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let’s take an example: if we have a glove-box filled with helium at 5 bars at room temperature which is far beyond its critical point, what actually it would be like ? would it be like a normal gas ? would it be like a liquid ? or it will be like a condensed vapors ? can this supercritical fluid dissolve salts for example ? would it quench hot metals like water ? if I put may hands inside this fluid would it feels like putting my hands inside a water ? or it feel like a normal air ?

is supercritical fluid a special types of a liquid ? if no, why then some SCFs used as a solvent ?

I searched a lot but really can’t find any clear answer explains what they ar exactly.

Note: this is not duplicate, I asking about the nature of SCFs while the "duplicated" question was asking about room temperature SCFs which is clearly different.

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    $\begingroup$ Just to make it clearer: are you familiar with "phase diagrams" or not at all? $\endgroup$ – SteffX Feb 22 '19 at 16:29
  • $\begingroup$ If you search supercritical carbon dioxide on Youtube, you will see very nice videos of supercritical carbon dioxide in transparent cells. I work with supercritical CO2 for some purposes but never saw it in real. $\endgroup$ – M. Farooq Feb 22 '19 at 17:23
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    $\begingroup$ 5 bars of He at room temperature is a gas. Not even close to a critical point. The question is unclear. $\endgroup$ – Jon Custer Feb 22 '19 at 18:00
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    $\begingroup$ my question is not duplicate !! I see that video about supercritical CO2 and it seems like a free moving liquid which make me confused. by definition critical temperature is the temperature at which gas can not liquefied no matter what how much pressure is applied , so 5 bars at room temperature exceeding critical point . $\endgroup$ – Johnny Feb 22 '19 at 20:35
  • $\begingroup$ @JonCuster Technically it's supercritical, because He has low crit. pressure, but yeah, in practice it's just a gas. $\endgroup$ – Mithoron Feb 23 '19 at 0:10
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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 https://www.youtube.com/watch?v=GEr3NxsPTOA), 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 supercritical state. It is just like thinking what does a plasma look like? It very much appears a like an ordinary flame which is extremely hot.

Response to the comment "is supercritical fluid a special types of a liquid ? if no, why then some SCFs used as a solvent ? what if a small pot of liquid heated inside a large pressure chamber to its critical point? would it expanding fill the whole chamber like a gas or stay in its container like a liquid ?"

I am afraid you are mixing several concepts here. The key question is, how does one distinguish a liquid phase and a gas phase? Supercritical fluid is not a liquid because its density and viscosity are not close to liquids, nor it has a definite boiling or melting point (a liquid does, right?). Think of supercritical fluid as a state or condition of a gas where it is above its critical temperature and pressure. That is all. We cannot say supercritical carbon dioxide has a boiling point of x Celsius or a freezing point of y Celsius.

Coming to your second part on SFC extraction: First, let us talk about gases. Can the gases act as a solvent or carrier of molecules? The answer is yes, and this is the very basis of gas chromatography. How do you detect odors? The gas molecules are carrying the odor causing molecules from a far away place to your nose i.e., gases also have some capability of "dissolving" substances. In the same vein, if I use a temperature of Tc, and a pressure Pc for carbon dioxide (or any gas), we can use it for extraction. It just turns out that supercritical carbon dioxide has higher dissolving power than gaseous carbon dioxide and certainly less than liquid carbon dioxide.

Third point: Let us do an imaginary experiment: If you have a sealed container of fixed volume containing nothing but carbon dioxide at very low temperature. Also assume a pressure gauge is also attached. You have filled in enough carbon dioxide in the chamber that the gauge shows a value of 85 atm. Now you start heating the chamber until the temperature of the chamber reaches 35 Celsius. Imagine you can see the insides of that chamber with a camera: You will see nothing because there will be no liquid in that chamber. It will look empty to your eyes. Given the fact you know that an empty glass is not empty, it is filled with gases (nitrogen, oxygen etc). In the same way, our chamber is also filled with a carbon dioxide which our eyes cannot see. We will still call this carbon dioxide as supercritical carbon dioxide because we know from our experimental settings that we are above critical temperature and pressure of carbon dioxide. You can go higher temperature to 45 Celsius, still you are in supercritical state. Thus in a phase diagram, there is a supercritical region.

Don't let the word fluid confuse you. Since we cannot call a gas above its critical temperature and pressure a typical gas, nor we can call it a liquid, it is best to call it a fluid. Hope that clarifies your confusion.

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  • $\begingroup$ that video is actually what make me confused. when CO2 reaching critical point it appears like an expanding liquid or a condesed vapor which rise a question : is supercritical fluid a special types of a liquid ? if no, why then some SCFs used as a solvent ? what if a small pot of liquid heated inside a large pressure chamber to its critical point? would it expanding fill the whole chamber like a gas or stay in its container like a liquid ? $\endgroup$ – Johnny Feb 23 '19 at 2:15
  • $\begingroup$ See the edited answer above. $\endgroup$ – M. Farooq Feb 23 '19 at 3:18
  • $\begingroup$ thank you for explanation. but it is not yet clearly understanded. when I watched this video:youtube.com/watch?v=QHcqyFm0i9M , the supercritical CO2 seems like an expanded liquid or a liquid that act like a gas . it appears like any liquid !! I know that SCF is not a true liquid and it is a fluid combining liquid and gas properties but the video show it is a liquid-like fluid which expanding like gases. $\endgroup$ – Johnny Feb 24 '19 at 5:19
  • $\begingroup$ Could you mention what grade you are in-this will help in explaining? At 2:35 min of the video, this is the supercritical form of CO2. Let me ask a question. Why do you see a glass filled with water? The reason is refractive index RI of water is different from glass and air. You are able to see liquid CO2 b/c its RI is different from the container walls. Now they heat it up. I can see some ripples after 2:35 min of the video, this is due to refractive index changes in the supercritical state. Have you seen stars twinkling? This is also due to refractive index changes of the air in the sky. $\endgroup$ – M. Farooq Feb 24 '19 at 6:03
  • $\begingroup$ I studying pharmaceuticals.I did not know a lot about refractive index but I think if the supercritical CO2 differ from normal CO2 it should also differ in refractive index . I think also that filling glass with a the fluid after transition is not related to refractive index . CO2 expanded as its density decreased by increasing temperature, until it filled the glass. I did’nt say supercritical fluid is a liquid, but the video shows that it appears like a liquid sharing some properties of gases "i.e. expanding" . $\endgroup$ – Johnny Feb 25 '19 at 6:54

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