Tuesday, January 12, 2010

If CO2 is so heavy, why doesn't it sink and suffocate us?

A note about the author (added October 6, 2016):

 Thanks for visiting this post, penned some 6 years ago. It's still attracting 15-20 visits per day! (Maybe it's that graphic below of those thermally-agitated  gas molecules that attract visitors, rather than the wordy science  per se - yup, I'm a down-to-earth realist).

Before proceeding onto your perilous journey into the intricaies of how molecules - light versus heavy-  respond  (or don't) to the Earth's gravitational field, please be advised that this investigative blogger/retired scientist has made other contributions to knowledge (or as some unkind folk would say - fantasy). There are two in particular I wish to flag up:

1. Stonehenge. I believe it to have been designed initially for SKY BURIAL, or as I prefer to call it, AFS (avian-facilitated skeletonization). 

See most recent posting on this site.

Ugh! Yes, I know - hardly the stuff of after-dinner conversation.

2. The Shroud of Turin. I believe it to be a medieval fake, consistent with the radiocarbon dating (1260-1390). I believe it was intended to reproduce what Joseph of Arimathea's 'fine linen'  - used simply to transport the crucified Jesus from cross to tomb -  would have looked like 13 centuries later if it had captured a body imprint in SWEAT and BLOOD. I have demonstrated in my own home how the enigmatic body image could have been imprinted onto wet linen using an adult male volunteer.  The lucky fellow was first smeared with vegetable oil, then coated with a dusting of white flour.  Heating in a bread-making oven or similar (the linen, not the bloke) would have produced the sepia body image, later attenuated  to its final 'ghostly' appearance by vigorous washing with soap and water.

See this recent posting on my specialist Shroud site.
Naughty, I know,  inserting this 'commercial'. Thanking you for your forebearance (but do please visit those two sites if you're interested in cutting-edge stuff that's still to be picked up by the meeja)

 Back now to those bustling molecules.

Thanks to kinetic energy, there will be no appreciable 'unmixing' - or even partial stratification,- at least under  normal g values. That's the case even if one type of molecule is much heavier than the other

The question in the title was inspired by a comment on a site I visit a lot, one to which I've posted a lot in nigh on three years. More about that later. Suffice it to say that I shall reply to here, rather than there, and attempt to place a link on the other site. There will be no more posting to that site until such time as its moderation policies are given a thorough overhaul! Nuff said for now...

The question was essentially this:  given that we all know that petrol fumes sink to the ground at a filling station, why doesn't CO2 - which we also know is denser than air - also settle at ground level? Why are we not suffocated by the stuff - or does it only come up to ankle or knee level?

Imagine one were to trap gases inside balloons - one for hydrogen, one for oxygen, one for nitrogen, one for carbon dioxide - and then release them. The four balloons would behave exactly as the questioner supposes. The hydrogen balloon would quickly ascend, the CO2 balloon would rapidly descend, and the nitrogen and oxygen  balloons  would probably hover or sink slowly - due mainly to the weight of the balloon rubber - not the contents. The relative densities of hydrogen :nitrogen: oxygen: air::carbon dioxide are approximately 1 : 7 : 8 : 7.2 : 22. Gases lighter than air rise, those heavier than air sink. No surprises there.

When petrol fumes are released, they too sink quickly, at least to start with. A typical molecule in petrol is  one of the isomeric octanes,  general formula C8H18, with a relative vapour density of 57 - some 4 times greater than air.

But the petrol fumes would  not stay for very long at ground level. Convection current carry them upwards, and gaseous diffusion would cause mixing with air even without convection. That's because gas molecules are in a state of constant motion, colliding with other molecules, millions of times a second, causing them gradually to diffuse ("spread") in all directions. The fumes gradually spread into all the space available - which could be a jar, a garage, a hangar, the entire atmosphere. Once the space is evenly occupied, the molecules then show no tendency to unmix. Why not? Answer: because the 1g force that acts on all molecules in air at sea level is insufficient to overcome the kinetic forces due to collision between molecules. Put more simply - a molecule that gets a strong bump from below will be knocked upwards, against the weaker force of gravity.

This is true for g=1, but is not true for progressively higher g forces.

