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Q & A: vinegar vs.lemon for dissolving eggshells

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Most recent answer: 02/04/2015
Q:
Hi, My 7 year old and I just did the vinegar and egg experiment. He knew vinegar was acidic so he wanted to compare with other acids. So he put an egg in orange juice and an egg in lemon juice, because he knew both were acidic. Then, we compared the PH of the three (lemon, orange, vinegar). The vinegar and the lemon juice were the same PH. So, what made the egg change so much more in the vinegar compared to the lemon juice? Not only was the egg shell dissolved, as you discuss on your website, from the acetic acid, but also the egg because "bouncy and smooshy" to quote my son. The lemon juice dissolved the egg shell less than the vinegar, and the egg was less "bouncy and smooshy." Thanks!! We are excited to hear your answer.
- Amanda Rice (age 45)
Berkeley, Ca, USA
A:

Your son's results make alot of sense. Ordinary 5% vinegar is a little les than 1 Molar acetic acid. Because acetic acid is a weak acid,only a small fraction of it (about 1/2 %) actually comes apart into Ac- + H+ at that concentration. The pH is about 2.4. If you have citric acid from a lemon at that pH, it's mostly dissociated into citrate- + H+. Once you put the eggshell in, it starts reacting with the H+, as we described in the earlier answer (). That would raise the pH, causing more of the acetic acid to dissociate. There isn't much more of the citri acid left that isn't already dissociated. So all that spare acetic acid is available to react with the eggshell, and then with the egg.  If you've used say 1/2 liter of liquid, you'll get almost a mole of H+ available for reactions from the vinegar but only about 0.01 mole or a little less from the lemon juice. So it's not just the pH that determine how much of the reaction will happen but also how much of the non-dissociated acid there is in reserve.

You had enough acid left to also pickle the inside of the egg, a your son described so well. I believe that the acid denatures the protein (albumin) in the egg white. The protein chains sort of unravel and get tangled up with each other. They can't just roll past eah other any more. That makes it bouncy. It's similar to what happens when you boil an egg.

Mike W.


(published on 02/04/2015)

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