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I've noticed that adding a chunk of ice cream to soda makes the soda fizz slightly near the soda-ice cream interface. I thought it was a physical effect due to the temperature, but adding ice has no effect.

It still can be a physical effect due to solubility, or it may be a chemical effect.

I'd like to know which it is, and the details of the mechanism.

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The reason for this is mainly because of a factor called Nucleation. Although it is not exactly obvious what is the source of the site for the nucleation to occur, what is clear is that there are sites present on ice-cream that are not present on ice alone. The carbon dioxide (CO$_2$) in the drink nucleates and forms bubbles (sometimes even a frothy foam) on the drink.

There are several possibilities some of which are discussed in the excellent answer by Ashu.

I list some of them here starting with what I feel is the most likely.

  1. Air in ice-cream
  2. Ice crystals in ice-cream
  3. Other chemicals found in the ice-cream
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    $\begingroup$ Could be the fat globules(that are part of milk) coupled with the temperature form nucleation centers :\ $\endgroup$
    – ManishEarth
    Commented Jun 6, 2012 at 3:54
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Soda contains $\ce{CO2}$. Over time the gaseous $\ce{CO2}$ comes to the surface. The ice cream increases the rate of this $\ce{CO2}$ from the soda causing increased fizzing. This increased rate is a result of the solvation of ice cream particles in the liquid. The particles serve as nucleation sites for the growth of gas bubbles. The bubbles get large quiet fast thus you observe fizzing.

After a bit of experimentation I found the following:

  • Effect of adding milk — There is a bit of extra bubbling and a curdy precipitate is formed. (Couldn't dare to drink it.)

  • Effect of adding salt (yes ice cream does contain salt) — Lots of bubbling around the salt. After the salt settles to the bottom, bubbles collect around it become big and then come to the surface.

Only thing left to try is sugar and vanilla essence but I ran out of soda.

So we come to know that the fizzing effect may be due to milk or salt or a combo of both.

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    $\begingroup$ why wouldn't it be mainly due to the lower temperature of the ice cream, thereby decreasing the solubility of carbon dioxide? $\endgroup$
    – Chris
    Commented May 10, 2012 at 19:34
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    $\begingroup$ @Chris: The solubility of gases in liquids decreases with increasing temperature. $\endgroup$
    – Aesin
    Commented May 11, 2012 at 1:03
  • $\begingroup$ your argument is true at equilibrium conditions. In non-equilibrium conditions the ice cream frozen at 0 F will begin mixing with the warmer solution, pop. Upon mixing, an ice sheet will form in some regions for some amount of time at the ice cream/water interface. CO2 hardly dissolves in solid water. This is why it isn't entirely clear to me. Perhaps milk mixed with pop at the same temperature would be more convincing. Time for me to try an exp at home! $\endgroup$
    – Chris
    Commented May 11, 2012 at 2:07
  • $\begingroup$ @Chris: Yeah, I think it's the milk. Ice doesn't work on its own. $\endgroup$
    – ManishEarth
    Commented May 11, 2012 at 8:39
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    $\begingroup$ This still doesn't answer the question--what's the actual reaction going on? $\endgroup$
    – ManishEarth
    Commented May 18, 2012 at 4:01
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I would say that it is a chemical reaction due to the fact that it is irreversible. Aside from that, the carbon dioxide in the drink reacts to the fat and the bubble (which make ice cream light and fluffy) and wha-la. There is still not solid research as to how it works, but this is my guess due to the fact that most soft drinks have no fat in them, and ice cream has a fair amount. There is research however that states that the carbon dioxide is able to grow in the small air bubbles in the ice cream and this causes the froth. Personally, I think it's both

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  • $\begingroup$ Could you give the references to which you refer to? $\endgroup$
    – tschoppi
    Commented Feb 17, 2014 at 14:57
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    $\begingroup$ I too think this is a spurious answer. A simple gas-fired piston works by the expansion of a gas. Once it's happened it doesn't reverse spontaneously but we wouldn't call the process of gas expansion 'a chemical reaction'. $\endgroup$
    – user7232
    Commented Aug 30, 2014 at 22:15

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