|MadSci Network: Chemistry|
This is an excellent question, and I will start by saying that I don't KNOW the answer for sure in the sense that I have never seen this written up in a book or article. But I will tell you what I think are two reasonable explanations, both of which probably play a role.
In general, the solubility of gases in liquids increases with a decrease in temperature, so you are correct in saying that if the only effect of adding ice to soda was to chill the temperature of the solution then the CO2 should in fact become more soluble, not less soluble as experimentally indicated by effervescence.
Bottled carbonated beverages are gasified under pressure and then tightly capped in order to maintain an amount of gas dissolved in the liquid that is greater than what would normally be in equilibrium with the partial pressure of CO2 in the atmosphere. A simpler way to say this is that the soda is supersaturated with CO2. I am guessing that effervescence is caused by physically disturbing this state of supersaturation, and the reason I think that is as follows. Think about pouring some room-temperature soda into a glass that has been sitting in a cupboard for days. The soda and the glass are at the same temperature, and still the soda effervesces. Clearly then temperature is not the cause of the effervescence. Rather, this is probably due to one or both of the following effects. First, it is possible that when the soda is sitting in the bottle only the part of the solution that is in contact with the atmosphere has equilibrated with it, and this part of the solution is in slow exchange with the rest of the soda such that it has less CO2 than the rest of the soda. When the soda is poured, new parts of the soda with higher CO2 are brought in contact with the atmosphere and give off gas (effervesce) in order to equilibrate. Alternatively, or perhaps additionally, when the soda is poured into a glass, perhaps the solution does not completely "wet" the inner surface of the glass, and microscopic bubbles form. The interior of each of these bubbles is an "atmosphere" with which the CO2 in the soda must equilibrate, and as more gas evolves into the bubble from the soda it gets bigger until it gets big enough to float to the surface and escape. This would result in effervescence as well. When ice cubes are dumped into a glass of soda they would (i) cause physical mixing of the solution and thus could generate the first effect, and (ii) they provide surfaces which would also be improperly wetted and thus could cause the second effect as well.
I hope this is a useful answer for you!
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