What you SEE is not completely different from what HAPPENS, and that is that carbon dioxide, like most heavy gases, sinks to the bottom of any column of air. Convection and turbulence can mix things up a little bit, but not enough to completely offset the net effect. The same thing happens with helium released into the atmosphere: it tends to rise very quickly, because the heavier gases (e.g. oxygen and nitrogen) sink beneath it.
Nope, and I have PhD in physics.
Cool story, bro... but this is
chemistry.
You might want to get a refund on that PhD.
White stuff is water fog, if it had any significant amount of CO2 you would not like breathing it at all, probably would have died from spasms or something.
That's not a typical effect of CO2 poisoning, even at high concentrations, which dry ice will not produce. More significantly: the foggy condensation you see will form where the very cool gas comes in contact with the moisture in the air around it, which means most of that CO2 will remain much colder than the air it comes into contact with until thermal equilibrium is reached. This takes a few minutes, especially in a relatively still volume of air, and the condensation effect is informative in allowing you to see (approximately) where the colder gas is distributed.
A similar thing happens with sulfur hexaflouride in a container: as a heavier gas, it will sink to the bottom of just about any volume of air its exposed to, and at most temperatures it's even lighter than CO2. If you fill a fish tank with sulfur hexaflouride, the foggy condensation from dry ice sublimation will float on top of the gas layer like a cloudy sky; this, too, gives you some idea of where the carbon dioxide is and where it isn't.
Heavy gases will not stay at the bottom, depending on how open the place is, they will mix rather quickly and occupy couple of kilometers of height.
Eventually, sure. How long that takes depends on weather conditions, mainly ambient temperature, sunlight and wind conditions. But if you introduce a large volume of gas -- or ANY fluid, really -- to a volume of another gas with a lower molecular mass, the first thing the heavier fluid will do is
sink, and the first place it will begin to dissipate is laterally along the bottom-most surface of the container. You see a good example of this when you pour water into a vessel of oil (the water immediately sinks to the bottom and spreads out from there) or, as above, when you introduce carbon dioxide to a volume of air or sulfur hexaflouride to a volume of carbon dioxide. In the latter case, the "water vapor" you keep insisting has nothing to do with the presence of carbon dioxide will actually "jump" on top of the growing pool of heavier gas. It's a pretty trippy effect actually.
Basically height is inversely proportional to molecular weight. So if your gas is 5 times heavier than air (Sarin) then it will get to 2 km roughly...
... eventually. Again, that takes a lot of time to occur and how long that takes depends on a lot of things.
