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AsocialSurvival
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Gases seperation (by gravity)
If i close empty bottle (full of air of course), and let it stay settled for long period of time, without disturbing, moving, and opening it, what is
going to happen?
a) nitrogen and oxygen will stay homogeneous
b) nitrogen will concentrate at bottom because it is heavier, and oxygen above it
(If we ignore trace gases and know that oxygen is denser and heavier than nitrogen.)
[Edited on 9-10-2014 by AsocialSurvival]
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Texium
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Well, the gases actually separating seems extremely unlikely. Also, oxygen is actually both heavier and denser than nitrogen.
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AJKOER
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OK, a better larger scale example is earth's atmosphere.
If anyone thinks there is not more O2 near the earh's surface, than invite them up to the top of one of the tallest mountain without an oxygen mask.
Those acquainted with cave exploration also known that going below the earth's surface, one is more likely to find not more O2, but instead heavier
gases like CO2. The latter is a signficant danger to cave explorers.
Similarly, with respect to your bottle, the bottom of the vessel more likely has CO2 from your breath and our copious use of fossil fuels.
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Little_Ghost_again
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Surely a gas dosnt stay still? molecules must keep bumping into each other therefore mixing the gasses? I have no idea but a gas dosnt stay still
still or does it?
Actually the chances are if you leave a closed bottle for a very long time then microbes will eventually use the oxygen all up, then start on other
gases.
I know from biology and my dad that there is a experiment for fish tanks where you close up a vial of water. you can measure the oxygen before and
after, this is called the biological oxygen demand and is an indication in rivers etc of the biological loading
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blogfast25
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Quote: Originally posted by AJKOER  | If anyone thinks there is not more O2 near the earh's surface, than invite them up to the top of one of the tallest mountain without an oxygen mask.
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The air pressure up there is much less: there is BOTH less oxygen AND less nitrogen.
Fill a bottle carefully half with CO2 and then carefully add the lighter hydrogen on top, then close off and allow to stand undisturbed. Diffusion
soon gets rid of any concentration gradients, as Entropy dictates (the homogeneous state being far more probable than the unmixed, orderly state and
so of higher Entropy). Gases are infinitely miscible.
[Edited on 9-10-2014 by blogfast25]
Exactly.
Some separation between lighter and heavier gases based on the MM of the gases is of course possible: see the use of centrifuges to separate gaseous
U<sup>235</sup>F<sub>6</sub> and U<sup>238</sup>F<sub>6</sub>.
[Edited on 9-10-2014 by blogfast25]
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Praxichys
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| Quote: Originally posted by AJKOER | OK, a better larger scale example is earth's atmosphere.
If anyone thinks there is not more O2 near the earh's surface, than invite them up to the top of one of the tallest mountain without an oxygen mask.
Those acquainted with cave exploration also known that going below the earth's surface, one is more likely to find not more O2, but instead heavier
gases like CO2. The latter is a signficant danger to cave explorers.
Similarly, with respect to your bottle, the bottom of the vessel more likely has CO2 from your breath and our copious use of fossil fuels.
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No, no, no.
Brownian motion causes gases to diffuse into each other. Read up on entropy. Initially, 'pools' of heavy gases can form, but eventually they will
spread. While it is technically true that gravitational acceleration will tend to concentrate gases of different weights, the effect is negligible
when you compare intermolecular forces (which are very weak in gases, and would be the driving factor in concentration) against entropy. Entropy wins
by a wide margin, and the difference in concentration from top to bottom is immeasurably small.
There is less air at high altitudes. It's still 21% oxygen, but at a lower overall pressure which prevents you from absorbing as much. In
caves, it is the confined and ventilated space that accumulates CO2 from natural undergound sources... along with methane, H2S, etc. - little to do
with the weight of the gases. Consider methane found in the ground, which is lighter than air.
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phlogiston
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The gasses in earths atmosphere are not homogenous, but that is due to other causes.
CO2 is produced near the surface.
Ozone is produced at a certain height by radiation from the sun which doesn't pentrate deeply.
Water vapor is formed from liquid water at the surface and condenses at colder temperatures at altitude.
etc. etc.
