Eggshells in Vinegar - What Happened?

WHY DOES AN EGGSHELL DISINTEGRATE IN VINEGAR?
- Anonymous (age 13)
St. Francis, Bakersfield, CA, USA

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 10/22/2007)

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

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 10/22/2007)

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

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 10/22/2007)

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

Nothing we could say would be as vivid or accurate as what you can observe yourself 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, goggles, ventilation, etc. Some care is also good with the other cleaners.

Mike W.

(published on 02/22/2014)

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

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 02/24/2014)

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

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 04/28/2014)

Would toilet bowl cleaner devestate the egshell since its Potential Hydrogen is 0 and would oven cleaner do nothing since its PH is 13 and waters is 7?
- landon (age 13)
United States

That sounds right to me. You can check by doing the experiment. Be careful with that toilet bowl cleaner, you'll need goggles and plastic gloves to work with it. 

Let us know how it comes out. 

Mike W.

(published on 12/08/2014)

I think you should answer the questions instead of directing the questioner to "do the experiment". Or at least you should have the answers somewhere we can look up.As much as I am interested in this as an experiment for my 8 year old son, and I am a science-lover myself, I currently have a garbage disposal that is jammed with egg shells and I am trying to come up with an environmentally responsible way to get those egg shells out of the places I can't get to with an auger.I am not alone, this seems to be a pretty common issue with garbage disposals.I read a posting about someone else trying to do the same, and I did everything she did except the vinegar.http://boards.straightdope.com/sdmb/showthread.php?t=545815By posting on the web you are part of a larger community. Please answer the questions.
- Rachel (age 55)
Blue Bell, PA

I can see why you might be frustrated to come to this page and not find "the answer." You're the first to mention a clogged garbage disposal or any other practical application—the previous questions were pretty clearly related to science fair-type projects. We usually encourage people to try those themselves first (after all, that's the point of a science fair) although we're happy to try to help explain their results. I'm jumping in for Mike here, so he can feel free to add anything he wants to say.

Consider this, as well: we're volunteers who do this for free to teach people about physics. There are a lot of questions we don't have time to respond to at all. More importantly, we may not know the answer to the question. I don't have the specific background knowledge to predict which household chemicals will or will not dissolve eggshells. Maybe a chemist would. If I had to know, I'd probably do an experiment.

Good luck with your garbage dispoal. Maybe I'll stop putting eggshells down mine...

Rebecca H.

Nothing to add to Rebecca's philosophy. On the eggshell issue, they're definitely not recommended for disposal, because they turn into a sort of heavy sand that sits in the pipes. They do make great garden compost. Vinegar should be good if it doesn't make problems for the pipes etc. It's environmentally benign. /Mike W.

(published on 03/23/2016)