I've heard people say many times that one gas is heavier than the other so the heavier gas sinks. Now this must certainly be true to a certain extent (I think of gas used in WWI that sunk into trenches). However, this idea of the weight of a gas being a driving force seems unusual because, in my experience, entropy is the dominating motive for particles in the gas phase. A good example of this is that gas phase clusters are studied extensively in the literature due to how unusual they are.

So, why would some gases "sink" and give up their entropic gain for what doesn't seem to be much of an energy minimization? Could the answer be simply that this concentration of heavier gases is quite short lived and they soon float away to their entropy filled wonderland?

I would also give as an example the fact that CO2 certainly gets up high in the atmosphere quite well and that is more than twice the "weight" of other particles in the air. So what's different about that than about the examples given previously?

  • $\begingroup$ I think you're right with your answer. $\endgroup$
    – Mithoron
    Aug 22, 2015 at 19:37

1 Answer 1


Without significant mixing, diffusion takes a long time to mix gases. Our understanding of entropy tells us that we will indeed finish with mixed layers, but that doesn't give us a time frame for that mixing, only an outcome.

Given a slow but steady production of a dense gas, layers absolutely will form due to density differences. There are plenty of places where this can easily be seen, from your WWI example to the ability to pour carbon dioxide or hexane vapor into a container. They're not stable over the long term, but they do exist for substantial periods of time, days in some cases.

If confined by an open container, huge areas can experience this effect, as at Lake Nyos. Heck, even small temperature based density differences drive atmospheric layering that has major impacts on weather and pollution.

It's not an enormous effect, but you may be underestimating the strength of the forces involved here. The same pressure differential that forces a helium balloon upward acts on an open volume of helium as well, and more strongly due to the missing mass of the balloon. A body of a gas like $\ce{NO2}$ has about as much force pushing it downward (in air) than an equal volume of helium has pushing it upward. Without outside mixing to cause it to disperse, it will happily form a short term layer until diffusion carries it away.

  • $\begingroup$ No diffusion does not take a long time. You can smell a fart across the room in seconds. Thermal layers is a different thing. $\endgroup$
    – paparazzo
    Aug 23, 2015 at 2:53
  • 4
    $\begingroup$ @Frisbee Diffusion is fairly slow. The reason you can smell a fart quickly is the presence of large scale air currents in the room. Sure, "fairly slow" is qualitative, but for an easy example showing that it takes a significant amount of time for gases to diffuse through less than a meter of air, look at the famous demonstration of the reaction between ammonia and hydrochloric acid in gas phase. That's just to get the two gases in contact, not to fully mix the contents of the tube. $\endgroup$ Aug 23, 2015 at 5:28
  • $\begingroup$ youtube.com/watch?v=1PJTq2xQiQ0 $\endgroup$
    – Curt F.
    Aug 23, 2015 at 18:49

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