Here's an example - always a a controversial one. Enrichment of the fissile uranium isotope U-235 needed for atomic power stations OR Hiroshima-type A bombs, requires separation from the more abundant U-238. This can be accomplished in gaseous diffusion plants, or in centrifuges that generate an intense g force. Either process requires that solid metallic uranium  first be converted to the gaseous uranium hexafluoride (UF6).

There is a well known experiment that is done in schools, at any rate, those that still have  a fume cupboard, to demonstrate that dense gases and/or vapours gradually diffuse to fill the space available, and then do not subsequently unmix from air.

One places of few drops of elemental bromine, Br2, a fuming red liquid in the lower jar, which is separated from the upper jar by a glass divider. One waits for the lower jar to fill completely with red-brown fumes. One then removes the separator. The fumes gradually fill both jars evenly, despite bromine vapour being 5 times denser than air.

There's a variant on the experiment that I devised while teaching to demonstrate the petrol vapour effect. One places a jar of bromine on top, and then removes the divider. Most of the bromine fumes sink immediately into the lower jar, behaving as if they were enclosed in a balloon. But the fumes then gradually diffuse back upwards to produce the same end-result as before.

The short term behaviour of the petrol fumes is called a bulk phase effect. It's the temporary behaviour of heavy molecules in close proximity, which behave briefly as if enclosed in a balloon. But once diffusion has caused mixing of heavy molecules with the lighter molecules of nitrogen and oxygen, unmixing does not occur at normal values of g.

Think then of a gas before diffusion and mixing as a kind of ghost fluid, with its own density and  buoyancy characteristics. In fact, while the term "fluid" in everyday life is synonymous with "liquid", in physics it applies to both gases and liquids. But a gas  loses the distinguishing characteristics of its original 'fluidity' once it's had time to spread sufficient for its own kind of molecules to become separated and  irreversibly mixed with other kinds of molecules.

Further reading:

Will mixed gases spontaneously unmix?


Here's another that's quite thought-provoking, once you get past the unpromising preamble

It has some useful qualifying material at the end re altering the composition of  atmospheric gases with increasing altitude, which I've quoted below:

Finally, even if the air were completely and 
perfectly still, the carbon dioxide would 
not form a pool on the surface. There is a 
"dynamic equilibrium" set up between 
gravitation -- the tendency for the denser 
material to go to the bottom -- and diffusion 
– the tendency for a material not to 
concentrate in one place, but to spread 
itself out. The atmosphere we have contains 
roughly 78% nitrogen, 21% oxygen, 
0.93% argon, and 0.036% carbon dioxide.
Its composition does not vary until 
you get above 80 km in height. If the air 
were perfectly still,
its composition would be
Ground level: 75% nitrogen, 23% oxygen,
1.3% argon, 0.055% carbon dioxide 
10 km high: 79% nitrogen, 20% oxygen, 0.75% argon, 
0.026% carbon dioxide
20 km high: 82.5% nitrogen, 17% oxygen, 0.43% argon,
 0.012% carbon dioxide
So even in these circumstances, the 
heavier gases like carbon dioxide would 
have higher concentrations
lower down, but could not form a lethal pool.

So there is an effect (of sorts) that relates to molecular mass ("heaviness"),
even if there is no unmixing as such. Does that contradict anything that precedes
it in this posting?  Discuss. :-)

Here's my own interpretation, for what it's worth, of the grading by molecular size/mass with increasing altitude (added September 17 2014)?

As one gets higher, the air thins (this being due to decreasing gravitational pull on everything, gases included, such that most gas is held relatively close to the Earth's surface (a few tens of  kilometres). But another effect can then operate that discriminates according to molecular mass. It's to do with the average spacing between molecules and their average speed (best measured we're told as the root mean square velocity). As the molecules become further apart they can travel further using their own intrinsic motion before colliding with another to be deflected off in a different direction, impeding upward progress.

 But a light molecule travels faster at a given temperature than a heavier one. It's one of the givens of kinetic molecular theory. It explains why hydrogen gas diffuses faster than carbon dioxide. Thus there is a greater probability that a lighter molecule like hydrogen will be able to cross a given transient empty space faster than a heavier one before the gap, so to speak, closes up.  Ipso facto, light molecules have a greater probability of  "winning the race" to the top of the Earth's atmosphere.  But having got there they will find they are still held by the Earth's gravitational field  albeit much weaker than at ground level, unless exceptionally light, like hydrogen and/or helium atoms which we are told can and do escape from the Earth's atmosphere, leaking off into interplanetary space.

