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Anyone know of any examples of gas mixtures that phase separate, analogously to the way immiscible liquids do?

They would have to be extremely non-ideal. I might guess one might have to be much higher molecular weight than the other and at least one if not both would have to have significant intermolecular interactions (although of different kinds).

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    $\begingroup$ Can an "extremely non-ideal gas" still be called a gas? One could argue that a liquid is a type of extremely non-ideal gas. Depending on how much you relax that condition, something might eventually work, but it might not resemble a gas much by then. $\endgroup$ – Nicolau Saker Neto Jan 1 '16 at 23:32
  • $\begingroup$ I'd guess you could get phase separation as you approach the critical point of one (or both) of the fluids as you suggest. This is obviously a bit of a fluffy question but I'm curious about things that a kid would identify as a gas. $\endgroup$ – ericksonla Jan 1 '16 at 23:42
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    $\begingroup$ That's about what I thought you were trying to go for with your question. Unfortunately in that case I'm pretty sure the answer is that there is no simple way for gasses to spontaneously phase separate, so you'll probably have to bring in some rather heavy considerations (e.g. a gas of photons, a plasma, some external potential, intrinsically quantum mechanical effects, etc) which aren't exactly intuitive. $\endgroup$ – Nicolau Saker Neto Jan 2 '16 at 0:02
  • $\begingroup$ Side question, you assumed that the molecular weight would have to be imbalanced between the gases. If I was looking at liquids to guess if the were miscible, I would be looking at the surface tension. Are there 2 liquids with similar surface tensions that will not mix? Are there liquids with similar weights that do mix, don't? Ahh, whoops, excellent question mate. +1 $\endgroup$ – DigitalDesignDj May 4 '16 at 2:43
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    $\begingroup$ Can liquids be immiscible? Sure they can. Now what is the difference between a gas and a liquid, if you can smoothly walk from one to another without any transition - that is, if you walk around the critical point? $\endgroup$ – Ivan Neretin Nov 23 '17 at 7:14
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This question provides an interesting thought experiment. The rub comes with the "analogously" part of "...analogously to the way immiscible liquids do."

You could ask, for example, are dogs analogous to jellyfish...but the answer would be "yes and no."

Gases do "settle out" on planetary scales. Helium can (and will) escape the earth's atmosphere. So if you made a bottle big enough to hold the earth inside, the helium would be the "top layer."

However, when we think of the miscibility of liquids, we are often thinking about the interactions between molecules that force congregation or separation (polar non-polar interactions, for example) and not gravity related thermodynamic-style considerations. If you compress a gas out of a mixture of gases, then you made a liquid, or tricked a gas into separating using Graham's law.

If we mean, miscibility as the property of substances to mix in all proportions. Then yes, we can mix gases in a bottle at standard state. Dalton's law would hold.

If we mean miscibility as a strict homogeneity only seen under conditions of rest then nothing is absolutely miscible, really.

But...I love your question and it got me researching some fun articles.

Check out atmospheric concentration and vertical structure, it is fascinating! http://ruc.noaa.gov/AMB_Publications_bj/2009%20Schlatter_Atmospheric%20Composition%20and%20Vertical%20Structure_eae319MS-1.pdf

It might also interest you to look up some articles on mixture settling. http://webserver.dmt.upm.es/~isidoro/bk3/c07/Mixture%20settling.pdf

Hope this is useful to you!

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    $\begingroup$ Random baloney, this. Gases mix perfectly, under practically all conditions except in ultracentrifuges and >100km. As do miscible fluids. $\endgroup$ – Karl Apr 4 '16 at 19:56
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Good question but there is no such thing as immiscible gases. Immiscibility is caused by surface energy/tension. Gases do not have surfaces and therefore lack surface tension.

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The composition of a mixture of gases is due to gravity, which separates, competing with thermal motion which mixes. Gases are totally miscible but that does not mean that the concentration has to be uniform throughout a mixture. Both gases have a lower concentration lower down, but the gradient is less for the lighter gas, so hydrogen would be almost uniformly distributed in a room, but chlorine will be far more concentrated near to the floor but don't try hydrogen and chlorine as they explode in strong light!

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Yes. Not at standard conditions, but at elevated temperatures and pressures there can be gas-gas immiscibility. See Liquids and Liquid Mixtures by Rowlinson for more details. Some examples include argon+ammonia, methane+ammonia, ethane+water, carbon dioxide+water, and benzene+water.

At the pressures where these occur the densities are comparable to liquids, but the temperature is above the critical temperature of both components, so gas-gas equilibrium is an appropriate term.

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This answer is not completely my own and uses some ideas from Mcruggs' answer. I think that at 21°C and 1 atmosphere, the answer is no but it might be possible when either the pressure is 800 atmospheres or the temperature is 3 K. Even if a gas at this temperature and pressure is exactly at its boiling point, its atoms will still be widely spaced apart because according to Nelson chemistry 11, its volume for a given amount and pressure varies linearly with its Kelvin temperature and not with the amount above its boiling point. You can have 2 gases that can't mix in every proportion because they react with each other on mixing but no two gases have a tendency to have a sharp surface between them and be in solubility equilibrium with each other. Since the molecules are so widely spaced apart, their attractions to each other are negligible so when 2 gases that don't react with each other are put together a container, the tendency for molecules to diffuse greatly exceeds the tendency for them to them to molecules to attract molecules of the same type even if the gases are the polar HCl and the nonpolar He gas. According to Bubble's answer at https://physics.stackexchange.com/questions/76842/the-statistical-nature-of-the-2nd-law-of-thermodynamics, the second law of thermodynamics is probably not an absolute law. The statement that all gases that don't react with each other or condense upon mixing in a closed container at this temperature will almost evenly mix which can be deduced from the second law of thermodynamics turns out to be true anyway. From the presence of gravity, the proportion of each gas might vary continuously with height at a really low rate. If you start with a heterogeneous mixture of gases of different density, the denser gas might sink to the bottom in the near future but they're eventually uniformly diffuse into each other just like in my video at https://www.youtube.com/watch?v=ay0mJHPvj88, tea can sink to the bottom of a cup of hot water after a short time but will eventually uniformly diffuse into the water.

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