Expert: John Locke - 11/6/2010

Question
I need some expert advice.  My son did the egg in vinegar then distilled water experiment.  He measured the egg after vinegar (but not before) and after distilled water and his egg got smaller rather than bigger.  Of course he could have mis-measured but I was wondering if there was some other explanation, also.  Could it be that vinegar osmosed out (since it also permeated the membrane to get in)?  Vinegar out vs water in equilibrium not achieved?  I'm a chemist so this is gnawing at me a bit.... Thanks.

Answer
Thanks for using AllExperts. As you seem to be somewhat familiar with the experiment already, I'm sure you know that the initial step of putting the egg in vinegar dissolves away the eggshell's calcium carbonate via this reaction:

CaCO3(s) + 2CH3COOH(l) -->Ca(CH3COO)2(s) + H2CO3 (l)

from which the carbonic acid, H2CO3, decomposes into water and carbon dioxide gas which then evolves:

H2CO3(l)-->H2O(l)+CO2(g)

When the egg is removed from the vinegar and allowed to sit, the excess vinegar should evaporate and the egg will shrink slightly over the course of hours. Acetic acid is fairly volatile, as you know.

This would also be true of the egg after removing it from the distilled water (the excess water would evaporate, though more slowly, and the egg would eventually shrink), but there is something that doesn't quite fit: when the de-shelled egg is placed in distilled water, it would normally swell as the hypotonic water moves into the hypertonic yolk and albumen. Because it now lacks a restrictive shell, the egg would normally swell until it bursts. I think that either a) the shell was not entirely dissolved in the initial vinegar step, and so the egg was unable to swell when placed in the water--the egg appeared to shrink later because it was losing water, when in fact it never swelled in the first place; or b), the egg burst when placed in the water but in such a way that you did not notice it, and so when it was removed from the water, the excess water simply leaked away and the egg appeared to shrink.

I would suggest--in the true spirit of science--that you repeat this experiment again to confirm the results. You may wish to try a slightly different protocol and compare the results. There is a link for this below. Good luck!

http://www.edinformatics.com/math_science/science_of_cooking/naked_egg_experimen...