 Summary: what's operating is not settling out of heavier molecules in response to gravity. It's the speedier motion in an otherwise unfavoured direction (upwards) of molecules that are LIGHTER and thus able to DIFFUSE faster!   (They would diffuse anyway, whether or not a gravitational field was present, so gravitation becomes a secondary consideration).

Even further reading: see the excellent wiki entry on 'Atmospheric Escape" which also focuses on differential rates of gaseous diffusion.

Note: I'm pushing the limits of my physics in offering the above interpretation. If folk find it faulty, and/or can offer a better explanation for the sorting-by-size effect with altitude, then please feel free to comment. However, I do not consider the phenomenon is serious enough to challenge the generalization that gases do not spontaneously and efficiently unmix of their own accord, at least in a natural gravitational field around a planet-size object. Random molecular motion with constant collisions always ensures that molecules will never completely unmix, while accepting there can be concentrating effects of the kind described under normal or elevated g forces.

Here's a handy link to 'Physics for Dummies' with a section entitled "Using the Kinetic Energy Formula to Predict Air Molecule Speeds".

The takeaway message is the inverse square law that relates molecular mass to average speed at particular temperatures. A nitrogen molecule, N2, can be calculated to have an average speed of 508 metres per second  at 28 degrees C (301K), though it would have to be in a perfect vacuum to be able to traverse that distance. A molecule that was 4 times as heavy would travel at half that speed, a half being the inverse square root of 4. A molecule that was half as heavy would travel at, er, darn, where did I put that pencil? Off the top of my head I think the answer is root 2 times as fast. That's approx 1.4 times as fast, i.e. 40% faster or thereabouts. A hydrogen molecule, H2, has 1/14th the mass of a nitrogen molecule, N2, so would travel  (1/root 1/14) i.e. approx 3.75 times faster.

Addendum, 18th September 2014

I'm not sure I've adequately explained the difference between having a gas confined within a balloon or not, especially as regards the 'thought experiment' of taking the balloon away to watch the disappearance of density characteristics, "sinking" etc,

Here's a home-made diagram that will be used to describe my current (and still evolving) thinking on the subject:

Brace yourselves. More to come

First, look at the right-half of the balloon, and imagine the left were the same, i.e. an intact envelope enclosing gas all the way round. In that situation, the normal laws of buoyancy would apply, which as a revision exercise I shall now show in three diagrams (A-C)  filched from the internet, of increasing detail and complexity.

 Diagram A:

 Diagram A above shows an object (only) partly immersed in a fluid, which is subject to two forces: gravity, pulling it down, and "buoyancy" pushing it up. But what is the nature of the buoyancy force? The diagram does not explain. Let's look at another which does.

Diagram B:

 This diagram shows an object fully immersed in the fluid (I wish that Diagram A had too, but beggars/filchers can't be choosers). Note that the fluid exerts pressure on the object, that the pressure acts in all directions, that being the nature of pressure as a result of billions of random molecular collisions per second that have no single directionality. Note the upwards pointing arrow in the middle. Why is the nett force upwards ("buoyancy"). Again, the diagram does not explain. For that we need to go to the next diagram.

Diagram C:

 What this diagram shows is the imbalance of forces acting on the immersed object. The pressure at the bottom (pressure being force per unit area) is greater than at the top, and indeed greater than at all points between top and bottom. In other words, the nett force is upwards. the nett upwards force is called the UPTHRUST.

For the object to float, the upthrust needs to be greater than the weight of the object in air. For the object to sink, the upthrust must be less than the weight of the object. Upthrust can be measured as the weight of fluid displaced (handy for calculation, while not giving insights into the mechanism of upthrust which as explained is due to increasing pressure with depth producing an imbalance of forces between highest and lowest points).

Now let's return to our sealed/soon to become leaky balloon and compare with the three diagrams above:

Hopefully, dear reader, you can guess what is coming. While that balloon is intact, with the same kind of molecules all packed together, exerting their particular density characteristics, whether smaller or greater than the surrounding air, then the balloon goes up or down, following the laws of buoyancy, the gas behaving just the same as any other fluid.

