July 18, 2019, 03:38:53 PM
Forum Rules: Read This Before Posting


Topic: Does CO2 sink in air due to density, or does it mix due to like dissolves like.  (Read 4324 times)

0 Members and 1 Guest are viewing this topic.

Offline delta609

  • Regular Member
  • ***
  • Posts: 22
  • Mole Snacks: +3/-1
I understand all of the elemental basics of understanding density and solvation.  I really just want an explanation of what really happens in air. 

I would guess it just mixes, unless maybe the air was completely still.  Then I would guess it would sink.  However, kinetic energy is constantly being exchanged between molecules in air, so I figure air is never still?  Am I correct on this notion?

Offline Arkcon

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7360
  • Mole Snacks: +533/-146
Yes, someone asked a similar question on this forum before.  The gasses that make up Earth's atmosphere don't stratify by weight.  There is too much motion mixing them for that to happen.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline delta609

  • Regular Member
  • ***
  • Posts: 22
  • Mole Snacks: +3/-1
Thank you.  I probably should have used the search function.  I will next time.

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
even if the air was completely still on a macroscopic scale, the gases still would mix up over time anyway (as to be observed in wineries: carbon dioxide will not stay in the cellar forever), as gas particles at the usual temperatures are in perpetual motion, and hence "self-mixing".
as a consequence, on small scales ( some cubic meters, if you wish) gas mixtures seem to be homogeneous, and stay so forever  (for example, for a diver, the gas feed nitrogen:oxygen from his bottle will be the same, no matter if he'd dive upside down or not)

however, on lager scales ( i.e. some 10th of kilometers, like in the atmosphere) a different effect will show up distinctly: in a gravitation field, each gas will have it's own height distribution as a direct function of its MW.
hence, indeed nitrogen:oxygen will have less oxygen the higher you went in earth's atmosphere, and talking like a hundred kilometers ( or some kilo-g's like in ultra gas centrifuges), this will show.

I wouldn't call this "stratify" though, as there is no sharp boundary : this is more like a differential effect.

There is stratification in earth atmosphere nevertheless, no question - but this is due to completely different effects, and next to independent from gase's tendency to mix up over time and to show individual height distribution all the same.


regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline billnotgatez

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 3778
  • Mole Snacks: +208/-55
  • Gender: Male
Just to add to the question here, since many can be confused (including me).

If you take a container of hydrogen and open it up to the air and wait a few minutes people believe that it will rise quickly and go to the top of the atmosphere.
I have not done the following experiment but I have seen a few videos where hydrogen is released and it appears none is left to catch fire.

I suspect if you open a container downward of carbon dioxide and wait a few minutes a flame will not be quenched at the open the container opening.

Maybe if I searched better I might find videos to back up what I am suggesting.

comments??

EDIT
by the way, It is said that if you release Helium, it rises to the top of the atmosphere and is lost forever.


Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
there is a huge difference between the behaviour of an encapsulated gas body (like, for example , in a balloon) leading to buoyancy (in an external gas body of  different density), and the behaviour of "free" gas particles: don't confuse the physics of the one phenomenon with the other.

whenever two (initially separated) gas bodys are allowed to mix, they'll do so, and, on a macroscopic scale, will result in a homogeneous mixture (entropy rules!) with no difference with respect to composition wherever you went, horizontally.
Vertically, with respect to some tens of kilometers, the composition will change however, following the individual height distribution of each gas in the respective gravity. Hence, if you had an air/hydrogen mixture , there always will be hydrogen even at the bottom , i.e. not all the hydrogen will disappear to the top (in fact, there always will be a higher hydrogen density at the bottom than at the top).
However, the height distribution of hydrogen will reach out much much further into space (as it is a function of the MW of the respective gas), and eventually gravity will become too weak to capture hydrogen forever (on earth, that is: Jupiter would be something completely different, and hence has a hydrogen rich atmosphere)
The same is true for He , and as a consequence, Earth's atmosphere is low on both gases (though they're the most abundant elements in the universe): we're "bleeding" them away  (just like Mars did with water, oxygen nitrogen...), and that's why Moon doesn't have any atmosphere worthy to talk of (besides that Earth always would try to steal it)

regards

Ingo




« Last Edit: August 08, 2013, 03:51:18 PM by magician4 »
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline billnotgatez

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 3778
  • Mole Snacks: +208/-55
  • Gender: Male
I like your answer so far
Still pondering
especially
Quote
in fact, there always will be a higher hydrogen density at the bottom than at the top

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
Quote
especially
Quote
    in fact, there always will be a higher hydrogen density at the bottom than at the top

imagine a planet's atmosphere to consist of hydrogen exclusively: it should be obvious that here, it must be so

now, for a lot of gas phenomena you're allowed to consider the properites of the mixture as an additive result of the individual behaviours of each single gas.
composition of the atmosphere with respect to height is such an phenomenon: each gas will behave as if it was alone in the room at the partial pressure it causes.

so, the height distribution of for example hydrogen would be the same , and hence at the bottom always richer in hydrogen, no matter what other gases would be around


regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline antimatter101

  • Full Member
  • ****
  • Posts: 163
  • Mole Snacks: +9/-25
You know global warming is mainly caused by CO2 trapped in the atmosphere? That is because the carbon dioxide is denser than air, settling down and forming a layer of gas close to the surface to the earth. This gas layer is what causes global warming and its consequences. (The upper atmosphere is barely affected by global warming)

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 2674
  • Mole Snacks: +430/-20
  • Gender: Male
  • A lover of spectroscopy and chocolate.
@antimatter

This is wrong on a number of levels. Starting with: the most abundant greenhouse gas in the atmosphere is water vapor.  It is also more strongly absorbing than carbon dioxide and atmospheric scientists usually estimate it can account for well over 50% of the greenhouse effect on Earth.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
@ antimatter 101:

as this is a scientific forum, I'd strongly recommend to take a look at completely different insights, also.

One of the best to lecture on those, in my opinion is prof. BoB Carter, and you might wish to listen to what he has to say:
http://www.youtube.com/watch?v=FOLkze-9GcI
(pt. 2 through 4 availale on the sidebar)

..and to be completely clear about my personal position, also: after years of listening to all arguments published, I am deeply convinced that  "man made climate change" is nothing but a Hoax

regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Sponsored Links