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Q & A: Eggshells in Vinegar - What happened?

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Q:
WHY DOES AN EGGSHELL DISINTEGRATE IN VINEGAR?
- Anonymous (age 13)
St. Francis, Bakersfield, CA, USA
A:
Vinegar has, among other things, a chemical called acetic acid (about 3% of it is acetic acid). Egg shells contain calcium carbonate. Calcium carbonate in the egg shell reacts with vinegar to form carbon dioxide (can be seen as bubbles in the vinegar).

The reaction is: CaCO3+ 2H+ -> Ca+2 + H2O +CO2

(published on 10/08/07)

Follow-Up #1: dissolving eggshells

Q:
Besides vinegar, what else can dissolve an eggshell? I tried mixes of various things such as soda, wines, beer, and even 409. Why did some of them dissolve the eggshell?
- Danielle
Rialto Ca, USA
A:
If you look at the reaction above, you'll see that the only contribution of the acetic acid is 2H+, two protons. Any acid in water can contribute protons, so the key thing to look for is whether the chemical is very acidic. Some materials, such as cleaning fluids, may contain powerful solvents for oils, but that's not what dissolves an eggshell. Others, like colas, may be acidic enough. You tried the experiment, so you know which work better than I do. Probably, you could measure the acidity (i.e. the pH) of the liquids (maybe using cheap litmus paper) and predict which will dissolve the shell just by which have a low enough pH.

Mike W.

Lee H


(published on 03/03/07)

Follow-Up #2: egg shell in vinegar

Q:
This is not a question but rather an answer and info on the process whereby an eggshell is dissolved in vinegar (as in osmosis expts)...the eggshell (calcium carbonate) is not water soluable so to make it water soluable so that the shell dissolves, leaving the membrane, you put it in vinegar (acetic acid). The vinegar (acetic acid) reacts with the eggshell (calcium carbonate) to produce a water-soluable compound, calcium acetate, and carbon dioxide gas (the bubbles on the eggshell). Water is NOT produced from this reaction--vinegar has water in it as in diluted to 5-10% acidity. The answer given to the 12 year old questioner was not exactly accurate.
- Theresa Estes
Walker, MO USA
A:

I’m not sure your answer is as different as you think. Calcium acetate is somewhat water soluble, as you say. It goes into solution primarily as calcium ion Ca+2 and negatively charged acetate ions. So that’s what we showed. Since the acetate was there to begin with, it doesn’t appear in the formula showing what changed in the reaction. I don’t see how you get the reaction formula to balance without making an H2O, which would join the other water molecules already there. It’s true, however, that several different complexes of CO2 and water and its ions are in equilibrium in solution, so the  CO2 produced isn’t all in the simple form.

Mike W.


(published on 10/16/07)

Follow-Up #3: Effects of liquids on egg shells

Q:
could you describe the effect of Windex (pH of 9), Toilet Bowl Cleaner (pH of 0) and Oven Cleaner (pH of 13), water (pH of 7) and vinegar (pH 5) on eggshells?
- Gabby (age 13)
Canada
A:

Nothing we could say would be as vivid or accurate as what you can opbserve yourslef by doing the experiment. We do have something important to add, however. That oven cleaner can be quite nasty, so be sure to carefully follow all the handling guidelines- gloves, ventilation, etc. Some care is also good with the other cleaners.

Mike W.


(published on 02/23/14)

Follow-Up #4: do eggshells completely dissolve?

Q:
If HCl is added to ground up eggshell, would the eggshell dissolve fully or would there be a residue?
- Arielle (age 18)
America
A:

That is an excellent question to answer by doing the experiment. If I had to guess, some of the membrane that usually sticks on the inside of the shell wouldn't dissolve. But don't take my word for it.

Mike W.


(published on 02/24/14)

Follow-Up #5: calcium acetate?

Q:
This website is inaccurate and has no evidence
- Steven Lyod (age 19)
Tulsa, Oklahoma, united states of america
A:

It's unclear what you think is inaccurate. So far as I can tell, we say what everybody says except that we describe part of the product as being Ca2+ rather than as calcium acetate.  So I guess the issue is to what extent the calcium ion is free in solution and to what extent it's bound to the acetate ions. This website http://calcium.atomistry.com/calcium_acetate.html says on that issue  "The electrolytic dissociation of the salt in solution, calculated from the freezing-point lowering, is much greater than that indicated by the electrical conductivity". That sounds like many of the calcium ions may be loosely bound in hydrated complexes with acetate ions. (That's a little like the BeSO4 that I did some of my thesis on. Some of it goes into solution as free ions, some as a non-conducting hydrated neutral complex, and some as neutral molecules.) At this point you probably need a chemist if you want a more detailed description of what state the calcium ion is mostly in in vinegar. 

The main point, however, is that other acids (e.g. HCl, citric, ...) also dissolve the eggshells. There's no special role played by the acetate. The acidity is the key. You can find a full discussion with lots of data here:  http://en.wikipedia.org/wiki/Calcium_carbonate#Solubility_in_a_strong_or_weak_acid_solution. The solubility depends mostly on the pH and much less on the details of what other ions are around. So the common description of what's in solution as "calcium acetate" is misleading. It's clearer to think of it as the calcium ion.

Mike W.


(published on 04/29/14)

Follow-up on this answer.