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Q & A: gases and semi-permeable holes

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Most recent answer: 09/20/2013
Assume a small box, filled with small molecules, e.g. hydrogen. The box is placed in a vastly larger box filled with bigger molecules, e.g nitrogen. Pressure and temperature are equal at start. I punch a tiny hole in the small box, so tiny only hydrogen molecules can pass it. In the small box the molecules are whizzing around and some molecules will leave the box through that small hole into the vast space, so the chances of returning into the small box are near to zero. Will there be a near vacuŁm in the small box after some time? If not, why? Maybe pressure and thickness of the wall must be chosen so that the mean free path is long enough.
- Michel Uphoff (age 61)

Sure, after a while the H2 will be in equilibrium, distributed as a very sparse gas in the two boxes. The example might a little more realistic with He, which really can leak very much more easily through some things than other gases.

Mike W.

(published on 09/20/2013)

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