Even during maximal exertion expired air has:
O₂   17-18 %
CO₂   3-4 %
N₂   79.04 %

Molecular atomic masses are:
O₂   32
CO₂   46
N₂      28

CO₂ is denser than room air. Expired air is slightly denser than room air.
For this demonstration, we will cover the bottom of a 10 gallon aquarium with baking soda and pour about 500 ml of vinegar on top of it. The carbon dioxide produced by the ensuing reaction should remain at the bottom of the aquarium due to its density. Of course it will eventually diffuse and mix with room air. A student blowing soap bubbles which drift slowly down into the aquarium should see the bubbles "hover" at the interface of carbon dioxide and room air.

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