Light gasses such as Helium and hydrogen boil off into space, and their mass plays a role in that but it is not because a gradient is formed by
gravity. Rather, molecules of these molecules are light enough that they can reach escape velocity when near the edge of the atmosphere.
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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MrHomeScientist
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Lots of misinformation at the beginning of the thread, geez. Blogfast and most everything after is good info. The key point is entropy, as blogfast
mentioned. Entropy wants everything mixed evenly, and since gases have very little force between atoms entropy wins. So you can get density columns
with liquids but not gases, for example, because liquids hold together. You can fill a fish tank with sulfur hexafluoride because it's so much more
dense than air, but even this diffuses out completely in tens of minutes (I've done this myself).
Radon accumulates in basements because it is produced there, not because it's heavier than anything else. It's a product of radioactive decay of the
concrete that makes up the foundation, and once produced it has nowhere else to go. Same with CO2, methane, helium, etc. in caves. It
didn't sink there, it was made/trapped there.
At the top of a mountain there is less of everything, not just oxygen.
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blogfast25
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And you can't even make a decent cup of tea because the water boils at about 70 C (Everest)!
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AJKOER
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Per Wikipedia (http://en.m.wikipedia.org/wiki/Atmosphere_of_Earth):
"This layer (referring to the Exosphere) is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including
nitrogen, oxygen and carbon dioxide closer to the exobase. "
So, the density of air decreases with altitude and apparently also the composition, at least at extreme altitudes relating to molecular weight, with
questionable others so arguing as well (see, for example, https://www.google.com/url?sa=t&source=web&rct=j&... and also
https://www.google.com/url?sa=t&source=web&rct=j&... .
So, I remained confused, but with some company.
[Edited on 9-10-2014 by AJKOER]
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unionised
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As far as I can tell from a quick look, this is the only post that gets it right.
[ Edit. Ajkoer also got it right the second time- we cross posted]
The two gases behave largely independently of each other and come to equilibrium. The gas is denser at the bottom of the bottle than it is at the top.
This effect is bigger for oxygen, because it's more dense
The concentrations are proportional to the pressures (at a given temperature) and the expression for calculating that pressure is the second one on
this page
http://en.wikipedia.org/wiki/Barometric_formula
(Using the molar masses of the two gases instead of the average value for air)
The effect is tiny.
The corrections due to non ideal behaviour of gases are even tinier.
[Edited on 9-10-14 by unionised]
[Edited on 9-10-14 by unionised]
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AsocialSurvival
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Hmmm... If there is ~ 21 % Oxygen in air, meaning 1/5 of air, I can collect it by using two bottles where one at the bottom is 5x or more smaller than
inverted bottle at top (if bottom one is 0.5 L, then top one is 2.5 L or more ), and just sill them with chewing gum at their necks, and after some
time remove the top one, and test the bottom one with glowing splint - which is good enough test for oxygen.
OK, I will post the results here!
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Dan Vizine
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The proof that separation must occur is already known to all: H2 and He escape to space. Therefore gravitational separation occurs.
In a bottle at room temp. simple kinetic energy overwhelms that.
Simple fact that a lot of people don't know* ...Want to know how the densities of two gasses compare? Just compare the MWs. This approximately is
nearly always good enough to give meaningful comparisons.
*or know, but just have never really thought about it that way....
[Edited on 9-10-2014 by Dan Vizine]
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blogfast25
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| Quote: Originally posted by unionised | As far as I can tell from a quick look, this is the only post that gets it right.
The two gases behave largely independently of each other and come to equilibrium. The gas is denser at the bottom of the bottle than it is at the top.
This effect is bigger for oxygen, because it's more dense
[Edited on 9-10-14 by unionised] |
Would you care to calculate the effect for us? 
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unionised
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| Quote: Originally posted by AsocialSurvival | Hmmm... If there is ~ 21 % Oxygen in air, meaning 1/5 of air, I can collect it by using two bottles where one at the bottom is 5x or more smaller than
inverted bottle at top (if bottom one is 0.5 L, then top one is 2.5 L or more ), and just sill them with chewing gum at their necks, and after some
time remove the top one, and test the bottom one with glowing splint - which is good enough test for oxygen.
