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extro…
Guest

 Posted: Sat Jan 13, 2007 11:15 pm    Post subject: 1 Saw this cool video on youtube: http://www.youtube.com/watch?v=1PJTq2xQiQ0&eurl= It shows a "boat" made of aluminum foil floating in a tub of a heavy gas, sulfur hexaflouride, a very non-reactive gas that is 5.1 times as dense as air. Now, air is over 7 times as dense as helium. So that makes sulfur hexaflouride over 35 times as dense as helium. If I were to thoroughly mix helium and sulfur hexaflouride (50/50) in a sealed chamber, would they quickly seperate? I was thinking it would be cool to mix them in a sealed chamber, like a large capped glass bottle, and have inside a bunch of lightweight ping-pong balls (perhaps helium filled) - light enough that they'd float on the heavier gas, but sink in the helium. The appearance would be a layer of ping pong balls floating in mid air. Would it work?
worm
unregistered

 Posted: Sun Jan 14, 2007 12:26 am    Post subject: 2 generally, a mixture of two gases with different densities always "separates" somewhat...i.e. there are more atoms/molecules of the lighter gas at higher points and more atoms/molecules of the heavier gas at lower heights. however, there usually isn't a well-defined layer between the two (like between oil and water) but a gradual change in relative amounts. with a large enough difference in densities, though, they should separate fairly well. based on the movie, air could probably substitute for helium pretty well. i don't know for sure about the speed of separation...my guess is that it would be noticeable by the naked eye...fast enough that you shouldn't need time-lapse photography and slow enough that you shouldn't need high-speed cameras. what would be cool is if you could get the amounts right so that the balls go to the bottom when you shake the bottle and as the mixture settles, they start to rise (or maybe that's what you were going for all along).
Quailman
His Postmajesty

Posted: Sun Jan 14, 2007 4:03 am    Post subject: 3

 worm wrote: what would be cool is if you could get the amounts right so that the balls go to the bottom when you shake the bottle and as the mixture settles, they start to rise (or maybe that's what you were going for all along).

Wouldn't they go to the top?
extro...*
Guest

Posted: Sun Jan 14, 2007 4:06 am    Post subject: 4

Quailman wrote:
 worm wrote: what would be cool is if you could get the amounts right so that the balls go to the bottom when you shake the bottle and as the mixture settles, they start to rise (or maybe that's what you were going for all along).

Wouldn't they go to the top?

I think the answer is "it depends". The balls can be lighter than the heavy gas, and heavier than the light gas, but that alone doesn't tell us if their lighter or heavier than a 50/50 (or any other ratio) mix of the gases.
extro...*
Guest

 Posted: Sun Jan 14, 2007 5:32 am    Post subject: 5 And if you got it right, the balls might be equal in weight/volume as the mixed gasses, and just float about randomly. As the two gasses seperate, they'd line up on the boundary between the two.
worm
unregistered

 Posted: Sun Jan 14, 2007 7:23 am    Post subject: 6 yeah, if you bought some SF6, you might as well make a few bottles with different ratios that give different results. fyi, i saw a comment on the youtube page about inhaling gases heavier-than-air making your voice deeper (cf: the chipmunks voice you get from helium). this would work, but the gas would linger at the bottom of one's lungs, so try not to breathe too much of the stuff.
Daedalian Member

Posted: Wed Jan 17, 2007 1:04 am    Post subject: 7

 worm wrote: yeah, if you bought some SF6, you might as well make a few bottles with different ratios that give different results. fyi, i saw a comment on the youtube page about inhaling gases heavier-than-air making your voice deeper (cf: the chipmunks voice you get from helium). this would work, but the gas would linger at the bottom of one's lungs, so try not to breathe too much of the stuff.
According to wikipedia (who would, like, never lie to me ) the lungs are sufficiently good at mixing gases that that wouldn't be the problem but there's the problem of "asphyxiation" if that's the correct medical term (near as I can tell, equivalent to carbon monoxide poisoning with the filling-up-of-red-blood-cells) so really, breathing any is damaging, but in small quantities it might be just one of those things your body can handle without a bother.
worm
unregistered

