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So I want to know do gases have the same concept as water and oil ? Like if I put some hydrogen and helium in a room with 0% air will they float on each other like the hydrogen go down and the helium goes up or will it mix?

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    $\begingroup$ They mix, via diffusion and convection. Think perfume and air: perfume would be useless if if was only located near the floor. $\endgroup$
    – Ed V
    Oct 26, 2021 at 12:39
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    $\begingroup$ Why not? $\endgroup$ Oct 27, 2021 at 2:01
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    $\begingroup$ While there is no anything similar to layering of liquids in terms of long lasting stability I've decided to add this comment as for important SAFETY implications. Release of gases in confined environment such as cabinets but even rooms is dangerous. Do not expect anything to leave and mix so fast. Think of the many accidents with CO2 involved. And even a leaking pressure bottle can pose dangers. Asphyxia is particularly sneaky because we feel lack of oxygen indirectly by means of CO2 in the blood stream. Gradually reducing oxygen does not prompt us to breath, just makes us confused/ sleepy. $\endgroup$
    – Alchimista
    Oct 27, 2021 at 7:46

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There is difference between liquids and gases, as all gasses are mutually mixable.

Hydrogen and helium would do both mixing up and layering, with hydrogen going up and helium down.

Layering is just a temporary thing until all is mixed up. Similarly as ethanol and water can form 2 layers, if brought together carefully,as mixing would make a single phase from them.

Note that gas mixing up happens much faster and easier than for liquids. There is much smaller density difference and more intensive diffusion and convection.

The bigger the density difference is, the more easier is forming layers and the slower is their mixing.

The equilibrium condition in static scenario builds very slight concentration gradient where gravity induced sedimentation is balanced by gradient net diffusion. Real life scenarios in air context bring strong vertical and horizontal convective mixing.

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    $\begingroup$ And that SF6 leak will find the low spots and make them potentially hazardous, but not forever. Confined spaces are nasty things. $\endgroup$
    – Jon Custer
    Oct 26, 2021 at 13:02
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    $\begingroup$ maybe worth noting that in everyday life, density differences due to temp differences are much more common than density differences due to molecular composition, and layering of (less dense) warm air above (more dense) cold can be stable for quite a while $\endgroup$
    – Andrew
    Oct 26, 2021 at 13:52
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    $\begingroup$ @Andrew Surely the differences because of temperature are much more important in the air context, forming labile, conditionally stable and stable temperature gradients ( I have background in air force field meteorology). But we do not speak in the context of air here. $\endgroup$
    – Poutnik
    Oct 26, 2021 at 13:56
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    $\begingroup$ It depends on how much force the particles experience. Proteins in solution don't settle in a beaker on the benchtop, but they do in an ultracentrifuge, and isotopes in the gas phase may be separated by centrifugation as well. Hydrogen can escape from the upper atmosphere (Jeans escape, en.wikipedia.org/wiki/Atmospheric_escape#Jeans_escape). Maybe the latter does not count as settling. $\endgroup$ Oct 26, 2021 at 20:11
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    $\begingroup$ @KarstenTheis I am well aware about centrifuging of proteins and uranium. But it is not like forming 235UF6 and 238UF6 layers. It means upper layer is at equilibrium just slightly enriched by 235U. As at some concentration gradienct, settling is compensated by diffusion. Hydrogen escape is about escape velocity. BTW, I have said atmospheric. composition very slightly change with altitude, what implied increased hydrogen few hundreds km above sea. $\endgroup$
    – Poutnik
    Oct 26, 2021 at 20:26
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Yes, gases do mix but if you leave them overtime they will slowly settle down less dense one on the top and more dense one at the bottom (Of course if there not chemically reacting.).The example you have selected is so hard-the density difference of hydrogen gas STP is $\ce{0.09 kg}$ $\ce{m^{−3}}$ and helium gas is about $\ce{ 0.178 kg m^{−3}}$ which gives a density difference of $\ce{ 0.088 kg m^{−3}}$ which is very low might take so much time to settle down. Gaseous have very low inter molecular interactions that they mix up fast but separate out very slow due to the mobility of denser gas is less in closed environment.

You could try few examples,

  1. Electrolysis of salt water and taking Hydrogen and Chlorine gas to the same container Chlorine will sink to the bottom and Hydrogen will go to the top. (Hint you might need to fix two pipes to the top and bottom to take gaseous out and test).

  2. Try putting bit of dry ice (Frozen $\ce{CO_2}$) in to a warm water jar slowly poor it to a candle near by the gas will extinguish the fire. I am sure there is lot of YouTube videos about that

To see the effect in more depth as others have explained try to get the difference of the densities of the gaseous higher. like $Chlorine$ gas and $\ce{CO_2}$ is a good example for every day use. Even the fun experiment $\ce{SF_6}$ gas (the Aluminum boat floating in air experiment).

You should check this video for what happens to gaseous when there is no gravity. The denser gas sticks to where ever it is created in zero gravity. Layers will be observed if there is gravity due to buoyancy force.

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    $\begingroup$ You use the adjective gaseous as a noun. // Rate of gas diffusion and convection is many orders higher than the rate of settling down. At equilibrium, composition changes with altitude very slightly.,// What changes with altitude is air pressure and therefore oxygen partial pressure, not air composition. $\endgroup$
    – Poutnik
    Oct 26, 2021 at 17:48
  • $\begingroup$ Yeah probably right atmosphere is not a closed system after all. I will delete that example $\endgroup$
    – Avon97
    Oct 27, 2021 at 7:57
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    $\begingroup$ No, that first sentence is wrong. You can create temporary layers of gases with very different densities but, if left over time, they will mix, they won't settle out. $\endgroup$
    – matt_black
    Oct 27, 2021 at 16:35
  • $\begingroup$ Then what about the buoyancy force acting upon the less dense gas from more dense gas ? Yes Hydrogen and Helium gas isn't a good example to explain those gaseous will mix. $\endgroup$
    – Avon97
    Oct 28, 2021 at 6:35
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To add a point not covered; Although they generally mix , hydrogen has such a low density it tends to rise or float and concentrate. So, when inside, flammable levels of hydrogen can collect at a ceiling. Industrial sites will use explosion proof light fixtures and other precautions when hydrogen is present. I was impressed by this situation at my first job ( about 1958 so most precautions were not yet developed ) when a hydrogen explosion blew out a brick wall. The room contained about 50 conventional steel bottles of hydrogen that were connected by manifolds ; my job was to go into the room each day and open and close valves as needed to maintain the hydrogen pressure/ flow for a process. When I heard fire engines , I went to see what was happening ; my boss was very glad to see me as they went going through the broken brick looking for me.( I don't get to tell that story much).

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