Carbon dioxide (CO2) is heavier than most other molecules and elements in the air. In the same way that Helium rises (think of party balloons), CO2 sinks.
With a very large bowl or container and lots and lots of vinegar and baking soda, you can make a little “puddle” of CO2. When soap bubbles - which are filled with normal air - hover over this puddle, they float in place like beach balls sitting on top of a pool. You can see this in the picture, but of course it’s much weirder-looking in person: bubbles serenely unmoving in midair.
This Four Gas Tester, designed to check if air in a workplace is safe to breathe, showed that the air was normal above the bowl. But when I dipped the sensor under where the bubbles were floating, red alarm lights indicated that the air was not breathable - too much CO2!
A back-of-the-napkin calculation shows that one big box of Baking Soda is good for about six big bottles of vinegar. Their reaction produces CO2 gas and a big mess. The classic recipe is 12 parts water to 1 part blue Dawn dish detergent. A few tablespoons of glycerin (which can be bought at a pharmacy) help the bubbles last longer.