h a l f b a k e r ySugar and spice and unfettered insensibility.
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The video in the link has an interesting thing at the end (at the very end) - they float some tin foil in mid-air.
An interesting art display would be to fill a glass case with different gases of different densities, and then place tin foil boats in them like in the video. Thus you could have several
layers of gases, floating large tin foil boats.
A few heavy gases are Sulfur Hexafluoride, Xenon (similar to SHF in weight), and Radon (radioactive).
Sulfur Hexafluoride
http://i-am-bored.c...k.cfm?link_id=21534
[DesertFox, Feb 07 2007]
Xenon breathers
Xenon_20Breathers
More dense gas hijinks. [bungston, Feb 08 2007]
All gases are miscible
http://www.driedger...e6_v&t/CE6_V&T.html
[MaxwellBuchanan, Feb 08 2007]
Gravity just ain't enough.
http://www.freepate...ne.com/6454837.html
Patent for using zeolites to separate SF6 from air or nitrogen. [MaxwellBuchanan, Feb 08 2007]
(?) The 50% Solution
http://www.youtube....watch?v=4Upu6P2QNaQ
They'll mix with time, which we don't have much of. [reensure, Feb 11 2007]
[link]
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So, the idea is to reproduce what's in a video? |
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Sounds a lot more inventive than what's
in the video to me. The video just
demonstrates the properties - this idea
takes full advantage of them + |
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what kind of displacement are we looking
at versus say water? could we get tin foil
boats with people in them? |
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@tcarson - with a huge huge vat, and an extremely sturdy piece of tinfoil... The problem wouldn't be supporting the weight of a person, it would be supporting the weight of a person concentrated in a single place. You can't exactly lie on a sheet of tin foil suspended in air without ripping through and falling down. |
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I once did a vaguely similar trick by filling a jug with saltwater halfway up, plain water the rest of the way, and floating an egg on the layer where they met. It seemed like half the people who saw it didn't realize there was anything odd about what they were seeing, while the other half instantly figured out what I had done. |
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So this art project may be less impressive than it first seems, although boats floating in air should be better than an egg in water. |
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Oh, and watch out for the gasses mixing over time. |
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So I'll know that there's another halfbaker around when all the SF6 has gone missing from the switchgear at work?
If you had a snow globe half filled with heavy gas, and an object that floated on that gas, but sank when the heavy gas and air were mixed, then by shaking the snow globe the object would sink to the bottom, and magically float up the half way position as the heavy gas settled. |
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Very very cool. The effect would make for a startling Orrery. |
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I don't think the SF6 would 'settle' after
mixing - it'd just stay mixed (just like
shaking saltwater and plain water - you
get brine which doesn't repartition). I
think. |
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... and I'm pretty sure you'd be wrong in your thinking. |
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SF6 settles out very well. It pools at floor level and can be a lingering engulfment hazard in confined spaces and especially in cable pits located near switchgear during gassing/maintenance/install work. CO2, on the other hand, which IS miscible in air, will pool at first but will eventually dissipate. |
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the analogy with salt and plain water is that they are both essentially the same substance, but that one has a contaminant. by combining the two you are simply lowering the concentration of the contaminant (entropy will force this to happen) over a larger sample. With SF6 and air, there is no common element. SF6 does not normall occor in air, and is not miscible. |
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Thanks, [Custard]. Well explained. |
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Oooh, Oooh, so I can keep my snowglobe? |
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Only on Wenesdays from 4 to 9 PM and every-other weekend. Thanksgiving and Christmas visitation will be split among the two parties involved and alternate each year. |
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I'm intrigued - I had always assumed that
all gases are completely miscible (eg,
petrol vapour and air; or the various gases
that comprise air). Time to google.... |
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Googling done - I thought maybe I was
going mad*. As I'd thought, all gases
are mutually miscible in all proportions.
Yes, SF6 will "pool" by being denser, but
it will mix by diffusion or by agitation,
sooner or later. (In the same way, CO2
and many other gases can 'pool' in pits,
inpection shafts and so on, but are still
completely air-miscible). The pooling
tendency of SF6 will be greater than
that of CO2 (and it will take longer to
dissipate if indisturbed) simply because
it is so much denser. But, once mixed,
it won't repartition.
