Does such a pair of substances exist, that in certain physical conditions (temperature, pressure) when both are placed in the same conditions, one will be a liquid, the other - a gas, and the gas density will be higher than that of the liquid?

(let us exclude special states of matter - e.g. don't count superfluid as liquid, or plasma as gas.)

  • $\begingroup$ Maybe there is some really light "solid" material which is a supercooled liquid, like glass but much more lighter. Of course you won't get a thermodynamically equllibrium state, but mechanical equllibrium might be possible. $\endgroup$
    – Yrogirg
    Nov 26 '12 at 14:58
  • $\begingroup$ I wonder if the liquid and gas were both in a high pressure environment whether the gas density could exceed the liquid's, since liquid densities aren't much affected by pressure? $\endgroup$ Nov 26 '12 at 15:56
  • $\begingroup$ @MichaelLuciuk but at high pressures most gases would become liquid or solid. Even helium is solid at 25Bar $\endgroup$ Nov 26 '12 at 17:04
  • $\begingroup$ @Yrogirg - it's comparatively easy to make a solid almost arbitrarily light. If you make soemthing with lots of internal spaces and don't include the mass of the gas filling the spaces. eg. aerogel $\endgroup$ Nov 26 '12 at 17:05
  • $\begingroup$ @MartinBeckett I thought of this, but such materials would be better qualified as foams, not as liquids. I was looking for the lightest paraffin, but they are still quite heavy. $\endgroup$
    – Yrogirg
    Nov 27 '12 at 4:30

I've just remembered that there was once a suggestion to use a mixture of xenon and oxygen under high pressure to allow people to float/fly/swim in it. It was also stated that water could be lighter than such a mixture.

According to Smithsonian Physical Tables the critical point for xenon is

$16.6\,\text{C}^{\circ},\quad 60\,\frac{\text{kg}}{\text{cm}^2},\quad 1.155\,\frac{\text{g}}{\text{cm}^3}$

while the density of water is around this conditions according to this online calculator is $1.0015 \frac{\text{g}}{\text{cm}^3}$ and it is still liquid ($60\,\frac{\text{kg}}{\text{cm}^2} \approx 58.84\,\text{bar}$).

One may have a look on phase diagrams of water and xenon on wolfram alpha:

I suppose by lowering a bit the pressure under xenon critical point one can actually have a gas heavier than even water.

The original suggestion on high density xenon-oxygen breathable mixture should be in Ariadne section of New scientist, July 6, 1967. However this issue is not on Google books (yet?).

  • 7
    $\begingroup$ This doesnot work. The xenon dissolves into the water and freezes it. Proof : youtube.com/watch?v=AsP4yMY-a6U $\endgroup$
    – Tejas Kale
    Jul 16 '17 at 6:29
  • 2
    $\begingroup$ 75% ethanol (25% water) doesn't let xenon dissolve and floats on top of xenon near its critical point. $\endgroup$
    – SF.
    Jan 15 '18 at 3:17

At its critical point of 318 K and 3.8 bar, SF$_6$ has a density of 742 kg/m$^3$. This is certainly denser than some liquids. Of course we'd have to go for a slightly lower pressure to have an unambiguous gas.

Heptane is liquid at this temperature and pressure and has a density (at STP) of 697 kg/m$^3$. This looks good but SF$_6$ is likely to be soluble in heptane given that it's "very soluble in ethanol, hexane, benzene" (wikipedia). Isopentane is lighter still but has problems with the boiling point and solubility. So we have to look for something more exotic.

I can't find a phase diagram but pentaborane has a boiling point of 333 K and a density (again at STP) of 618 kg/m$^3$, meaning it will be liquid at the critical point of SF$_6$.

I wouldn't fancy trying the experiment though: Pentaborane was found to be too dangerous to use as a rocket fuel, and it took decades to figure out a safe way to dispose of it.


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