If gases assume the volume of the container that they are in, then wouldn't a gas that isn't enclosed in a container have an infinite volume?

But if the volume was infinite for gases not enclosed in a container, then how would the ideal gas law even function?


Thanks to anyone who can point out the piece of this puzzle I am not considering!




I give you a case. Suppose you opened an airtight jar containing an ideal gas. Suppose by some mysterious power, you are able to observe the molecules of the gas. What do you think you'll see on opening the jar?
Obviously you'll see the molecules dissipating in space(the correct word would be diffusing). Initially they suppose on opening occupied 100ml, then they occupied 200, then 300... Thus at every point of time the molecules have a finite volume, the value may be very large, but still its finite. Thus we say a gas "will occupy infinite volume" but only after "infinite time".


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**Yes. But with several complications you don't normally use the gas laws to deal with**


The ideal gas law is useful in laboratory situations where you can control the various components in the law (volume, pressure, amount of gas, temperature). And it usually describes the *equilibrium* reached when those factors are held constant. In the situation you describe, while the laws still work, you may never be able to observe the equilibrium state of the gas.


Consider practical experiments such as releasing a container of hydrogen held while you were spacewalking in the vacuum of space. The lack of a container means the hydrogen molecules will spread out eventually occupying an *infinite* volume. But you can never **see** this state as the individual molecules travel at a finite (and not very fast) speed. Even at the end of the lifetime of the finite universe, they may not have spread far enough to reach the equilibrium predicted by the gas law when the volume is infinite.


The more constrained case of opening a container of hydrogen to the atmosphere in a laboratory has a similar constraint. The gas sees a nearly infinite volume (the entirety of the earth's atmosphere is pretty large compared to one cylinder of hydrogen). But there are more constraints than in space. The hydrogen will diffuse into the atmosphere (more slowly than it would expand into space) and perhaps constrained even more by the walls of the lab. The expansion is also limited slightly by gravity which might contain the ultimate volume occupied to be the same as the volume of the atmosphere (but we normally ignore the complication in the ideal gas law). But, again, you would never really see the final equilibrium predicted by the gas law as it would take a long time to reach and other factors would prevent it being reached (some molecules in the upper atmosphere would be swept into space rather than being constrained by the atmosphere). 


In both these cases the final equilibrium state predicted by the gas law is still the same. But you can never observe it as it takes too long to happen and other complicating factors will become more important and will dominate the outcome. 


The gas law is not wrong but it is sometimes irrelevant.


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