jeet, let's imagine a very low pressure environment in that tank. very low, 10 to the -6 tor, which i think is lowest we can currently measure in a chamber. if we were somehow able to achieve a space level vacuum of 10 to the -11 tor, no matter how rarified the atmosphere of the 10 to -6 level, the gas molecules would (using your language) bounce away from the denser layer into the complete vacuum layer. there are two further issues for you here: (1) according to you there is no tank, or container (2) as the highest, less dense, lightest gases bleed into the vacuum, the lower heavier gasses would be rapidly bleed off as there would be no layer above them providing counter pressure, albiet a small amount of counter pressure.
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