However, imagine that balloon envelope suddenly becoming permeable, with gas escaping and mixing, then one no longer has a homogeneous fluid of characteristic density and buoyancy. As the escaping molecules begin to mix with the surrounding molecules of air, then the bulk effects disappear, the molecules then behaving more or less independently from their neighbours, now increasingly different.  What matters now are not the original bulk properties that respond to gravity, and/or the contingent pressure differences that depend on gravity,  but the behaviourof the individual particles comprising the originally-enclosed gas, which is now determined by their intrinsic molecular speed, which is in turn a function of temperature, kinetic energy, mass and velocity, summed up in the term diffusibility.

Interestingly, there's a transition period between release from a confining receptacle and complete mixing (whether by slow diffusion, or aided by air currents etc) when the body of gas is still sufficiently discrete to continue behaving as a fluid. Some of us recall the demo experiment in school chemistry labs where teacher takes a jar full of CO2 gas and "pours" it over a candle or lit Bunsen burner, the flame being instantly extinguished in both cases.

Come to think of it, might the idea that CO2 "sinks and suffocates" be based on reports where the gas has been released from underground, say,  or under water (as in the 1986 Lake Nyos disaster in Cameroon) , in both instances in regions of volcanic activity where the gas has vented from subterranean magma, and then flowed as a 'fluid' for a considerable time before there was time for mixing to occur?

Lake Nyos disaster, 1986
 But that is not CO2 settling out from a mixture, needless to say, which by now I hope is an idea that no one will entertain. The lethal invisible blanket of gas issuing from the Earth's bowels has been able to retain its density characteristics in the period between initial venting and  subsequent mixing with other gases in the atmosphere, notably oxygen and nitrogen.

Update: September 22 2014

I discovered today why this posting attracts far more visitors each day than any of my other postings, despite having been written some 5 years ago. Assuming that most visitors were finding it via their search engines, I tried entering strings of search terms that correspond with the title, and then whittling them down to a core set. To my surprise, I find that one has simply to enter (CO2 heavy)  and this posting tops the list of returns! It's clearly achieved that virtuous circle, aka critical mass, where its present prominence helps ensure continuing prominence!

Never one to rest on laurels, I've been making some additions by way of afterthoughts, and picking up on points that others have raised elsewhere, notably on science discussion forums where the content comes chiefly in serial additions from the participants themselves, starting with someone's primer question. In fact there's just such a forum that arrived three years after this one, posing essentially the same question, and is now third in my list of Google returns.

From 'spoogington' some 9 months ago, currently with 109 comments:

If CO2 is heavier than O2, why is our atmosphere not stratified with a layer of CO2 closer to earth?

Looking at the points made, I'm more than ever convinced that I was right to raise the question, since clearly there is some confusion in people's minds (as there was initially in my own) as to the importance or otherwise of bulk density v molecular weight where the behaviour of 'heavy' gases is concerned, before and after mixing in a gravitational field.

At the risk of giving this post an intimidating length, I  might try supplying my own answers to some of the points raised. Or there again, it might be wise to create a separate follow-up  post so as not to overload this one.

Oh, and here's a link to a climate change sceptic, maybe denialist even, who seems to think that CO2 is too heavy to get into the upper atmosphere.  In fact,the faux science  gets worse as one reads on, much worse.

Sample: (my italics)

 "How mad with power does a group of people become that they now want to control, CO2, a naturally occurring colorless, odorless, incombustible gas formed during respiration, decomposition of organic substances, volcanic emissions, decay of plant and soil organic matter? A gas that was intelligently designed to be heavier than air for a purpose. How crazy is that?

I invite him to read this posting and reconsider.

October 5, 2014:

Have just come across this blog, with a brief mention at the end of a delightful reductio ad absurdum argument. Look for the term "layer cake atmosphere".

October 6, 2014

And here's a must-see paper from a kindred spirit (an Italian caver at Turin University) who unlike myself has the maths to support the theory. The title says it all. Click to enlarge if you wish to read the abstract.

It's available as a pdf:

Note the date of first publication: April, 2009, i.e. some 9 months before I penned this posting. But I'm not a plagiarizer, honest, no, really, HONEST, not having spotted this paper until just a few days ago.

New addition

November 8th 2014

Here's an additional 'thought experiment'  using CO2 and helium filled balloons to demonstrate that once mixed, gases do not unmix.

I composed it yesterday as a new blog posting, and intended to add a brief summary here. Being somewhat busy right now, here's a cut-and-paste of the entire posting, which I shall endeavour to prune when I've some fee time.

