Well, on Earth you /usually/ see different chemicals separating by state of matter.

Occasionally you see gasses separating this way...but it usually only comes up in emergencies.

Note: If everyone in a room is unexpectedly unconscious, you leave and get help...because rushing might get a practical demonstration of how X gas is heavier than oxygen. I once heard a story where it took /two or three/ downed 'rescuers' before someone came along that had enough sense to phone for help, instead of running in and keeping over.

I also saw a mystery that used this once: everyone in town dropped dead at the same time...excepting a little girl at the doctor's office and a man asleep in the steeple.

It turned out they'd cracked open a pocket of gas in a mine, and it had flowed downhill and asphyxiated everyone. The man in the tower was too high, and the little girl had a Disability Immunity due to a medical condition, and the gas dispersed before it reached anyone further away.

I guess I'd frame the gasses as lakes or seas...maybe rivers or currents if there were regular transfers or something analogous to Earth's water cycle.