Argon walks into a bar. Barkeep says, "We don't serve noble gases here."
November 24, 2018 4:28 PM Subscribe
Scientists of Metafilter: Please explain to me how molecular mass affects gas behavior. Do heavier gases pool in low-lying areas at ambient pressure, and do lighter gases rise higher, or is this a myth? What about inside canisters, at higher pressures? Why do they or don't they?
Early in my piping career, other tradesfolk told me that if you somehow get a small amount of argon in your lungs, you have to stand on your head (or get your buddy to hang you upside down by your ankles) so that it can come out. Otherwise, they said, it will sit in the bottom of your lungs taking up space, since it's heavier than air.
But in my scuba training, I've learned that gases don't pool in cylinders. So an air cylinder or an enriched-air nitrox cylinder (for example) will both remain fully, evenly mixed throughout a dive. So is the argon thing a myth? And biologically, wouldn't my lungs be strong enough to expel the argon on an out-breath anyway?
Further confusion: At work, I was also warned that working in confined spaces, toxic gases can build up, which is why we use sniffers to test for air quality. This is obviously true, and air quality testing is the smart and safe way to go. But I've also used a sniffer when working in a deep trench, because of the risk of toxic gases building up in the trench.
If gases don't pool, how could toxic fumes build up in a deep (say 20') trench that's open to atmosphere along its entire length? If gases do pool based on molecular weight, why doesn't this apply to cylinders at pressure?
Obviously helium balloons float away in air (and an argon balloon would fall, or an air balloon, for that matter, due to the balloon weight itself) so the weight of the gas matters if you manage isolate it from the surrounding environment. So why do or don't the individual molecules behavior similarly? Do small chaotic molecular motions overcome any affect gravity would have?
I'm familiar with some basic gas concepts: PV=nRT, that the sum of the partial pressures equals the total pressure, and Henry's Law (at a constant temperature, the amount of gas dissolved in a volume of liquid is directly proportional to the partial pressure of that gas in the gas phase). But I'm having a little trouble figuring out/visualizing how these ideal gas laws would apply to gases separating or not separating based on mass.
*Punchline to title: Argon doesn't react.
Early in my piping career, other tradesfolk told me that if you somehow get a small amount of argon in your lungs, you have to stand on your head (or get your buddy to hang you upside down by your ankles) so that it can come out. Otherwise, they said, it will sit in the bottom of your lungs taking up space, since it's heavier than air.
But in my scuba training, I've learned that gases don't pool in cylinders. So an air cylinder or an enriched-air nitrox cylinder (for example) will both remain fully, evenly mixed throughout a dive. So is the argon thing a myth? And biologically, wouldn't my lungs be strong enough to expel the argon on an out-breath anyway?
Further confusion: At work, I was also warned that working in confined spaces, toxic gases can build up, which is why we use sniffers to test for air quality. This is obviously true, and air quality testing is the smart and safe way to go. But I've also used a sniffer when working in a deep trench, because of the risk of toxic gases building up in the trench.
If gases don't pool, how could toxic fumes build up in a deep (say 20') trench that's open to atmosphere along its entire length? If gases do pool based on molecular weight, why doesn't this apply to cylinders at pressure?
Obviously helium balloons float away in air (and an argon balloon would fall, or an air balloon, for that matter, due to the balloon weight itself) so the weight of the gas matters if you manage isolate it from the surrounding environment. So why do or don't the individual molecules behavior similarly? Do small chaotic molecular motions overcome any affect gravity would have?
I'm familiar with some basic gas concepts: PV=nRT, that the sum of the partial pressures equals the total pressure, and Henry's Law (at a constant temperature, the amount of gas dissolved in a volume of liquid is directly proportional to the partial pressure of that gas in the gas phase). But I'm having a little trouble figuring out/visualizing how these ideal gas laws would apply to gases separating or not separating based on mass.
*Punchline to title: Argon doesn't react.
Best answer: Do heavier gases pool in low-lying areas at ambient pressure?
Sure, if they are heavier than air.