Still they keep coming, to a posting I did nearly 5 years ago.

Posted on this site, Jan 12, 2010
It wasn't as if  CO2 and its behaviour in a gravitational field was a burning issue at the time. In fact that posting was a side-issue from my then preoccupation with the climate change/global warming debate (which I was glad to withdraw from, given the brow-beating denialist tendency on MSM forums).

It was provoked as I recall by a barmy comment to the effect that CO2 had been made "heavy" by a Benign Presence (Gaia? Guardian angel in attendance?)  so as to stay at lower altitudes, feeding our plants, not reaching higher altitudes where it might cause mischief!!!

Here's a snapshot from my sitemeter, showing that 11 of the last 20 visits have been to that posting  (13 is a more typical average).

The red tags are mine, pointing to the CO2 posting

  Quite why it still gets all the attention I haven't a clue. Maybe it's because  I'm a Londoner it's currently the second  listing one sees if entering (CO2 heavy) into Google. Sometimes it's the first he says in a rare moment of modesty-bypass.

To be honest, I've been somewhat embarrassed that a instant tutorial should now be seen as the first or second stop on a simple non-technical web search. So much so that I gave it a spring clean this last September, adding bits here and there to strengthen the case (as I'm only 99.9% certain about my conviction that  normal g forces - from the Earth's own pull - are insufficient to cause unmixing of CO2 and stratification, ONCE THE GAS has diffused and mixed with the nitrogen and oxygen of air).

With that as preamble, folk should perhaps understand why I'm back again, still fine-tuning, still whittling down that stubborn 0.1% of doubt.

What I wish to describe now is a thought experiment. (Yes, I  know it should or could have been a real one, but if thought experiments were good enough for Albert Einstein then they are good enough for me - that's my story and I'm sticking to it).

It's a development of the 'teaching aid' in the original posting, which began with a brief look at the behaviour of gases trapped inside balloons, where they do indeed show their heavier or lighter-than-air characteristics. That's before the gases have escaped from their balloons, then diffused and mixed with air and lost their ability to "sink", settle out, stratify, call it what you wish.

Last night I had a brainwave. Why not keep the gases trapped inside their balloons, and allow them to mix by diffusion (which may take a few minutes, possibly a lot of minutes for totally even and homogeneous mixing, but mix they will, such is the nature of gaseous diffusion).

 How might the behaviour of the balloons compare before and after mixing? Let's do that thought experiment.

Before opening the valve: the two attached balloons may ascend, descend, or stay put, depending on the relative size of the two balloons, and the average density of the two gases compared to that of the surrounding air. If the average density is less, the system ascends etc. (And it won't matter a jot whether the gases are separate or pre-mixed  or post-mixed for that to be true - important for what follows).

But one thing's for certain. The two balloons will remain oriented with respect to each other, as in the diagram, with the blue helium balloon on top. That's because it always experiences more upthrust than the red balloon, displacing a greater volume of air for a given weight of enclosed gas. If one attempts to alter the stacking geometry, the system will self-correct when released.

Now let's picture what happens if the valve is opened, or the fusible wax plug is melted, allowing the two gases to mix. One could allow mixing by diffusion only, which means a lot of waiting. Alternatively one can speed up mixing by inverting the balloons as shown below.

The lighter gas helium, now underneath, will ascend; the heavier CO2 will descend, and being a two-way countercurrent system there will be faster mixing than if it were by diffusion alone. A series of inversion, re-righting, re-inversion etc should result in a homogeneous distribution of gases between the two balloons.

What happens when one releases the two (still attached) after mixing?


If the original system ascended, so will the new one.

If the original system descended, so will the new one.

If the original system was perfectly balanced, neither ascending nor descending, so will the new one. 

But there will be a difference. Both balloons are now equally buoyant (or non-buoyant, depending on the proportions of the two gases). So there will be no tendency for one balloon to be above the other. In other words, the two balloons can adopt any configuration through 360 degrees (with a slight tendency maybe for the smaller balloon to 'lead the way'  if rising or sinking, due to aerodynamic differences).

So the two attached balloons might go up, go down, or stay put, looking like this:

or like this:

or any angles of rotation in between.

One thing's for certain. Restoring the original configuration to blue on top, red underneath will not cause reversion to the original self-correcting orientation, since that would require that the gases unmix, with CO2 going back into the red balloon, helium going back on top.That as we've seen. simply does NOT happen at normal values of g. Which is where we came in...