The miscibility of all gases is such a
basic phenomenon that it's hard to find
it stated explicitly, but see [link] for an
example (scroll down - "Since all gases
are miscible with each other in all
proportions...."). Also, various mixtures
of SF6 and air (or SF6 with other gases)
are used in various contexts. An SF6/
air mixture won't settle out any more
than CO2 settles out of the air. |
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See also, for example, the Wikipedia
article on SF6 ("In fact, the lungs mix
gases very effectively and rapidly, such
that SF6 would be purged from the
lungs within a breath or two."). |
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Googling also throws up articles which
talk about "air:SF6 interfaces", which
made me take a second look. However,
these interfaces are similar to those you
get between (say) brine and plain water,
or between water and ethanol - ie,
eventually they will dissipate (and
especially if you mix the two phases -
they won't repartition). |
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I'm not sure what you were getting at
about "common element" - nitrogen
and oxygen have no 'common element'
but are perfectly miscible. |
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If it helps, you might be able to restore
the partitioning in the snow-dome by
cooling it far enough to liquify the
gases - I don't know whether liquid SF6
is miscible with liquid nitrogen/oxygen/
CO2. |
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*this is, of course, still possible; but
this is a different issue. |
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And another. See this patent summary,
which mentions the use of SF6:air and
SF6:N2 mixtures in various contexts, and
then describes a zeolite-based process to
separate the SF6 from these mixtures. |
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Perfectly happy to stand corrected; my experience with SF6 is limited to the safety aspect; there is no limitation placed on the time it takes for SF6 to disperse, unlike any other gas I know of. |
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My point regarding the brine still holds: brine is simply water containing salt, and there is an entropic tendency for the brine to mix with the water. A better analogy would have been alcohol and water: totally miscible, different densities. |
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I suppose I should ask about the tendency for hydrogen and helium to rise through the atmosphere and disperse. when kept in a vessel it will undergo brownian mixing to some extent, but if left very still? any practicing chemists handy? I'm a little rusty with my gasses. |
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I haven't researched this but it feels correct intinctively: |
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If you have 2 substances of different densities which do not chemically bond when mixed, they will seperate in a gravitational field. |
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For solids, this often requires external agitation such as panning for gold or the sinking of buildings during an earthquake (liquifaction). |
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For gases, they are already agitated so I see no reason for them not to seperate via convection as a hot gas would. |
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If you burst a helium balloon I would expect the helium to rise. I believe this was part of the problem with CFCs. |
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Anyway, couldn't you achieve the same effect in a normal room with a helium boat? You would probably need very still air and a large temperature gradient (heated ceiling, chilled floor). |
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Custard - yes, you're right about the
brine, and ethanol/water is the better
example. I'm not sure about the rising
of helium/hydrogen in the
atmoshphere. My understanding was
that they don't tend to rise (I mean,
once they are mixed with air they don't
tend to separate out towards the top),
but they do preferentially escape from
the earth's atmosphere simply because
their lighter molecules are more likely
to acquire a higher velocity in
intermolecular collisions, and hence
have a greater chance of reaching
escape velocity. Therefore, at a guess I
would expect the upper atmosphere to
be slightly *poorer* in these gases as a
result, though the difference would
probably be minuscule since the escape
rate is incredibly slow compared to the
speed of mixing of the atmoshphere. |
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Anyone know if the upper atmosphere
is (mole-for-mole) richer, poorer or
neither in light gases? |
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[marklar] //If you have 2 substances of
different densities which do not
chemically bond when mixed, they will
seperate in a gravitational field. // |
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See above and below. As long as the
two substances are miscible (eg,
ethanol/water, or any two gases), they
won't tend to seperate out afer mixing.
You can look at it from either a
diffusional viewpoint or (as Custard
mentioned, I think) an entropy
viewpoint, but it comes to the same
thing. This is why there is no point
asking for the first shot from a bottle of
whisky - the top of the bottle contains
no more or less ethanol than the
bottom. |
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OK your references, in order. |
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the first that mentions the total miscibility of all gasses is not a discourse on gas law or gas behaviour, and the comment was unreferenced. The comment bears little importance in the context of the paper and I question whether this article was subject to peer review. In other words, this was not a critical claim or salient point from the paper, so I don't know if I'd go banking anything on the validity of the claim. |
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The second made no refernce that I saw to the miscibility of gasses, it simply outlines an industrial process (an unverified patent mind you) for efficient extraction of SF6. This provides no proof that SF6 won't settle out. |
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Incidentally, to be fair, if the
gravitational field is strong enough, it
*will* tend to overcome diffusion and
concentrate the denser gases toward
the bottom. But you need a huge force
- it won't happen in a terrestrial living
room. This is the idea behind (as I
understand it) gas-phase uranium
isotope enrichment (using UF6, a
gaseous comounf of uranium). |
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But, again, this won't happen to any
perceptible degree for an SF6/air
mixture under 1G of gravity. |
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I've seen smoke pour through a ceiling vent, onto the floor and down some stairs. I've even seen a smoke projector screen that uses a constant waterfall of smoke. I can't find a scientific statement of this but I know it to be the case. |
