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Re: Ice cream sodas

On Tue, 6 Mar 2001, Glenn A. Carlson wrote:

Another possible effect is the difference in solubility in water of carbon
dioxide and ice cream.

Thoroughly melt some ice cream, skim off the foam, chill the remaining
liquid, and see if it still triggers foaming in cola. I bet it doesn't.
I've convinced myself that the "nucleation" is almost always caused by
tiny, preexisting air bubbles.

An even more dramatic effect occurs when table salt
is added to soda. Though nucleation may also contribute to the resulting
fizz, I theorize that NaCl "displaces" the CO2 in solution because NaCl is
much more soluble in water than CO2.


I discovered this:

Dissolve salt in water, then dump it into the cola. It fizzes.

Dissolve salt in water, wait 1/2 hr, then dump it in. NO FIZZ.

Wet a pile of salt, wait 1/2 hr, then dump it in. NO FIZZ.

Dissolve salt in water with a bit of detergent. After a few minutes
a white layer of microbubbles collects on the surface.

I conclude that salt + water creates bubbles, and it's these bubbles which
are the "nuclei." If you wait awhile, these bubbles rise to the surface
and burst, and thereafter the salt water won't cause fizzing.

Is there any situation where the nucleation is provably caused by
something other than tiny bubbles? I suspect that the received wizdom is
wrong, and nucleation is not triggered by sharp crystals or rough
surfaces, but instead is caused by trapped air.

One possibility: scratching the immersed surface of a beaker full of
superheated water. This is supposed to trigger boiling. But is it
because of the roughened surface? Or did the creation of surface cracks
also cause cavitation (and the cavities immediately fill with H2O vapor?)


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