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As far as I know there are no two gasses that don't mix (excluding demixing by gravitational effects). For me, as someone working with fluids and surface tensions a lot, this means that the surface tension between the gasses is small or even non-existent.

I saw in this post that mixing will be governed by the Gibbs free energy: $\Delta G = \Delta H - T\Delta S$ and that the only thing stopping fluids from mixing would be the enthalpy term which arises from repulsion.

Is it correct to think that the enthalpy will always be small in gasses, because of their low density and thus low interaction? And that this is the cause that gasses will always mix?

(just a footnote: I am talking about 'everyday' gasses here, not gasses compressed at thousands of bars with densities close to liquids)

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I agree with John Rennie but I would phrase it differently: gases are defined as those fluids that have negligible interactions between the individual molecules. So as long as the thing deserves to be called a gas, the molecules fly pretty much freely through space and may be combined simply by adding the partial pressures. There's nothing that stops one gas compound from spreading to the whole vessel even if the vessel is filled by other gases. When the obstructions are significant, the stuff is a liquid. – Luboš Motl Feb 27 '13 at 7:20
The division of fluids to liquids and gases is gradual and is governed exactly by criteria like yours: when the surface tension and similar things is negligible, it's gases. When it's not and when the interactions between the molecules become significant, it's a liquid. – Luboš Motl Feb 27 '13 at 7:21
Surface tension is exactly zero at the critical point. Above the critical point one could define a negative surface tension. If one has seen the critical opalescense (due to immense density fluctuations) in experiment, one knows that for the rest of life :=) – Georg Feb 27 '13 at 19:07
up vote 2 down vote accepted

At STP most gases are nearly ideal, that is the interactions between molecules can be ignored and the gas molecules treated as non-interacting points. This means the enthalpy of mixing is negligable and the mixing is dominated by the entropy.

Obviously this is only true far from a phase transition, and small effects such as the Joule Thompson effect can be measured even at modest pressures.

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Do non-interacting points collide with one another? – Asphir Dom Feb 27 '13 at 11:03
No. Non-interacting means the collision cross section is zero. Interestingly collisions between e.g. air molecules are frequent enough that the mean free path at STP is well under a micron. However as long as the collision cross section is small compared to the average molecule spacing the gas behaviour remains approximately ideal. – John Rennie Feb 27 '13 at 11:07

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