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This
beautiful experiment illustrates the principles of buoyancy, semi-permeability,
and interference.
Blow bubbles so that they float down into the chamber. The bubbles
will descend, and then hover on the denser layer of carbon dioxide
gas. After a few minutes, notice that the bubbles begin to expand
and sink. Notice the color bands on the bubbles. Watch how some
of the bubbles freeze on the dry ice. As dry ice turns from solid
to vapor, or sublimes, it produces carbon dioxide gas. Carbon
dioxide is denser than air. (Carbon dioxide molecules have an
atomic mass of 44 amu. Air is made up of nitrogen, 28 amu, and
oxygen, 32 amu.) The denser carbon dioxide gas forms a layer on
the bottom of the aquarium. A bubble is full of air. It floats
on the carbon dioxide layer just like a helium balloon floating
in the air. You might expect that the air in the bubble would
cool and contract near the dry ice, but the bubble actually expands
slightly. The soapy wall of the bubble allows carbon dioxide to
pass through, but does not allow air molecules to pass through.
Initially, the concentration of carbon dioxide gas is low inside
the bubble and high outside the bubble.
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