Here’s a classic demonstration of floating a small boat on clear sulfur hexafluoride gas, which is heavier than air and stays in a vessel that you pour it into.
More complicated versions layer several gases of different densities like a tequila sunrise layered drink, and have a different little boat floating at each interface.
posted by SaltySalticid at 4:51 PM on November 24, 2018 [2 favorites]
Sure, if they are heavier than air.
Here’s a classic demonstration of floating a small boat on clear sulfur hexafluoride gas, which is heavier than air and stays in a vessel that you pour it into.
More complicated versions layer several gases of different densities like a tequila sunrise layered drink, and have a different little boat floating at each interface.
posted by SaltySalticid at 4:51 PM on November 24, 2018 [2 favorites]
Best answer: caveat: all this is As I Understand It, and is at the very least a big generalisation
gasses will remain separated if they're already separated and of sufficiently different molecular weights, but won't separate if already mixed - brownian motion and small-scale turbulence are enough to keep mixed gasses mixed. all the demonstrations of floating a little boat on a fishtank of xenon or sulfur hexafluoride involve carefully pouring the heavy gas carefully to minimise mixing. the heavy gasses pooling in a trench are coming from somewhere in pure or pure-ish form, not separating out from some mixture.
on a larger scale, there is separation. for example, the proportion of hydrogen and helium is elevated in the very high atmosphere, and the majority of gasses lost to space from the top of the atmosphere are the very light gasses. but for the scenarios you suggested, mixed gasses stay mixed.
with respect to clearing heavy gasses from the lungs, here's a video that debunks the story you've been told.
posted by russm at 6:20 PM on November 24, 2018 [2 favorites]
gasses will remain separated if they're already separated and of sufficiently different molecular weights, but won't separate if already mixed - brownian motion and small-scale turbulence are enough to keep mixed gasses mixed. all the demonstrations of floating a little boat on a fishtank of xenon or sulfur hexafluoride involve carefully pouring the heavy gas carefully to minimise mixing. the heavy gasses pooling in a trench are coming from somewhere in pure or pure-ish form, not separating out from some mixture.
on a larger scale, there is separation. for example, the proportion of hydrogen and helium is elevated in the very high atmosphere, and the majority of gasses lost to space from the top of the atmosphere are the very light gasses. but for the scenarios you suggested, mixed gasses stay mixed.
with respect to clearing heavy gasses from the lungs, here's a video that debunks the story you've been told.
posted by russm at 6:20 PM on November 24, 2018 [2 favorites]
Best answer: Heavier gasses (higher density than air at STP, Standard Temperature and Pressure) do pool, but they do not separate once mixed. Lighter gasses do rise, until they are mixed. Mixed gasses can only be separated by doing work. See diffusion.
Yes, argon in your lungs would be easily expelled by a forced, full exhalation, if you were conscious. I don't think there is any way that the gentle force of one gravity on your inverted lungs would be more effective. In this respect I have some experience with a gas that is denser than argon.
I don't know what the protocol would be if you were unconscious, however.
The higher the molecular weight of a gas, the longer it takes to diffuse into air, all other things being equal.
A finite amount of any gas released once in the bottom of an open trench will eventually dissipate entirely, through diffusion. A gas that is being continuously produced in the bottom of a trench, from industrial or geological processes (carbon dioxide, methane, sulfur dioxide, etc) will form a relatively stable gradient of concentration between where the gas is being emitted and the top of the trench, most concentrated to least concentrated.
The sniffer alerts you to regions in the gradient where the concentrations are high enough to endanger you.
posted by the Real Dan at 6:21 PM on November 24, 2018 [2 favorites]
Yes, argon in your lungs would be easily expelled by a forced, full exhalation, if you were conscious. I don't think there is any way that the gentle force of one gravity on your inverted lungs would be more effective. In this respect I have some experience with a gas that is denser than argon.
I don't know what the protocol would be if you were unconscious, however.
The higher the molecular weight of a gas, the longer it takes to diffuse into air, all other things being equal.
