What is Alka Seltzer? How much CO2 can be evolved from a single
tablet? How much does it cost?
The most dramatic demonstration of the different speeds of sound in
air and CO2 I ever saw was at the Exploratorium in 1980. I have seen
it since at a couple of other science museums, and I don't know where
it originated.
The demonstration employs a large weather balloon (almost three
meters in diameter) filled with CO2 and placed between benches on
which participants (the Exploratorium is especially partial to
interactive demonstrations) sit. The participants can whisper to one
another over a great distance, their voices imaged by the giant
acoustic lens.
I think this demonstration can be reduced to less heroic dimensions
for classroom purposes and used with a mechanical wristwatch to
determine the focal length, and thus the acoustic index of refraction
relative to air, of a smaller balloon filled with CO2. Perhaps a
balloon inflated by Alka Seltzer (or, more likely, dry ice) can be
used. If one gets ambitious, a compound lens consisting of a CO2
balloon and a commercial helium-filled Mylar balloon can be
constructed to measure the speed of sound in helium as well.