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This question left me wondering what other very light (condensed) fluids could exist, at any environmental condition.

Obvious candidates for the list are (at their Bp./1 bar)

  • LH2 (0.07 g/ml)
  • LHe (0.125)
  • LNe (nope, 1.21)
  • methane (0.42)
  • ethane (0.54)
  • propane (0.58)

plus those mentioned in the other question

  • $\ce{4NH3 \cdot Li}$ (0.48)
  • isopentane (lightest "normal" liquid at 25°C, 0.62 g/ml)

To give a definit boundary on possible answers, let´s say the density must be below that of isopentane.

The question is, is there anything else? Perhaps at very high temperatures?

(The term "fluids" of course also includes supercritical fluids and gases, those are obviously out. You can always reduce the pressure, and thereby density, on those, until you either have a regular liquid, or you hit the bottom line of your phase diagram at $p=0$. That makes no sense, so I want actual condensed matter.)

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    $\begingroup$ What are LNe, LH2,LNe? Searching on google yields the value of $ \ln e$. Do you mean liquid Ne, $\ce{H2}$, He? $\endgroup$ Commented Aug 7, 2020 at 8:03
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    $\begingroup$ I believe that's pretty much the size of it. Maybe you want to add liquid diborane (at low T) and liquid Li (at high T), but those won't break the record. $\endgroup$ Commented Aug 7, 2020 at 8:10
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    $\begingroup$ What about the supercritical regime? The question needs to be bounded. Define "fluid". $\endgroup$
    – Buck Thorn
    Commented Aug 7, 2020 at 10:53
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    $\begingroup$ Well, the boundary between liquid and supercritical fluid is not well defined as far as I am aware, so it would appear to remain a question of semantics :-) In fact, I would be surprised if the theorem of corresponding states didn't have something universal to suggest about this. $\endgroup$
    – Buck Thorn
    Commented Aug 7, 2020 at 16:52
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    $\begingroup$ @BuckThorn If you are somewhere within the supercritical regime, you can always reduce the pressure, which will inevitably reduce the density, until you are definitely in the liquid, or the density approaches zero. The former case then is relevant for this question, the latter sort of doesn´t matter. Sorry for the pun. ;-) $\endgroup$
    – Karl
    Commented Aug 7, 2020 at 20:54

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The least dense fluid is liquid Hydrogen $\ce{H2}$, at $-255$°C and $1$ bar. Its density is $0.071$ g/mL.

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    $\begingroup$ Actually, helium-3 has a density of $\mathrm{0.059\ g/mL}$ at its normal boiling point of $\mathrm{3.19\ K}$. For comparison, in helium-4 the respective values are $\mathrm{0.1247\ g/mL}$ at $\mathrm{4.23\ K}$. $\endgroup$ Commented Jul 12, 2023 at 11:28
  • $\begingroup$ @NicolauSakerNeto No idea where I got the 0.18 for helium in the question. Tnx! $\endgroup$
    – Karl
    Commented Jul 12, 2023 at 19:27

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