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Inspired by Is there any type of liquid, other than mercury, that PTFE teflon is known to "float" to the surface in?, are there gases that are more dense than liquids? (So bubbles would sink down instead of bubbling up?)

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    $\begingroup$ The critical point of Xenon is at a temperature of 16,6 °C and a pressure of 5,84 MPa (~ 58 bar) with a density of 1.1 $\mathrm{g\, cm^{-3}}$. Water is liquid under this conditions. I guess that water drops would float up in gaseous Xenon slightly below the critical point ($p < p_{crit}$, $T < T_{crit}$). $\endgroup$ – aventurin Feb 10 '16 at 18:21
  • $\begingroup$ @aventurin thanks, this gives rise to another question, but that would be physics: would liquid droplets form and raise, or gas bubbles form and sink? $\endgroup$ – Gyro Gearloose Feb 10 '16 at 18:26
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    $\begingroup$ What about sulphur hexafluoride? It can allow a solid boat to float on it, but I'm not sure about how a liquid would work with it. Also, Just shake up some oil and water to see how the two separate. My guess would be that the gas separating from the water would do the same. $\endgroup$ – N A Feb 11 '16 at 1:48
  • $\begingroup$ @NA A boat can float on SF6 because the boat shape holds air. A solid block of the same material would sink. At standard temperature and pressure, there is no known liquid less dense than SF6 or even WF6. $\endgroup$ – f'' Aug 20 '16 at 6:46
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    $\begingroup$ @aventurin Amazingly, YouTube user Cody Don Reeder (of Cody's Lab fame) managed to implement this after some trial and error. Watch this video to see liquids floating in gaseous xenon at room temperature. Mind blown! $\endgroup$ – Nicolau Saker Neto Jul 16 '17 at 5:19
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It depends on the conditions. Let's start decomposing your question in two related questions:

Denser gasses at SATP?

There are indeed some gasses that are quite dense. Sulfur hexafluoride has a density of 6.17 g/L while tungsten hexafluoride of 12.4 g/L. But usually, they are not so dense compared to the density of liquids.

Lightest liquid at SATP?

The density of liquids largely depends on the atomic mass of the compounds. Hydrogen that is the compound with the lowest atomic mass has a density of 70.85 g/L, which is probably the lowest density you can find.

So at room temperature is not possible but if you increase the pressure the density of the gasses will increase while the density of the liquid won't increase much because liquids are not appreciably compressible. It is hence theoretically possible to achieve a gas with a greater density compared to that one of a liquid, and also the coexistence of the two phases should be possible in certain conditions but the pressure required won't allow you to observe it.

Bubbles sinking down?

That won't happen also if there was a denser gas compared to a liquid. Bubble formation is quite a different process in this case inverting the gravity would be better! ;-)

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