 Posted: Wed Jan 17, 2007 2:36 am    Post subject: 8 i think asphyxiation is a pretty generic term...the person is somehow being deprived of oxygen. when you breathe in air, there's a relatively large amount of oxygen, and much of it gets absorbed into your bloodstream. when you inhale another gas (helium, argon, SF6) that doesn't contain oxygen, it disturbs the equilibrium...your lungs are really low in oxygen, so they can actually remove some from your blood. on the other hand, carbon monoxide binds to hemoglobin more tightly than oxygen. hemoglobin carries oxygen to your cells so this is bad (it's reversible, though, unlike cyanide which binds to hemoglobin irreversibly). but back to what we were talking about. i don't think you'd have a problem from a little bit here or there...the folks in that video certainly inhaled "some" SF6 and the flow of air into one's lungs will help with the mixing, which will eventually get rid of it. mixing of gases is usually fast, but with densities this different i still wouldn't inhale it from a balloon like helium no matter what wiki says.
austinap*
Guest

Posted: Fri Jan 19, 2007 5:09 pm    Post subject: 9

 extro… wrote: Saw this cool video on youtube: http://www.youtube.com/watch?v=1PJTq2xQiQ0&eurl= It shows a "boat" made of aluminum foil floating in a tub of a heavy gas, sulfur hexaflouride, a very non-reactive gas that is 5.1 times as dense as air. Now, air is over 7 times as dense as helium. So that makes sulfur hexaflouride over 35 times as dense as helium. If I were to thoroughly mix helium and sulfur hexaflouride (50/50) in a sealed chamber, would they quickly seperate? I was thinking it would be cool to mix them in a sealed chamber, like a large capped glass bottle, and have inside a bunch of lightweight ping-pong balls (perhaps helium filled) - light enough that they'd float on the heavier gas, but sink in the helium. The appearance would be a layer of ping pong balls floating in mid air. Would it work?

I say no. Well, the correct answer is "what temperature are you going to do it at?" Gasses diffuse quite quickly and in the experiment shown, if it were let to sit for a while, the SF6 would diffuse out into the air creating a relatively even mixture.

Even in a very heavy gas like SF6, the molecules are moving at incredible speeds (i'd guess ~120 m/s). While these speeds are slow compared to that of most gasses in air (or helium, at a speed of ~780 m/s at room temp), there is still enough motion to mix evenly and quickly. There will be some density gradient to be sure, but at room temperature and at earth's gravity, I don't think it would be enough to detect using such a simple method.

The problem is that there isn't any real 'cohesive' force holding gasses together, so they mix freely. Liquids can separate because there is some intermolecular force keeping them close. If you want a clear example of this, think about smoke. Even the smallest ash particles in smoke are much heavier than SF6, but smoke still diffuses readily.

There is one case where I might be wrong: that is the case where the critical point of SF6 is at a point on the curve where the other gas is still a gas. In this case, the SF6 would effectively be behaving like a liquid, while the other is still a gas and thus they would be able to separate relatively well. Unfortunately, I don't have a phase diagram for SF6, so I can't tell you if this is what's happening or not.
austinap*
Guest

 Posted: Fri Jan 19, 2007 5:13 pm    Post subject: 10 Oh, and worm is right about the asphyxiation thing. SF6 is very non-reactive, so I think it would be very likely to coordinate with the heme group to cause biological problems in that way. Instead, it's just the problem of not getting air. This is the same problem with just breathing nitrogen or helium: though they don't directly harm you, they just prevent you from getting something you need. This isn't the same as carbon monoxide, NO, or cyanide where the gas is actually able to coordinate with the heme with higher affinity than oxygen. Because of this, you can die of CO poisoning even after the CO source is removed and you are placed on pure oxygen.
worm
unregistered

Posted: Sat Jan 20, 2007 7:03 am    Post subject: 11

 austinap* wrote: There will be some density gradient to be sure, but at room temperature and at earth's gravity, I don't think it would be enough to detect using such a simple method.

i agree. for the ping pong ball experiment, i think the questions are:

1. how sharp is the gradient?
2. how big is your bottle?
3. how close is the density of the gas mix to that of the ball?

honestly, i'm not sure what the results would be. but unlike the video's ezperiment, we're talking about a closed system, so an equilibrium will eventually be reached. why should diffusion and the velocities of the gas molecules matter at equilibrium?

on a different note, i think if a ping pong ball were used in the open system they had in the video, it would fall. the shape of the aluminum boat (nice and flat) made it effective for pressing down on a large area of gas molecules. theoretically, the ball's surface would only have one point that was pushing straight down, opposing gravity. it's like the difference between sticking your hand out a car window with your palm out vs. palm in.

-edit

 austinap wrote: it would be very UNlikely to coordinate with the heme group

extro...*
Guest

Posted: Sat Jan 20, 2007 11:52 am    Post subject: 12

 worm wrote: why should diffusion and the velocities of the gas molecules matter at equilibrium?

I think it's a question of what that equilibrium, between two ever-present opposing forces, will be - those forces being gravity which will tend to seperate them, and the movement of gas molecules associated with their heat, which will tend to mix them.