OK, I will post the results here! |
There's no point trying.
The effect is, as I said, tiny.
Far too small to be detected that easily.
Dan Visine, re.
"In a bottle at room temp. simple kinetic energy overwhelms that."
Nope.
It's not.
It's enough to make the overall effect really small.
Blogfast,
Re "Would you care to calculate the effect for us?"
No thanks
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AJKOER
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A major omission, so far, at least with respect to planet earth, is the total disregard for the effect of light leading to the formation of active
species and ozone. To quote one source (see, for example, https://www.google.com/url?sa=t&source=web&rct=j&... ):
"Major steps of Chapman mechanism: 1) Above about 30 km altitude, molecular oxygen absorbs solar radiation (wavelength < 242 nm) and
photodissociates: O2 + hν -> O + O 2) The oxygen atom, O, reacts rapidly with O2 in the presence of a third body, denoted M (M is usually another
O2 or N2), to form ozone: O2 + O + M -> O3 + M "
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phlogiston
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| Quote: Originally posted by AJKOER | | A major omission, so far, at least with respect to planet earth, is the total disregard for the effect of light leading to the formation of active
species and ozone. |
| Quote: Originally posted by Dan Vizine | | The proof that separation must occur is already known to all: H2 and He escape to space. Therefore gravitational separation occurs.
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| Quote: Originally posted by phlogiston | | Light gasses such as Helium and hydrogen boil off into space, and their mass plays a role in that but it is not because a gradient is formed by
gravity. Rather, molecules of these molecules are light enough that they can reach escape velocity when near the edge of the atmosphere.
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I admit I also often don't read the entire thread first.
[Edited on 9-10-2014 by phlogiston]
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Dan Vizine
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| Quote: Originally posted by unionised |
"In a bottle at room temp. simple kinetic energy overwhelms that."
Nope.
It's not.
It's enough to make the overall effect really small.
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???
Yes, the miniscule separation due to gravity is absolutely overwhelmed by the simple motion of the molecules due to their kinetic energy. I didn't say
that it didn't exist, it's just that it's utterly insignificant for all practical purposes.
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phlogiston
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This 2009 paper from the Journal of Cave and Karst Studies is about this very topic, and includes a calculation of the concentration gradients that
might be expected for different gasses as a cause of gravity:
The legend of carbon dioxide heaviness
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Praxichys
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Aw dang. I was just readying up to crunch the numbers this weekend. Great find!
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unionised
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| Quote: Originally posted by AJKOER | A major omission, so far, at least with respect to planet earth, is the total disregard for the effect of light leading to the formation of active
species and ozone. To quote one source
"Major steps of Chapman mechanism: 1) Above about 30 km altitude, ... |
How big do you think a bottle is?
Does 30 KM seem like a reasonable size to you?
Also, what do you imagine the bottle to be made from? Glass and plastic will both do a good job of absorbing that hard UV.
Did it occur to you that there might be a very good reason why we were ignoring the effect of light?
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phlogiston
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Just goes to show that the level of some of the members here matches that of professional chemists that publish in peer-reviewed journals. (You would
do this just for the fun of it, but assuming you do a good job apparently it would be worthy of publication in a peer-reviewed journal)
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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unionised
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| Quote: Originally posted by phlogiston |
Just goes to show that the level of some of the members here matches that of professional chemists that publish in peer-reviewed journals. (You would
do this just for the fun of it, but assuming you do a good job apparently it would be worthy of publication in a peer-reviewed journal)
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Some of us are professional chemists who publish in peer-reviewed journals.
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jock88
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Just wondering would it be possible to attain pure or close to pure Oxygen (for say burning in a flame) using a home made centrifuge?
Just a note to state my credentials.
I am a profesional amataur! 
[Edited on 11-10-2014 by jock88]
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Oxirane
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If gases are miscible, how can Uranium hexafluoride be separated by high speed gas centrifuge? In this case, any gas with density difference could be
concentrated using the same techniques?
This does not apply to liquids, I suppose? If we have water and alcohol mixture and we centrifuge it, there sould be no separation, shouldn't there?
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