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[Maklar] Yes, I don't doubt that. But if you
mix smoke and air up, they won't separate
out. (As it happens, smoke is basically a
bunch of very small particles or droplets in
in air, so it's a bit of a wild herring in the
present discussion). |
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The point is that if you mix two gases,
they won't separate out. |
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At the risk of sidetracking an interesting discussion, I found another use for heavy gasses. Lord Rayleigh, the great physicist, used to fill a balloon with carbon dioxide and use it as a lens to focus sound, mostly as a party trick. SF6 should work even better, and could be part of the proposed art project. |
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Speaking of party tricks, there was
something on TV last night which had a
guy inhaling SF6 - sort of a "reverse
helium" effect on the voice. |
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The simple reason that most gasses will
not tend to seperate is the kinetic
model of gas behavior. Gas molecules
have an inherent kinetic energy, and
move constantly. Under normal
conditions this energy is more than
enough to overcome the gravitational
potential energy for any given space. If
the gas temperatures were lowered
enough, the materials probably would
separate or remain separated. What
this low temperature is, I don't know,
and it may or may not be above the
liquid transition temperature. |
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[MechE] thanks for the clarification. As
far as I know (or can find out), the
separation point is always below the
liquefaction point (ie, all gases are
miscible - right down to the point
where they condense). |
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On reflection, it's hard to imagine how
any two gases could *not* be miscible:
molecules of gas A do not cohere to
other A molecules (if they did, A would
be a liquid), and likewise molecules of
gas B don't stick to molecules of B
(ditto), so there is no real mechanism
for partitioning like that seen in
immiscible liquids. |
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But the key point is that I thought the
boat thing floating on the SF6 ocean
was very pretty. |
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Under a sufficiently high G field, or sufficiently large difference in molecular weight, it is not always below the liquidification temperature. (At least the point at which they are statistically, if not totally separated). This is the principle behind gas centrifuge separation techniques. I just don't know how you find what qualifies as sufficient. I susppect Uranium Hexafluoride and helium meet the requirements, but a radioactive art display might prove a bit of a problem. |
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A piece of art that can only be viewed wearing radiation suits would be more intriguing. |
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Gasses at room temperature won't separate, because molecules have enough kinetic energy to stay diffused through Brownian motion. If you cool down the mixture to slightly above condensation point for one of the gasses, they will separate after a while through gravity alone. |
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Make a glass thermos bottle, fill it with hexafluoride and hydrogen, and cool it almost to the condensation point of one of the gasses (whichever is higher). The gasses would have a tendency to separate. All that remains is to find a way to fill the tinfoil (or styrofoam) boat with hydrogen and let it float on the hexafluoride without opening the bottle. |
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A waterless snow globe would be easier to make: fill the snow globe with sulphur hexafluoride, and make snowflakes out of styrofoam, made with helium instead of CO2 or whichever usual propellant gas used in making styrofoam. That would reduce its density. So, having low-density flakes in a high-density atmosphere would make them float more slowly and twirl and act like natural snowflakes, without the water. |
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EDIT: I kept writing "freezing" instead of "condensation". Wrong phase transition. I edited it now. |
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Yes. Thanks for the correction. I'll edit my anno. |
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[MechE] //The simple reason that most gasses will not tend to seperate is the kinetic model of gas behavior. Gas molecules have an inherent kinetic energy, and move constantly. Under normal conditions this energy is more than enough to overcome the gravitational potential energy for any given space. If the gas temperatures were lowered enough, the materials probably would separate or remain separated. What this low temperature is, I don't know, and it may or may not be above the liquid transition temperature.// |
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The temperature depends on the gravitational potential difference that needs to be overcome, which depends on the height of the space and the mass of each gas molecule. There wouldn't be a sharp separation. The light gas would be more evenly distributed through the space, and only slightly denser at the bottom than at the top, while the heavy gas would be more concentrated at the bottom. |
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According to Wikipedia, atmospheric pressure drops to 50% after about 5.6km. Something 6 times as heavy would have to overcome the same potential energy at 1/6 of the height, about 1 km. So the amount of settling out is tiny in containers under a metre high. |
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Reducing the temperature from about 300 K to 30 K would be needed to reduce the height to 100 metres, but I don't think it would stay a gas. |
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[Custardguts] // I suppose I should ask about the tendency for hydrogen and helium to rise through the atmosphere and disperse. when kept in a vessel it will undergo brownian mixing to some extent, but if left very still? any practicing chemists handy? I'm a little rusty with my gasses. // |
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I'm not a practicing chemist, but... brownian mixing should partially separate the gasses. I think with a 6km tall vessel the hydrogen and helium should be fairly evenly spread through the column, but the air should be twice as dense at the bottom, so the ratio of the light gases to air should double at the top. However, my calculations don't predict how long the process would take. |
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In some recent Browsing, I think I remembered that SF6 condenses at around -60C (normal pressure). So the snow globe would need to simulate extreme winter conditions to allow gas separation. |
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//extreme winter conditions// |
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