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When you go to a soda fountain and pour some soda into your cup, frothy fizz is generated at the top of the cup. Obviously, it has something to do with carbonation. However, when a cup of soda is just sitting and is not being filled or refilled, there is no fizz.

What causes the fizz? Is the fizz generated by the crashing of the soda into the cup?

I would also like to know how to stop the fizzing produced when the soda cup is being filled/refilled: Can I slow the rate at which the soda falls? Will spinning the liquid on the way down to the cup help attenuate the fizzing? What about changing the material from which the cup is made?

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I think this is a great question. Thanks for asking Seth! – Curt F. Feb 27 at 20:51
Please don't double-post a question; it's a chemistry question; so I would suggest you to delete the Phys SE question: – MAFIA36790 Feb 27 at 20:58
I'm seeing there exists an answer there; so I would suggest you to ask the mods there to migrate the question here. – MAFIA36790 Feb 27 at 21:03
They told me to ask here so I did. Sorry I don't know how to migrate. – Seth Kitchen Feb 27 at 21:49
Is this really just a chemistry question? I think it would fit in either place. One or the other, though. – SendersReagent Feb 27 at 22:07

The soda is supersaturated with dissolved carbon dioxide since in the sealed container the soda is under pressure using carbon dioxide. A number of things can cause the formation of nucleation sites which degas the solution by forming bubbles which float to the surface. The foam is due to the surface tension of the liquid solution. The lay understanding is that the soda is "flat" when the soda is no longer supersaturated with carbon dioxide.

It should be noted that even in a saturated solution most of the carbon dioxide is in solution as a dissolved gas and not carbonic acid. Since soda solutions are acidic anyways, this also favors the dissolved gas as opposed to carbonic acid.

To minimize fizzing:

  1. Use a clean glass. (Bubbles emanating from some sort on the glass indicate that the glass is dirty.)
  2. Use wet ice at the freezing temperature of water (not ice from fridge below 32 F).
  3. Cool the soda before pouring gently onto the side of the glass. In other words do not just dump it into the center of the glass onto the ice. Imagine trying to pour water into the glass without creating any bubbles.
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Thank you for the link to Foam Wiki, I didn't image this would be available. I don't think this answers the question completely. Is surface tension the only reason? If I dump soda into a surface-tension-free glass, there would be no fizz? I am hoping you could delineate on "formation of nucleation sites" – Seth Kitchen Feb 27 at 22:03
?!? Surface tension is a property of the liquid, not the glass itself. // "Nucleation sites" are essentially "irregularities" in the solution which would cause the dissolved gas to form bubbles at that site. So particulates floating in the solution, sharp edges on the ice cubes, dirt on the surface of the glass, a chip or crack in the glass and so on. // Added wikipedia links for "nucleation sites" and "surface tension" rather than trying to explain those concepts. – MaxW Feb 27 at 23:15
I'll point out in addition that "fizz" and "foam" are two different things. I was thinking of fizz as the bubbles floating to the surface. The "foam" results because not all bubbles pop immediately upon reaching the surface. – MaxW Feb 27 at 23:22
Thanks this is super helpful. Please tell me if I misunderstand at all. So the bubbles don't pop because of surface tension which is a characteristic of the soda that we won't be able to change unless we add a different liquid in? The only way to stop the foam is to create less bubbles. Which means creating less nucleation sites. The only ways to do this are decrease impurities, keep substances at the same temperature, and pouring more gently. What property is pouring gently fixing? Velocity, acceleration, angular momentum, force ? Generation of Nucleation sites are directly proportional to? – Seth Kitchen Feb 27 at 23:34
RE: "The only way to stop the foam is to create less bubbles." You could add various chemicals which would "kill" the foam. You wouldn't be getting less bubbles, it would just be that the bubbles would pop when they hit the surface. For beer drinkers having a nice "head" (layer of foam) is desirable since the bubbles busting release various smells which are detected by the nose. – MaxW Feb 27 at 23:56

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