Austinap had me convinced, until just now I recollected an ancient memory: I was at a cabin once, with a wood stove, in the winter. It was bitterly cold when I got there, and I started up a big fire in the stove. I sat down and read a book for a while while the place warmed up. A sharp temperature gradient developed, and there happened to have been a source of smoke in the room (cough), and I noticed after a while that the smoke had settled in a thin horizontal layer, across the entire room, about 3 or 4 feet off the floor. I remember that if I stood up and sat down again, it created these really cool slowly undulating wavelike movements of the entire smoke layer.

People use ceiling fans during winter for this reason - to bring the warm air down from the ceiling. The difference in density between warm and cool air would be much less, I think, than between CF6 and He, but we still need a fan to keep them mixed up. Of course, that's not throughly convincing either, as without a constant heat source I think the temperatures would equalize due to conductive or radiant transfer. So we have more opposing forces at play in that scenario - gravity and heat source contributing to seperation, and molecular motion, and temperature equalization through conduction or radiation (can a gas radiate heat?), contributing to mixing.

 Quote: on a different note, i think if a ping pong ball were used in the open system they had in the video, it would fall. the shape of the aluminum boat (nice and flat) made it effective for pressing down on a large area of gas molecules. ... it's like the difference between sticking your hand out a car window with your palm out vs. palm in.

I'm not sure that analogy fits. The shape will affect how fast it sinks if it does sink, not whether it ultimately sinks or floats.

I'm not sure traditional ping-pong balls would work (it's a matter of scale), but whatever was used would have to be enclosed, like a balloon, and not open like a boat. Boats capsize and sink.
austinap*
Guest

Posted: Sat Jan 20, 2007 3:23 pm    Post subject: 13

worm wrote:

honestly, i'm not sure what the results would be. but unlike the video's ezperiment, we're talking about a closed system, so an equilibrium will eventually be reached. why should diffusion and the velocities of the gas molecules matter at equilibrium?

 austinap wrote: it would be very UNlikely to coordinate with the heme group

Thanks for the fix! As for the first part, the velocity of the gas matters because it acts as a self-mixing system, and I'm willing to bet that this factor would mix the gasses better than gravity would 'unmix' them.

As far as extro's example, I can definately see the point, but there are so many other forces at play in that scenario that I'm not sure how relevant the results actually are. First off, it's a non-equilibrium system. You're pumping heat into one area, it's diffusing in another, etc.. This factor alone would significantly change the dynamics of the system.

I'll say one other thing about this: there's undoubtedly _some_ density gradient in any non-zero volume (height actually) of gas under a gravitational field. I'm not questioning this, I just doubt how large of a difference you'd see. In the example you gave, (be nice to me, I'm talking in hypotheticals now), there's a chance that the density of the smoke was almost the exact same as room temperature air, and therefore a very small density gradient kept it more concentrated in a certain layer. However, even this I think is pretty unlikely. The fact is, when you turn something as complex as a gas system into a non-equilibrium state, a lot of its properties change. You'll start to get weird and turbulent flows, complex thermal dynamics (convection, radiation, etc..).
worm
unregistered

 Posted: Sat Jan 20, 2007 8:55 pm    Post subject: 14 there would be a height-based density gradient even if we were talking about one gas. this gradient should be enhanced by a large difference in the masses of the gases. without data or at least some reasonable calculations, we can only speculate how much of a difference this does or doesn't make (question 1 above). if we had a tall enough bottle containing only SF6, a ball at the bottom containing air at 1 atmosphere should rise like a helium balloon rises in air. the ball should rise until the pressure difference between inside and outside the ball (driving the ball up) balanced the force of gravity (pulling it down). so, the height of the bottle matters (question 2) now, say we mixed air and SF6 such that a uniform, non-equilibrium mixture of the two had a density that could achieve the balancing act above. this is really the idea that extro... presented in post 5. if we let the mixture equilibrate and form a density gradient, the ball should rest at some non-zero height. (what i was getting at in question 3).
austinap*
Guest

Posted: Sun Jan 21, 2007 1:44 am    Post subject: 15

 worm wrote: this gradient should be enhanced by a large difference in the masses of the gases. without data or at least some reasonable calculations, we can only speculate how much of a difference this does or doesn't make (question 1 above).

