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I believe that at STP, there are no pairings of a liquid and a gas where the liquid will float atop the gas. Even considering very high-density gases, such as tungsten hexafluoride (12.4g/L), and very low-density liquids such as isopentane (616g/L), this seems clearly impossible at standard temperature and pressure.

However, I'm curious if there are any well-known (or maybe not-so-well-known) pairings at any temperature and pressure where this can occur.

Liquid hydrogen has extremely low density (70.99g/L) for a liquid, but tungsten hexaflouride is easily a solid long before hydrogen is a liquid (at least at standard pressure) and even so is still significantly less dense.

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    $\begingroup$ Do clouds count? Little water droplets floating in air? $\endgroup$
    – mmesser314
    Commented Aug 22, 2020 at 4:29
  • $\begingroup$ @mmesser314 hah well - not really what I was thinking but that is an accurate description of clouds. $\endgroup$
    – Jesse Williams
    Commented Aug 22, 2020 at 12:28

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At or near STP, most gases behave as near-ideal and their density is inversely proportional to the temperature.

OK, we need something with a low boiling temperature and a high molar mass for the gas part of the question.

WF6 is a good start at 300K (its molar mass being ~300).

Increase the temperature to 400K and you'l have to find something that is gas at 400K and have molar mass ~400 or more in order to have the same density as WF6 at room temperature. And it also has to be stable at 400K.

And it is still way off the lightest liquid at any temperature.

Going down - there are few gases at 200K.

We look here: enter image description here and see quite a solid limit of boiling temperatures vs molar mass for non-polar molecules. And its slope (2.5K/Da) can be converted directly to some 5g/l extrapolation for higher temperatures for any non-polar gas near (but above) its boiling point.

That gets us to the conclusion that we won't get much better than WF6 at any temperature.

Polar substances boil higher and have less dense vapors so they are not useful.

You are right.

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Maybe "liquid" He could float above "gaseous" tritium? I couldn't find enough data to tell.

As a gas, He is monatomic. $^3$He has a mass of 3 amu, and $^4$He has a mass of 4 amu.

As a gas, H is diatomic. Molecules of $^1$H$_2$ (ordinary H), $^2$H$_2$ (deuterium), $^3$H$_2$ (tritium) have masses of 2, 4, and 6 amu.

Deuterium and tritium are rare, but in theory you could make molecules of H$_2$ that are more dense than He as gasses.

The critical point of He is $5.2$ K. The critical point of H$_2$ is $33$ K. Above these temperatures, there is no sharp boundary between liquid and gas. At low pressure, they are gas. When compressed, they get denser and denser until they are something more like a liquid.

At pressures where they are liquid like, they are clearly not ideal gasses. I don't know how compressible they are. Perhaps there is a temperature and pressure where He is liquid like, $^3$H$_2$ is gaseous, and He is still less dense.

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At room temperature and pressure, no, but at room temperature and high pressure, liquid ethanol will float on gaseous xenon, as demonstrated by Cody Don Reader in his video Can You Float a Liquid on a Gas?.

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  • $\begingroup$ That's an excellent answer - thank you! $\endgroup$
    – Jesse Williams
    Commented Jan 11 at 17:53

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