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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.)

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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. – Yrogirg Nov 26 '12 at 14:58
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? – Michael Luciuk Nov 26 '12 at 15:56
@MichaelLuciuk but at high pressures most gases would become liquid or solid. Even helium is solid at 25Bar – Martin Beckett Nov 26 '12 at 17:04
@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 – Martin Beckett Nov 26 '12 at 17:05
@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. – Yrogirg Nov 27 '12 at 4:30
up vote 8 down vote accepted

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?).

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