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Straight Dope Classic from Cecil's storehouse of human knowledge21-Jul-1989
Dear Cecil:
Long ago I noticed that the bubbles in clear carbonated beverages seem to stream from
fixed spots on the bottom and sides of the glass containing them. Boiling water seems to
behave in a similar way. What's so special about these spots? --David Peterson,
Washington, D.C.
Dear David:
They've got a fancy name, for starters, The places from which the bubbles stream are known
as nucleation sites. They're microscopic defects or bits of crud on the glass. When water
is changing phase (e.g., boiling, condensing, freezing), it needs a place where the vapor
bubbles, droplets, crystals, or whatever can congregate until they're big enough to
survive. That's what nucleation sites provide. Snowflakes and raindrops, for instance,
typically form around dust motes. When water reaches the boiling point, the scratches in
the container provide havens where microscopic bubbles can collect long enough to become
big bubbles.
The carbon dioxide bubbles in beer and soft drinks work the same way. Before you uncap the
bottle, the pressure inside keeps all the CO2 in solution. After uncorking, the reduced
pressure enables the gas to slowly boil away, which is where the nucleation sites come in
handy. If you want to see some serious bubble action, try sprinkling salt in your beer.
The salt provides an abundance of nucleation sites, producing not only a fascinating
demonstration of physics but pots of fun besides.
--CECIL ADAMS
The Straight Dope / Questions or
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