I have some reasonable calculations in my first post based in statistical thermodynamics and the kinetic theory of gasses. Taking this a step further, let's say that we have a fairly large, 10m high container (the cross sectional area is irrelevant). Then, at room temperature (298 K), the average velocity of a molecule of SF6 is 225 m/s (I forgot a constant factor of 3 in my original calculations, so it's off by a factor of sqrt[3]), giving an average molecule a kinetic energy of about 6.14e-21 J. Similarly, the gravitational potential for a molecule of SF6 between the top and bottom of the container works out to be about 2.38 e-23 J, less than 1% of the kinetic energy of a molecule of the gas. To figure out the population differences between the top and the bottom, plug it into Boltzmann's equation and you'll get a ratio of the number density on the bottom to the number density on top is about 1.006. So, the bottom is approximately 0.6% more dense than the top. I made a few assumptions here, but there are your reasonable calculations.

 worm wrote: if we had a tall enough bottle containing only SF6, a ball at the bottom containing air at 1 atmosphere should rise like a helium balloon rises in air. the ball should rise until the pressure difference between inside and outside the ball (driving the ball up) balanced the force of gravity (pulling it down). so, the height of the bottle matters (question 2)

Yes to the first part, absolutely not to the second part. Assuming that a ping pong ball is a ridgid vessel (which I think is fitting in this case), then it's floating has absolutely nothing to do with the pressure inside the ball. It has to do with the density of the ball relative to the density of the fluid it is immersed in. This is archimede's principle in the simplest of forms. It can find bouyant equilibrium and still not be isobaric to it's surroundings. And yes, the high of the bottle matters. We already established that, and I did it again in a more rigorous way above.

 worm wrote: now, say we mixed air and SF6 such that a uniform, non-equilibrium mixture of the two had a density that could achieve the balancing act above. this is really the idea that extro... presented in post 5. if we let the mixture equilibrate and form a density gradient, the ball should rest at some non-zero height. (what i was getting at in question 3).

I know what extro... was getting at. The problem is that it won't work unless you get a pingpong ball of exactly the right density, cool the mixture down to very low temperatures, get a bottle of _huge_ height, or place the container in the sun's gravitational field.
worm
unregistered

 Posted: Sun Jan 21, 2007 4:46 am    Post subject: 16 the problem with people like you is...umm...just kidding actually, this reminds me a little of the old days when i'd go back and forth with hy about some problem with one important exception... ...i'm wrong here. i think what makes the idea seam plausible is that the video makes it seem like SF6 behaves like a liquid...it pools at the bottom of the tub and they can pour it out of a beaker. in short it doesn't mix with air that readily. the error in my thinking is worse than i'd like to admit. looking back at the pics in my mind's eye, i think i tried to merge the slow diffusion with the height-based gradient (if that makes any sense). thanks for the time with the calculations and all.
austinap*
Guest

 Posted: Sun Jan 21, 2007 3:11 pm    Post subject: 17 No problem at all (: It's been a while since I've had the chance to argue something that I'm pretty sure I'm right about! So are you a chemistry guy or what?
worm
unregistered

 Posted: Sun Jan 21, 2007 8:42 pm    Post subject: 18 yes, but i'm organic...we avoid such calculations for the most part. give me a molecule to make or a mechanism to draw out and i feel at home.
austinap*
Guest

 Posted: Sun Jan 21, 2007 10:11 pm    Post subject: 19 Ugh, organic chemistry Biophysical chem is where it's at! Do you work in industry or academia or are you a student or what's the deal?
worm
unregistered

 Posted: Mon Jan 22, 2007 3:23 am    Post subject: 20 academia, but it's a pretty small university and i'm not tenure-track. i wanted small...nice to actually know who your students are. the non-tenure track part is b/c i just finished up grad school last summer, so this actually a "teaching" postdoc. are you still in school? what year? and what are you going to do with that biophysical chem?
austinap*
Guest

 Posted: Mon Jan 22, 2007 4:22 am    Post subject: 21 Yup, I'm still in school and probably will be forever (: I'm just finishing up my undergrad in biotech, biochem, and math and just waiting to hear from grad/med schools. I'm coming from a pretty small school right now (or at least, a pretty small department) and its very nice knowing all of my professors quite well. What do I want to do? I'm still deciding that for sure. I have interests all over the place. As far as chemistry goes, all the biologically relevant (well, directly at least) chemistry interests me, as does molecular and cellular biology. As of right now, I'm thinking that academic medicine would be a great area, but there are a lot of awesome things being done now so I'm not going to narrow things down quite yet. I have a feeling that I'll probably go to grad school next year (I have a few months to be certain) as I got a pretty sweet offer from UC Berkeley that would be really hard to turn down. Anyways, it was a fun debate! If you feel like it, drop me a line on AIM (AustinPitcher) or msn (austinaphotmail.com). Have a good one!
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