MadSci Network: Other

Re: Why does Vanilla ice-cream react the way it does to Root Beer?

Area: Other
Posted By: Lynn Bry, MD/PhD Student, Washington University Medical School
Date: Thu Jun 19 17:50:58 1997
Area of science: Other
ID: 864353740.Ot

Dear Nancy

The effect you're seeing (lots of thick foam, I presume) comes primarily from the numerous gas bubbles released when you put the cold ice cream in the soda drink. Carbonated beverages are supersaturated with the gas carbon dioxide, or CO2. Supersaturated means you have far more gas molecules in the liquid than you would expect to find in the normal conditions under which we live (~ 25 degrees centigrade, "one atmosphere" of pressure). The gas molecules are forced into the liquid drink by placing the liquid under high pressures of the gas, higher than the normal atmospheric pressure in which we live. When you open the can, that first "spprurscht" represents some of the gas escaping. Over time, more gas escapes, as seen by the bubbles forming within the drink. However, the beverage is still "supersaturated" with regards to what a normal glass of water, or flat soda would have in it. Given enough time, it will equilibrate with the surrouding air, at which point it becomes flat and ends up down the drain..

When you add the ice cream, you drop the temperature of the drink. The lowering of temperature makes it even more inhospitable for the great excess of gas molecules to remain in solution (***), and you get more bubbles rising to the surface. The same effect happens when you add ice cubes, *except* that you don't see so much foam. In addition, some of the ice cream melts and "goes into solution," namely, it dissolves in the soda and can further displace gas molecules in solution.

So why the foam?

Ice cream is a mixture of substances, milk, sugar, eggs, flavors, many of which contain *proteins*. Protein-rich solutions are prone to frothing - just try mixing some egg-whites sometime. The bubbles released at the surface mix the protein-rich, melting ice cream and create small frothy bubbles - thus the foam that goes everywhere.

As for an experiment..

To examine the effect of temperature on the amount of foam produced, you could try adding fixed amounts of ice cream to defined amounts of soda opened at different temperatures - is there a difference in the *volume* of foam produced at 20*C versus 2* C? Is the amount of foam generated related to the amount of ice cream/soda used?

I should note that gasses are more soluble in cold solutions than in warm ones (a
search for "solubility gas" on our site will locate relevant files). The reason the addition of the ice or ice cream releases *more* bubbles of gas is due to the fact that the soda is supersaturated with CO2.

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