Anastasia F-B said...

Hey, Colin. It all looks good. :-) I tried to add myself to your followers but the connection is undergoing maintenance at the present, so I'll come back later. By the way there was one w too many on your link. It took me to a page on Metaphysics and Psychic Reading. :-))

sciencebod said...

Delighted to hear that my typo - of which there's an increasing number these days - took you somewhere interesting, Ana. I trust you found deep spiritual enlightment there.

Don't be surprised if my wife "Sheona" makes contact- cyber-contact that is. There's reason for thinking that the two of you are kindred spirits of sorts. It's that mention of yours re having attended a top girls' school- with thumbnail piccy attached. If it's the school Sheona thinks it is, she taught modern languages there for a number of years, but left probably a year or two before you arrived. I once had tea with the headmistress one Sunday, with the pupil creme de la wotsit in attendance. Fantastic grounds.

Have a look at my new site if you like to see the Byronic side of my nature:


I've left out the wwww(w) this time. ;-)

sciencebod said...

Now then, why didn't that appear as a proper hyper link? Maybe it does need those www. Let's try again:

Link to "A Rough Ride on MyT":


sciencebod said...

Oh well, never mind. Blogger doesn't do links it would appear. Our is not to reason why.

Btw, I envy you that full screen width on your blog, Ana. I assume you or a friend have done some tweaking on the HTML template to achieve that, since I don't recall any of the set templates offering a full-screen option.

Anastasia F-B said...

I just went to the customise option on Blogger, Colin, and played around until I got something I liked. Oh, BTW, Schooldays is also on Ana the Imp, with some pictures. :-)
I'll have a look at your link, thanks.

PS, I see I have incresed your following 100% :-) I'm surprised you don't have more. I've put myself around about; that always helps, assuming, of course, that you want to increase your following. You can check out some of the sites I've joined on Ana the Imp and my AnastasiaFB page on Blog Catologue.

Anastasia F-B said...

*Catalogue. Yikes!

Anastasia F-B said...

Oh, sorry; you've seen the picture of my old school. I thought perhaps you had only read this piece on My T.

sciencebod said...

A science site attracts a pretty narrow clientele at the best of times - especially a generalist one - strange though that may seem at first sight. But then there are lots of general science sites - New Scientist, BBC etc- for whetting the appetite for non-specialist scientific novelty.

This site serves a more personal archival role - especially to record my first reactions when new important announcements are made - sadly few and far between these days - but does get a passing readership, as revealed by the sitemeter, mainly from folk googling this or that.

One just wishes there were more like yourself Ana, taking the trouble to leave a comment - if only to say I'm bonkers!

You do realise that your own site is an exception to the vast majority of personal blogs, don't you Ana - as a peek at your meter shows? It's dazzlingly successful - in terms of content obviously - but especially re hits and followers. You are a class act - I believe I was the first to say as much on MyT while the cynics were muttering their inaninities about you being the front for an organization, or much older than your declared years!

Having taught, I've had time to adjust to the experience of being outshone by people a fraction my age!

Was Mrs. B****n your Head of Languages? ;-)

Anonymous said...

Not sure that this is entirely correct. There will always be some fractionation of the different components, or in other words the concentration of CO2 would always be slightly higher at lower elevations. The solution of gases can increase its overall entropy by mixing, but the number densities of each molecular component will still be distributed exponentially (at equilibrium) with height according to its mass density (http://www.shef.ac.uk/physics/people/rjones/PDFs/PHY101/PHY101_RALJ_lecture6.pdf). It is not precisely true to say “1g force that acts on all molecules in air at sea level is insufficient to overcome the kinetic forces,” rather 1g of acceleration is not sufficient to fractionate the mixtures to as noticeable a degree as you would see when placed in a centrifuge. There isn’t a “threshold acceleration“ where the solution all of a sudden separates, it’s a continuous process.

Unknown said...

Blogs are so informative where we get lots of information on any topic. Nice job keep it up!!

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Doug Lohre said...

But don't the collision forces from other molecules acting upward AND downward statistically cancel each other, leaving a net small force of gravity acting downward, causing the CO2 to sink? I mean I understand it DOESN'T sink, but doesn't this contradict your explanation?

sciencebod said...

