Edit: Eh, sort of changed the question part way though, so if a mod could change the title to "Question regarding gas flow from high to low pressure". Moderator edit: Done I'm about 99% sure I already answered this question for myself but just wanted to make sure my logic was correct, so if anyone could help me out it would be much appreciated. If an open vessel that was initially P=150atm of a pure gas is currently letting gas out and it closes at exactly 1atm, will any gasses or air from the surroundings some how mix with the gas in the vessel while its open? Also assuming surrounding atmosphere has P=1atm. I realize that gasses always flow from high to low pressure, but within this hypothetical opening, could any bit of other gases or air sneak into the tank?