Chemistry - Can gases be "immiscible"?
Solution 1:
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.
Solution 2:
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!
Solution 3:
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.
Solution 4:
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!