Aren't you forgetting something, Doug - ENTROPY? You start with a somewhat ordered arrangement, at least for a gas, namely a lot of the same molecules, all confined in the same volume of space. Once those molecular collisions begin to knock molecules into the surrounding space, the mixing effect is irreversible, because you know on probability and statistical grounds that once gases are mixed, they do not spontaneously ummix. That's because new collisions do not and cannot restore the initial arrangement that existed initially.

The example I used to give my students was that of their bedrooms. If they were perfectly neat and tidy (highly improbable scenario admittedly) then a gust of wind through an open window would create disorder. But if the initial state were disordered, then the probability of the same gust of wind creating order is essentially zero, there being an almost infinite number of other disordered arrangements compared with tidy conventional ones, with books in rows on shelves, not in rows on the bed.

The effects of gravity are minute when compared with the spontaneous mixing due to molecular motion.

Unknown said...

But what if the air column were long (high), would the effect of gravity have more of an impact on high columns?

I bet it would. The time and column height might be inversely proportional to g.

If so, we can capture carbon dioxide by just blowing air into a long column, and let gravity do the work.

Unknown said...

All being said on this matter, where should I place my carbon monoxide detector, on the first or second floor of my house?

sciencebod said...

Hi DanB

I don't know if there's any official advice on that. Have you tried goggling (carbon monoxide detector best location house)?

Commonsense would say you put the detector as close as possible to the most likely source of CO, which in most people's homes would be the gas central heating boiler or gas cooker. Don't worry about not being able to hear it!

Unknown said...


Can you help clarify this in the upper atmosphere for us.

If gases do not unmix till over80/KM how is there an ozone layer. How do CFC's accumulate at this layer, 10-50 KM's. Shouldn't this just be distributed through the whole atmosphere.

sciencebod said...

Interesting question Mike. I'll need a day or two to think about it.

sciencebod said...

I still need more time to research your comment, Mike. For now, I'd just say this. The reason for upper-atmosphere ozone being way up there, but below the 'unmixing' layer, may have something to do with the way it's formed. Incoming solar radiation has first to split O2 into its separate atoms. Those O atoms then have to find another intact O2 molecule to form O3, and to do so before they simply collide with the lighter O atoms to reform O2 ("futile cycle"). Maybe there's an optimal height for ozone formation and the subsequent uvb-absorbing oxygen-ozone cycle to operate.

As for CFCs, I wasn't aware they concentrated at a certain height. I thought they just got there from our old fridges etc by diffusion, a process taking months maybe years. Once there, they cause destruction of ozone by free- radical processes (initiation/propagation/termination). I seem to recall reading that one CFC molecule can destroy 19,000 (?) molecules before a free chlorine atom or other CFC-derived free radical meets another of its own kind to finally terminate the chain reaction.

sciencebod said...

Nicely put, Mike.119

sciencebod said...

Nicely put Mike.

Unknown said...

My comment concerns the relative concentration of CO2 in the air that we breathe while sleeping in a closed room. Assuming a CO2 exhalation rate of 200ml/min over 8 hours, 2 adults would produce a total of approximately 6.75 CF of CO2. In a 12x15x8 bedroom, 1440CF, this would result in increasing CO2 from 350ppm "fresh air" level to just over 5,000ppm by morning assuming no air leakage from the room.

Breathing, however, is an oscillating air flow (inhalation, exhalation) at a point (our nose). Thus the air that we breathe IN every 5 seconds is in part the same air we exhaled.

Besides gaseous diffusion, air currents due to breathing will cause some degree of mixing and dilution of CO2 levels. The assumption of perfect diffusion throughout the room, however, must be questioned.

This issue begs the study of CO2 concentration (near our nose) while sleeping versus the rest of the room space to get a better understanding of what is the percentage of CO2 in the air that we breathe while sleeping over the course of the night.

A low-speed ceiling fan would seem to offer a good solution to meeting the assumption of perfect diffusion.

Unknown said...

actually I thought your dual balloon experiment would result in whichever balloon is below the other becoming marginally denser as CO2 conc. would be higher further down as per your stats earlier regarding atmospheric concentrations in a static system. The gas filters down until it reaches equilibrium between gravitational forces and kinetic forces that you mentioned earlier, so that if the conc. lower down is reduced, say by plants, more CO2 will replace it from higher up.

PMcKay said...

