3

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.)

Karl
  • 12,053
  • 2
  • 30
  • 61
  • 6
    What are LNe, LH2,LNe? Searching on google yields the value of $ \ln e$. Do you mean liquid Ne, $\ce{H2}$, He? – Safdar Faisal Aug 07 '20 at 08:03
  • 1
    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. – Ivan Neretin Aug 07 '20 at 08:10
  • @IvanNeretin Diborane would be a nice answer, it seems. I found a hardly readable scan of a NASA report that would land it in third place, but can´t get at the references. – Karl Aug 07 '20 at 09:15
  • 1
    What about the supercritical regime? The question needs to be bounded. Define "fluid". – Buck Thorn Aug 07 '20 at 10:53
  • @BuckThorn Supercritical makes no sense, obviously. Perhaps I should have said "liquid", but I didn´t want to exclude anything that´s a bit out of the box. – Karl Aug 07 '20 at 12:17
  • 2
    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. – Buck Thorn Aug 07 '20 at 16:52
  • According to Physics Reviews (2013), the lowest density liquid in nature is He-3 adsorbed on alkali metals as a liquid layer (at least in theory) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.045303 – AChem Aug 07 '20 at 18:38
  • 1
    @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. ;-) – Karl Aug 07 '20 at 20:54
  • Well, these limitations were in original question to make it not overly broad. I think there may be need to make some other besides not gas or supercritical... – Mithoron Aug 07 '20 at 21:11
  • Fluid is anything that flows, i.e. liquids, supercritical fluids, gases and plasma. The least dense fluid, then would be lone atoms (or ions), in intergalactic space. – DrMoishe Pippik Aug 07 '20 at 21:12
  • 1
    Some alkylboranes are light, around ~0.7 . But some are not, closer to 1.4-1.6 – permeakra Aug 08 '20 at 10:45
  • @permeakra Trimethylborane would be a good candidate, en:wp gives it 0.63 at -100°C, and it only boils at -20. The adduct with trimethylamine might be another idea. I was thinking about $\ce{BeH2}$, but wasn´t able to find if it would melt without decomposing at higher pressure. – Karl Aug 08 '20 at 11:23
  • @Karl Triethylborane is 0.68 with bp +95 mp -93. Not isopentane, but close enough in my opinion and a nicer liquid phase range – permeakra Aug 08 '20 at 19:24
  • Okay, I need some clarification on this.
    1. Do you mean liquid (like water or oil) or fluid (basically anything that flows, including gas and plasma)?
    2. Do you mean density in the term of mass of mole?

    Because @DrMoishePippik is correct, if you mean fluid, then intergalactic medium would be the least dense.

     – Tensor Jul 13 '23 at 06:23

1 Answers1

3

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

Maurice
  • 28,241
  • 3
  • 29
  • 61
  • 3
    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 $\mathrm{4.23\ K}$. – Nicolau Saker Neto Jul 12 '23 at 11:28
  • @NicolauSakerNeto No idea where I got the 0.18 for helium in the question. Tnx! – Karl Jul 12 '23 at 19:27