That's a very clear question, Molly, but I'm not completely sure of the answer. If I had to guess, I'd say that the sugar was more important because I suspect you had a soft drink with more molecules of sugar in it than molecules of CO
2. (The reduction of the freezing point depends almost exactly just on how many molecules are dissolved in each cup.) Here's some things you can do to test what's important.
The CO
2 gradually comes out of solution, going into the air. (You can see the little bubbles of it form.) If you let both drinks sit open for a day before putting them in the freezer, does the soda still freeze more slowly? If so, the CO
2 probably wasn't very important.
Both the soda and the juice have sugar in them. You can take a cup of each and let them dry out. What's left will be mostly sugar. Which one had more sugar in it? If it's the soda, then that by itself would explain why it froze more slowly. If it's the juice, then you might have to look for other explanations, like the carbonation.
You could also try a diet soda. These have about the same carbonation as regular soda. but usually use a smaller number of very sweet molecules instead of a larger number of sugar molecules. Does the diet soda freeze faster than the regular one? If so, that would be due to sugar, not carbonation. You can also get simple carbonated water (seltzer water) without sugar in the store and try it.
This could make a great project. Please follow up to let us know how it turns out.
Mike W.
(published on 04/12/2011)