Fantastic blog. I've asked many Chemists over the years about this, and all talk about the 'buoyancy' of the gas causing lighter gases to rise. I've always thought the velocities would cause complete mixing over time. In my line of work I use many different mixtures of gases in cylinders (for calibrating industrial gas analysers) - these cylinders have laboratory certified concentration, none say" shake before use" despite containing mixtures of hydrocarbons ranging from Methane through to Nonane.

Phil (Yarra Valley - Australia)

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Julian said...

Great blog.
You should update your posts with interesting comments exchanges as mist prople dont read the comments.
Sldo, I would be interested to find out if you had any further thoughts on the Ozone layer and CFC question

Thank you for your time to share and educate young and old alike


Unknown said...

I found this post searching "is co2 heavier than air?". I was thinking about home brew beer what happens in the air between the brewing beer and the air-lock. It starts out as a stand mux of air. My original thought was that the CO2 generated as a byproduct of fermentation would be heavier than the rest of the air and push that air out through the air lock. So after enough fermentation only CO2 would be left. But now after reading your post I suspect there is still some other gases in the head room even after a large volume of CO2 is generated.

Another topic would be carbonation. I have heard that CO2 dissolves into a cooled liquid faster. I'm not sure if that is true or not. For making home made ginger beer, I let it ferment for a day or two in a sealed plastic bottle. Then put it in the fridge when it becomes under pressure. If I open it right away most of the CO2 escapes and it is flat. If I wait a few days in the fridge, the ginger beer becomes carbonated. The other benefit obviously is the fridge stops the fermentation.

Unknown said...
This comment has been removed by the author.
keith said...

i got here by way of searching carbon dioxide in balloons. Thank you for intelligent discourse.

Unknown said...

Greetings. I’m a 60 yo layperson who now enjoys reading science blogs (C-student in High School) and I have a question. While reading about arguments for/against a young earth, I came across the position that the levels of Helium-4 in Earth’s atmosphere should be much higher if the Earth was 4 billion years old. The writer states that the reason for the lower levels of He-4 found in our atmosphere is because of the ejection of He-4 at the poles caused by the Earth’s magnetic field, sort of a ‘polar wind’.

Here’s my question: Are other molecules also subject to the same forces at Earth’s poles? Does the Earth ‘cleans’ it self, keeping a ‘natural’ balance of nitrogen, oxygen, argon and carbon dioxide?

And BTW, thanks for writing and presenting your ideas the way to do so that the rest of us mortals can understand the principles being presented.

sciencebod said...

Fascinating Mark. Sorry - I plead ignorance. This blogger has some reading and catching up to do re He-4 - and then those other gases too.

Enjoy what's left of Christmas Day...

Colin Berry, UK.

whitefarmer said...

i think you are incorrect in saying that it is because of gravity decreasing the more one goes out into space that there is decreased atmospheric pressure there. that would not explain why venus has 92 bar at its' surface but is slightly less massive than earth. the actual reason is because of the weight of atmosphere resting on top of the lower layers.

also in your example of a balloon with helium and one with CO2 being connected and let mix you say that the balloons have a different upthrust (as if they were different sizes). but upthrust is the same since the atmosphere is the same for both. the difference however is the amount of downthrust that the CO2 balloon gives compared to the helium one.

whitefarmer said...

BTW I found your page when searching the density of Nitrogen in comparison to CO2 with reference to the HAVOC mission to Venus.

GrahamW said...

I agree with whitefarmer. After a quote discussing gas proportions up to 20km, you write: "As one gets higher, the air thins (this being due to decreasing gravitational pull on everything, gases included".
Your argument depends on gravity being significantly less at a radius of 6390km than it is at 6370km. I believe that comes out to about 0.6% reduction in gravity. That's surely trivial compared to the effect of pressure attributable to weight of air above, which is reduced from 1 atm at sea level to less than 0.1 atm above 12km apparently.

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Mike McKeaney said...

I have a related question. If CO2 does not sink in open space, due to mixing caused by air currents, then why is the ocean considered a massive CO2 sink? It seems that between the near-constant wind and the sloshing of waves at the surface, this would be impossible.

Also, how can 400 ppm of CO2, the equivalent of 4 people spread out through a 10,000 seat arena, trap heat? This has always intrigued me and I've never seen the topic discussed.

John W, Plymouth, MI said...

Thank you. I was wondering why the radon does't stay in the basement.

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