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Why do bubbles stream from fixed spots in glasses of beer or soda pop?

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.

Cecil replies:

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

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