Tuesday, January 12, 2010

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

The question 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.

15 comments:

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:

http://sciencebod.blogspot.com

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":

http://www.sciencebod.blogspot.com

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.

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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.

Jae Kwon 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.