Letter 33608

Why does soda dissolve a penny  

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Hi,

I need help on my biology project and i can't find real research, only on cleaning pennies but i have to find a real answer from you.

Can you tell me why soda dissolves a penny? And by doing that why 7up, Coke, Rootbeer, and Mountain Dew dissolves a penny also. thanks!

Joan B.
- San Leandro, CA, USA

Don't try to use science to explain why something happens until you have verified that it actually does happen, Joan. How long did it take for these four sodas to dissolve the pennies? What was left, if anything, as sediment when the dissolving was done?

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Coke and other sodas contain various acids and some of the strongest acids availiable on the market. For example phosphoric acid. This acid reacts with the copper and the zink of the coin and dissolves it. The hydrogen (H or H2) of the acid will connect with the copper(Cu) or zink(Zn) of the penny and become copper phosphate(Cu2PO4-I believe) or zink phosphate. Mathematically a chemist could define how much acid is necessary for dissolving a penny, time alone is not the issue. The chemical balance of ions and molecules to be connected has to be calculated and the possible loss of Hydrogen has to be taken into consideration too because the acid is not stable indefinitely. Please excuse my English I'm still learning.

Rolf Reiser
Old World Tradesman Inc. - Boulder, CO USA

PENNIES ARE MADE OF COPPER, WHICH WILL DISSOLVE IN THE PRESENCE OF ACID, ONE OF THE MAIN INGREDIENTS IN ALL SODA IS PHOSPHORIC ACID. THE pH of soda is quite low, due to the phosphoric acid content. a penny will eventually dissolve in any aqueous solution with a low pH...low pH is considered 0-6 on the pH scale...the scale goes from 0-14...with 7 being neutral (water) and acids on the lower side (0-6) and bases AKA alkalis on the higher side (8-14).

Jason A. Brown
Sanmina-SCI - Athens, PA, USA

Soda dissolves a penny because of its low pH level and its phosphoric acid content. I'm doing a science fair project on this topic!!!

Alexandra D
- Mechanicsville, MD

A good answer probably should include something with reduction potentials. An elementary table of reduction potentials shows that the potential for Cu++ to be reduced to Cu is (+0.34). Positive reduction potentials are spontaneous. The reduction potential for H+ to H2 (sorry, no subscript) is 0. Thus, it does not seem likely that Cu will be dissolved in a simple acid/base reduction system as the total reduction potential would be (-0.34) a non-spontaneous value. A quick check of the net shows that strong nitric acid is used to dissolve copper, and when one looks at the chemical equation for this reaction, it is apparent that other oxidation and reduction potentials enter the fray. Perhaps, phosphoric acid works similarly (or, perhaps not at all). A good answer to this question would involve demonstrating that the overall reduction potential is positive. Unfortunately, I don't have the tables at my fingertips to confirm or refute such a supposition.
I do know this. Once, when I was teaching junior high, we performed an experiment in which pennies were to be supposedly dissolved in lemon juice for a period of 24 to 48 hours. When the pennies were removed a steel nail (if my memory serves me correctly) was place in the solution which would supposedly then be coated by copper. Out of perhaps 25 student experiments, only a handful showed any deposition of copper on the nail--none, with spectacular results (I still have no idea why even these meager results were achieved given the issues stated above). Unfortunately, this isn't the first time one of the experiments in the glossy mass-marketed science experiment books available in American bookstores failed me. This is why I chose to go with a book UNESCO put out for 3rd world countries and disaster ridden where spending money for science education is scarce. They emphasized using models to teach science concepts and when they did present actual experiments, the experiments usually worked. You would think that the glossy books sold in the bookstores by the thousands or millions would have the better experiments, but unfortunately they don't. There are a lot of ambiguities in life.
That is why I endorse the first answer to this question. Try it before you buy it!

Marc Vander Vliet
- Grand Rapids, MI USA

Hi, Marc. We recently received a tip on letter 43238 that Myth Busters had busted the myth of Coke dissolving pennies. I didn't see the episode myself, so decided I'd have to do the experiment. I cut post-1982 pennies into eight pie slices and put some into Coke and some into vinegar. My first check, after 30 hours, shows the vinegar to be far more aggressive than the Coke. I'm guessing from what I see so far that the vinegar will dissolve all the zinc in less than 30 days and that the Coke won't do it even in months. Hopefully time will tell.

If any student or teacher has a workbook or textbook that says in writing that Coke will dissolve pennies, we'd love to have the publisher info.

Regards,

Ted Mooney, P.E. 
finishing.com
Brick, New Jersey

One problem that presents itself here is the fact that modern pennies are made of zinc that has been electroplated with a very thin coating of copper.
I believe that this transition occurred during the 1970s when pennies previously minted were of 90% copper content.
I do know that a 1974 penny (copper variety) will slowly but surely dissole into a solution containing a combination of 6N HCl and 35% H2O2. This is a modern form of aqua regia where the H2O2 replaces the nitric as the oxidizer. I use it to dissolve 99.99% gold metal. It is slower to work than aqua regia but you do not have to remove the nitrates before winning from the AuCl3.
Oh but back to the penny...copper and zinc are two different animals. Older published experiments assumed that a penny would always be made of copper...and that Coke would never be new and improved :-)

Peace and light ~

Don Nance
Alchemist - Tampa, Fl, U.S.A.

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