A finite amount of any gas released once in the bottom of an open trench will eventually dissipate entirely, through diffusion. A gas that is being continuously produced in the bottom of a trench, from industrial or geological processes (carbon dioxide, methane, sulfur dioxide, etc) will form a relatively stable gradient of concentration between where the gas is being emitted and the top of the trench, most concentrated to least concentrated.
The sniffer alerts you to regions in the gradient where the concentrations are high enough to endanger you.
posted by the Real Dan at 6:21 PM on November 24, 2018 [2 favorites]
Best answer: on a larger scale, there is separation. for example, the proportion of hydrogen and helium is elevated in the very high atmosphere, and the majority of gasses lost to space from the top of the atmosphere are the very light gasses. but for the scenarios you suggested, mixed gasses stay mixed.
Just to elaborate on this, the dynamics of many systems are described by a Boltzmann distribution. Under this probability distribution, the probability of a molecule being in a particular state is proportional to e-E/kT, where E is the energy of the state. A molecule's gravitational potential energy is mgz, where z is its elevation, m is its molecular mass, and g is the gravitational acceleration. Putting this all together, the density of a gas a height z is proportional to e-mgz/kT; and if you have two gases, the ratio of their densities decreases as e-(∆m)gz/kT, where ∆m is the difference between the molecular masses.
If you drop typical molecular masses into the above equation, though, you find that the relative density is doesn't change much until you get up to heights of a few kilometers; in other words, for differences in heights of a few meters, the relative density of the gases is pretty much the same at the top and the bottom of your volume. If you can effectively increase the strength of gravity, though, the concentrations can vary significantly over smaller scales. This is basically how gas centrifuges for uranium enrichment work; the more fissile uranium-235 is more concentrated near the center of the centrifuge, while the less fissile uranium-238 accumulates at the edges.
posted by Johnny Assay at 7:03 PM on November 24, 2018 [6 favorites]
Just to elaborate on this, the dynamics of many systems are described by a Boltzmann distribution. Under this probability distribution, the probability of a molecule being in a particular state is proportional to e-E/kT, where E is the energy of the state. A molecule's gravitational potential energy is mgz, where z is its elevation, m is its molecular mass, and g is the gravitational acceleration. Putting this all together, the density of a gas a height z is proportional to e-mgz/kT; and if you have two gases, the ratio of their densities decreases as e-(∆m)gz/kT, where ∆m is the difference between the molecular masses.
If you drop typical molecular masses into the above equation, though, you find that the relative density is doesn't change much until you get up to heights of a few kilometers; in other words, for differences in heights of a few meters, the relative density of the gases is pretty much the same at the top and the bottom of your volume. If you can effectively increase the strength of gravity, though, the concentrations can vary significantly over smaller scales. This is basically how gas centrifuges for uranium enrichment work; the more fissile uranium-235 is more concentrated near the center of the centrifuge, while the less fissile uranium-238 accumulates at the edges.
posted by Johnny Assay at 7:03 PM on November 24, 2018 [6 favorites]
Others are going over the science side here, but I've got to say that as soon as I read what your work colleagues told you (especially the bit about being held by the ankles) I immediately wondered whether they ever sent you out to purchase elbow grease, compass oil or striped paint.
posted by Acheman at 11:33 AM on November 25, 2018
posted by Acheman at 11:33 AM on November 25, 2018
Response by poster: Haha, good guess, Acheman. But no, no skyhooks here. It was actually in an OSHA-30 class and the welders teaching that *believed* it.
posted by cnidaria at 11:58 AM on November 25, 2018
posted by cnidaria at 11:58 AM on November 25, 2018
Others have covered the basic concepts well enough already, but I'd just like to add that carbon dioxide has a higher molecular mass than argon. And we expel carbon dioxide from our lungs with every breath.
posted by DevilsAdvocate at 6:34 PM on November 25, 2018 [1 favorite]
posted by DevilsAdvocate at 6:34 PM on November 25, 2018 [1 favorite]
This thread is closed to new comments.
posted by cnidaria at 4:50 PM on November 24, 2018