (or pressures).
    • My explanation so far has been rather abstract and based on thermodynamics. Actual seeing with the eyes of course is much more complicated, and involves the interacion of the system with light of different frequencies. I completely omitted this part, since 1) your question started off from thermodynamics, and 2) I simply can't say much about that.

    Let me add final comment that the supercritical state of a fluid will most likely not resemble the state near the critical point that one often sees in images (-> critical opalescence).
  4. Mar 20, 2012 #3
    Hi Timo,
    thanks for the great reply. Thing are much clear now...
    But I am still wondering about the appearance of this supercritical fluid.
    For instance, if we put some supercritical CO2 in a container, some say it will look like a liquid because of its high density but it will have the low viscosity of a gas. Supercritical CO2 is used for decaffeination.

    I understand that between a liquid and a gas there is only a density difference. But a gas occupies a finite volume in a different way than a gas. The gas looks "gaseous". Water and waver vapour don't look the same and also have different structures. We can all tell if a substance is in its gaseous phase or liquid phase

    Unrelated question: what is fog according to you perspective? Is it pratically equivalent to rain but with drops of much smaller size? However, the fog droplets are much more numerous than rain droplets, per cubic meter. Why?
  5. Mar 20, 2012 #4
    Hi fisico,

    glad you liked my reply, even though it may have appeared a bit exotic. Several comments of mine on your previous post:

    - I think that supercritical CO2 with a density like liquid will look like liquid while supercritical CO2 with a density of gas will look like gas. "Supercritical" does not -to my knowledge- constrain the densities. In fact, I don't even think that the term has a very precise meaning (as mentioned in my previous post). If the CO2 used for decaffeination happens to have the density of liquid water (at standard pressure), then it may well look like it. If you are really interested in this particular process, then perhaps you should see if you can find the actual density of the CO2 being used. Since Wikipedia sais something about "keeping the favorable density of liquid", I guess it will indeed have a typical liquid density.

    - High densities and low densities look different, of course. In first approximation, a gas looks more tranparent since it absorbs less light. I don't see how liquid and gas have different structures (except for the different density, of course).

    - It is well possible that other properties also undergo a huge change at the liquid-gas phase transition alongside with the density (those that are directly related to density, for example). However, the same reasoning why liquid and gas are not that different that applied to density also applies to them. If it didn't, the phase transition would not end in a critical point.

    - Note that what people usually conceive as "water vapor" is actually "steam", i.e. water vapor that has risen from your cook-pot and condensed into tiny droplets of liquid water.

    - Fog is small droplets of liquid. Structurally the same as steam, just not as hot. Raindrops are just larger droplets (and in particular sufficiently large such that gravity becomes the dominant force that acts on them).

    - The similarity of liquid and gas is somewhat abstract and fundamental and does not play out well in everyday experience. There, pressures are usually constrained withing a small interval around standard pressure, and it is highly unlikely for a random substance to have its critical point within this pressure interval. The critical point of water (the most important liquid) is at 200 times the standard pressure. So in practice, you can can think of liquid water and gaseous water as being fundamentally different in most cases.
  6. Mar 20, 2012 #5


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    Air at ambient temperature is supercritical. So you should have quite an idea how a supercritical gas/liquid looks like.
  7. Mar 20, 2012 #6
    Thanks everyone.
    I am getting there.

    So, the difference between gas, water vapor, and steam is the following:

    in this case the substance is water (H2O). Steam is just a technical name of water vapour that has a high temperature. Both represent water molecules diffused in space.

    A vapour is different from a gas: the term vapour describes the state of a substance when it's gaseous phase is in equilibrium with it's liquid or solid phases, below it's boiling point....
    But vapour and gas seem to have the same structure...

  8. Mar 21, 2012 #7
    On my chemistry book I read that if a gas is at its critical temperature but at a pressure below the critical pressure, no matter how high the pressure, it will not liquefy.
    Only if the pressure is equal or larger than the critical pressure the gas will pass from gas state to liquid state.....

    also, I read that a supercritical fluid will fill a closed, finite size container like a gas would, but it would have a viscosity close the one liquids have....

  9. Mar 21, 2012 #8


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    A liquid will also fill a finite size container, until its pressure gets to low and vapour forms. Just like a gas will fill a container until its pressure gets too high and liquid will condense. There is reallz no difference between the two phases in that respect (other than the liquid, having the higher density, will accumulate at the bottom and the gas at the top).
  10. Mar 22, 2012 #9
    Hello DrDu,

    you mention "A liquid will also fill a finite size container, until its pressure gets to low and vapour forms."

    Can you give me an example? 2 Liters of water in a 4 liter bottle don' t fill the whole bottle....
    A Gas would instead occupy the whole 4 liters....

  11. Mar 22, 2012 #10


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    No, only a supercritical gas will fill the whole bottle irrespective of its pressure. A sub-critical gas will fill the whole bottle until its pressure becomes bigger than the vapour pressure at the given temperature. Then it begins to condense and it will only fill that part of the bottle which is not filled by the liquid.
    Do you know Torricellis famous experiment?
    He filled a glas tube (closed at one end) with liquid mercury, the lower end of the tube sticking inside a bath of mercury. When he erected the tube, the mercury filled the whole tube until the pressure at the upper end became equal to the vapour pressure of the mercury. Then the upper end of the tube filled with mercure vapour.
    So in the pressure range below the vapour pressure, a container is completely filled with gas, while above the vapour pressure, it is filled completely with liquid.
    At the vapour pressure, in general it will contain both gas and liquid and neither of the two fills the whole container. So there is no difference between liquids and gasses in the way how they fill containers.
  12. Mar 22, 2012 #11
    I know this is a basic question but let me ask it anyway. I think there are some flaws in my thinking: vapour pressure.

    To me, vapour pressure is that pressure that the molecules of the substance exert of the walls on a closed contained where both the gas phase and the liquid phase are in dynamic equilibrium... is that correct?

    A liquid start boiling when the vapour pressure is equal to the external (1 atm) pressure...Ok, but I thought that we could talk about vapour pressure outside the liquid and not inside it, which is where the air bubbles form....

  13. Mar 23, 2012 #12


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    The liquid will also excert a pressure on the container and on the gas above it and in equilibrium this pressure has to be equal to the gas pressure. If not, the liquid will move (it it is free to) or evaporate.
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