# I If air is a mixture, why don't the gases separate?

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1. Nov 16, 2015

### AMan24

If you try to mix water and sand, the sand will mix around and eventually fall to the bottom. Sand and water can't make a solution, so they separate. However salt and water can make a solution, and they don't separate. If air is a mixture, why don't the gases separate?

1) So, pretty much what i'm asking is, is there a term for a solution of gases?
2) Is it just called a solution?
3) Or am i completely off track and is it something totally unrelated?

4) I was also thinking, why don't they separate because of density?

2. Nov 16, 2015

### Staff: Mentor

The gases in the atmosphere do separate somewhat, but because the atmosphere gets pretty mixed-up by wind, they don't separate that much.

3. Nov 16, 2015

### Staff: Mentor

I didn't know that! That is interesting. Do they just separate by density? There is no surface tension so I would imagine that any such separation would be gradual.

4. Nov 16, 2015

### Staff: Mentor

I believe free hydrogen in the atmosphere is an example. Most of it rose to the fringes of space and disappeared long ago.

5. Nov 16, 2015

### Staff: Mentor

At the laboratory scale, the main mechanism is rapid diffusion resulting from the high kinetic energy of the molecules. Even over most of the atmosphere, small scale turbulence combined with diffusion provides most of the story. Only high up in the atmosphere, where the mean free path of the molecules is much larger, does gravitational segregation play much of a role.

Chet

6. Nov 16, 2015

### sophiecentaur

Why wouldn't gravity have an equal effect all the way up (statistically)? It seems to me that, with or without collisions, the distributions of heavy and light molecules would have the same effect on each other, despite the random motions. Is there something 'different' at work when gases are well intermixed?

7. Nov 16, 2015

### Staff: Mentor

Hummm. Good question. I'm not sure. Maybe it has something to do with the gravitational force on each molecule staying about the same, while the frequency of collisions with other molecules decreases as the altitude increases.

8. Nov 16, 2015

### lightarrow

What do you mean with "the distributions of heavy and light molecules would have the same effect on each other"? I would say diffusion can't happen in absence of collisions.

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lightarrow

9. Nov 16, 2015

### Staff: Mentor

Even though we do not normally speak of buoyancy with respect to uncontained gasses, that is what is happening here on a molecular level. Lighter gas molecules are more buoyant than heavy ones. That is why a helium balloon rises. Pop the balloon and the helium gases rise anyhow.

10. Nov 16, 2015

### Staff: Mentor

This definitely doesn't sound correct to me. Are you saying that, in a sealed room containing helium in air, the helium will all segregate near the ceiling, and the air will stratify below?

Chet

11. Nov 16, 2015

### Staff: Mentor

Yes, that's what I'm saying.

Edit: Of course the boundary between the helium and air will not be sharp because of diffusion, but the layer should be there.

12. Nov 16, 2015

### Staff: Mentor

Not completely, but it has to separate a little, doesn't it?

I know due to Dalton's law we sometimes treat gases as completely separate(and therefore each occupying the full volume uniformly), but I don't think that is the reality for this context.

See: http://wordpress.mrreid.org/2014/08/01/the-composition-of-earths-atmosphere-with-elevation/

Now, virtually all of the variation is above 100km, which makes that hard to read, so I'd like to find some more detail of what those minor gases are doing down low.

13. Nov 16, 2015

### Staff: Mentor

From https://en.wikipedia.org/wiki/Helium

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million.[76][77] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes.[78][79][80] In the Earth'sheterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.​

The same applies to hydrogen. That is why we only find trace amounts of hydrogen and helium in the atmosphere today. It rose and drifted away into space in primordial times.

14. Nov 16, 2015

### nasu

15. Nov 16, 2015

### Staff: Mentor

I'm not saying that there isn't a slight effect in a sealed room. But I am saying that, in a sealed room at atmospheric pressure, the molecular agitation would be adequate to guarantee that the amount of segregation would be virtually undetectable. There certainly wouldn't be a mostly helium layer adjacent to the ceiling.

Chet

16. Nov 16, 2015

### Staff: Mentor

17. Nov 16, 2015

### Staff: Mentor

Also propane. Boats that allowed propane to leak to the low points in the bilge have exploded years after the leak was stopped. Despite limited air circulation, the propane remains layered in the low points.

18. Nov 16, 2015

### Staff: Mentor

We're agreed. I did not intend to imply the effect would be significant for a small container. Whether it is significant for the lower atmosphere depends on your definition of "significant". But since that issue was the thrust of the OP's question, I think it is significant enough to mention.

19. Nov 16, 2015

### Staff: Mentor

I didn't see anything in the OP's question about air over vertical atmospheric distances on the order of 10's of km. Maybe it's just my perspective as a ChE to think small, on a scale on the order of meters rather than km. (Although I do have some actual experience as an atmospheric scientist).

Chet

20. Nov 16, 2015

### nasu

I read claims of a similar effect in wine cellars. The CO2 will come from the fermentation, in these cases.

21. Nov 16, 2015

### Staff: Mentor

Since he asked "why don't they separate because of density", I wanted to start off by congratulating him for his correct logic and telling him he's right that they do/should, before hitting him with the big "but".

22. Nov 16, 2015

### Staff: Mentor

So, in a room filled with stagnant air, the majority of the oxygen molecules will eventually be situated in the bottom 20% of the room, and the majority of the nitrogen molecules will be situated in the upper 80 % of the room (even though air is nearly an ideal gas, with the individual molecules traveling very rapidly in all directions)? How does that reconcile with your helium balloon explanation?

Chet

23. Nov 16, 2015

### nasu

It does not follow. The difference in molecular weight between oxygen and nitrogen is much smaller than the difference between nitrogen and propane, CO2 or helium.
So if the diffusion is enough to equalize O2 and N2 in normal conditions it does not mean is enough in every case.
It would be interesting to have some quantitative estimate of the effect.

24. Nov 16, 2015

### DrStupid

That sounds plausible.

25. Nov 16, 2015

### Staff: Mentor

I sure don't want to try to breathe the air in the upper part of that room.