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Q & A: a straw full of baloney

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Most recent answer: 05/27/2013
Q:
http://www.physicscentral.com/experiment/askaphysicist/physics-answer.cfm?uid=20120117095017
This site gives a theory about why diet soda would get pulled up a straw/
- Mike W.
A:

This one is a disappointment, because it's posted by the American Physical Society, a reputable organization of which I'm a member.

Someone asked why the diet soda in a straw went a couple of inches above the level in a covered cup after all the ice melted in the cup. The answer claims that diet soda is less dense than water, and that after water evaporated from the straw this less dense stuff was so concentrated that the heavier soda pushed it up the straw.

Archimedes figured out how high things float in denser liquids over 2000 years ago.The mass of the floating object (here the liquid in the straw) just matches the mass of the displaced liquid, in this case diet soda that would fill the straw two inches less.  For the stuff in the straw to float 2" above the liquid in the partly filled cup (probably only about 5" deep) the hypothetical concentrated diet soda would thus have to be a lot less dense than water, maybe 40%. I just tried measuring the density of a diet soda with a crude kitchen scale, and got that it was within a few percent of the same density as water. (According to  the density of diet Coke is 0.997 gm/ml, very close to water.) The proposed theory couldn't predict an effect nearly large enough to account for the 2" rise.

One possible explanation, based on the facts given, would involve the melting ice. As the ice melted, the soda in the cup became a much less concentrated solution than the soda in the straw. This contrast would be enhanced by some evaporation of water from the open straw. There are strong chemical forces, called osmotic pressure, tending to equalize the concentration of solutes. It's forces like that that allow trees to beat gravity and pull sap up to the top, where the trees keep the solute concentration high. The more concentrated soda in the straw might pull in the more dilute soda in the cup by osmosis. You could imagine some other possible mechanisms.

There are ways to test these ideas. For example, the idea proposed on the APS site doesn't involve the ice, so it would work even if there were no ice. The osmotic idea does involve the ice, but could be reproduced by just adding water to the soda in the cup but not the straw. Maybe exploring these ideas would make a good science fair project.

Mike W.


(published on 05/27/2013)

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