Sciencemadness Discussion Board

Gas displacing one gas but not the other?

MeSynth - 4-8-2011 at 14:36

Is it possible for a gas to displace one gas but mix with another? An example of displacment is the CO2 fire suppression systems found in some labs and at gas stations. The CO2 forces all of the oxygen out of the room and in the case of gas stations the ground area around the pumps.

The question I thought up today is this (will sound dumb to some of you smarty pants people)...

Can a gas be miscible with one gas and not another? Like water and ethanol and oil? Take an oil that is soluble in ethanol but not water. Mix equal parts of oil and ethanol together and they will form a solution. Add enough water to completly displace the oil from the solution. That was just a poor example of what I'm trying to find out about gases. By gas I mean anything in gas form that doesnt react with eachother not just elemental gases.

Mr. Wizard - 4-8-2011 at 15:58

I'll go out on a limb here, and say that ALL gasses are miscible. They may not mix immediately on introduction to each other, but they will all mix eventually due to thermal mixing , wind, and convection. This may take a while, but once mixed they will stay mixed under normal circumstances.

If CO2 or Gasoline vapor in air is mixed , then left on it's own, it won't separate and recollect on the floor.

MeSynth - 4-8-2011 at 19:49

Quote: Originally posted by Mr. Wizard  
I'll go out on a limb here, and say that ALL gasses are miscible. They may not mix immediately on introduction to each other, but they will all mix eventually due to thermal mixing , wind, and convection. This may take a while, but once mixed they will stay mixed under normal circumstances.

If CO2 or Gasoline vapor in air is mixed , then left on it's own, it won't separate and recollect on the floor.


ok that makes sence. Since there are no strong bonds so there is nothing to keep them from mixing.

you say that gases dont separate due to wind and such that make sence. even in an area cut off from all outside air flow it wouldnt settle because gravity is not strong enough. there has to be some way to break these rules though.

what do you think about these two gasses separating under controlled conditions? Sulfur hexafluoride and helium. (not sure if they react or not but if they dont would they separate into a lower and upper layer if left undisturbed in an air tight room?)

they were mixed well and then left for a year.

ScienceSquirrel - 5-8-2011 at 03:54

There are some gases that will mix extremely slowly if conditions are right.
For instance it is possible for carbon dioxide to collect in the bottom of wells or old mines where convection, wind currents, etc are absent.
There have been accidents where people have descended into an area filled with carbon dioxide and lost conciousness.
The gas is sufficiently dense to flow along the ground for long distances eg in the Lake Nyos accident.

http://en.wikipedia.org/wiki/Lake_Nyos

Mr. Wizard - 5-8-2011 at 07:08

Quote: Originally posted by ScienceSquirrel  
There are some gases that will mix extremely slowly if conditions are right.
For instance it is possible for carbon dioxide to collect in the bottom of wells or old mines where convection, wind currents, etc are absent.
There have been accidents where people have descended into an area filled with carbon dioxide and lost conciousness.
The gas is sufficiently dense to flow along the ground for long distances eg in the Lake Nyos accident.

http://en.wikipedia.org/wiki/Lake_Nyos

Yes, there are conditions where gasses will mix slowly. The Brownian or regular diffusion of gasses will proceed at a very slow pace, in the absence of convection currents, wind or other mixing on a macro scale. The air in mines and enclosed spaces is dangerous because it often lacks stirring or motion to remove the slowly accumulating products of decay, emitted gasses, oxygen removal by slow chemical processes, and bacterial action. Lighter gasses such as Hydrogen and Helium tend to diffuse or mix more quickly because of their lighter atomic weight. The weight isn't the real point though. Since all gas molecules at the same temperature and pressure has the same average energy, the lighter gasses must have a higher velocity to account for their energy. E=1/2MV^2. So as the weight goes down the speed of lighter molecules goes up exponentially with the change. Faster molecules mean more chances to spread out.

The Lake Nyos incident, where large volumes of rather cool CO2 are released suddenly is more an instance of the gas not having time to mix.

[Edited on 5-8-2011 by Mr. Wizard]

White Yeti - 5-8-2011 at 07:36

All gases are miscible, but they diffuse through membranes at different rates. You can separate gasses based on their molar masses and with the aid of special membranes.

MeSynth - 5-8-2011 at 11:04

putting all theorys aside and using my ape brain to think of why they cant mix this is what I came up with. (I dont memory)

Alright so everything that is anything has matter and matter is effected by gravity. That is why when you drop some Dry Ice into a bowl of punch the gas stays in the bowl or falls to the ground.

My original question was what 2 gases will mix with each other and what 1 gas will only mix with one of the previous 2 gases and when mixed will displace the one it does not mix with. If this question is possible to answer that would be cool.

Mr. Wizard - 5-8-2011 at 15:40

I think we have an impedance mismatch here, somebody else give it a shot.

Neil - 6-8-2011 at 10:43

Quote: Originally posted by MeSynth  

My original question was what 2 gases will mix with each other and what 1 gas will only mix with one of the previous 2 gases and when mixed will displace the one it does not mix with. If this question is possible to answer that would be cool.


The answer is no.

MeSynth - 6-8-2011 at 14:01

We'll see...

Mr. Wizard - 6-8-2011 at 18:49

Quote: Originally posted by MeSynth  
We'll see...

That's the spirit, go for it.