Why does soda dissolve a penny

A. Don't try to use science to explain why something happens until you have done the experiment and 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? Did you make sure your baby brother didn't take the pennies and buy a candy bar?

If you didn't do the experiment, do it. I had read a hundred times or more that Coke dissolves pennies. So I simply put some copper pennies and some copper plated zinc pennies in Coke under a few different conditions and made some very interesting observations.

A. Coke and other sodas contain various acids and some of the strongest acids available on the market. For example phosphoric acid. This acid reacts with the copper and the zinc of the coin and dissolves it. The hydrogen (H or H2) of the acid will connect with the copper(Cu) or zinc(Zn) of the penny and become copper phosphate(Cu2PO4-I believe) or zinc 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.

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A. It won't! There is a VERY low concentration of phosphoric acid ... not at all "Some of the strongest you can buy" and it is VERY diluted. For example, citrus drinks like lemonade and orange juice are TWICE as acidic.

Theoretically, yes, it will dissolve a penny, if you left it in for YEARS, and kept replenishing the Coke with fresh Coke. But none of that matters in terms of drinking it because when you drink it, you don't hold it in your mouth for years, and you have saliva to protect it.

John K [surname deleted for privacy by Editor]
- Binghamton, New York, USA
April 16, 2013

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A. 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, Pennsylvania, USA
2005

A. 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, Maryland
2006

A. 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 placed 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, Michigan USA
June 2, 2008

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June 2, 2008

Hi, Marc. We recently received a tip on topic 43238 that Myth Busters had busted the myth of Coke dissolving pennies. I didn't see the episode myself, so I decided I'd have to do the experiment again myself to refresh my memory. I cut post-1982 copper plated zinc pennies into eight pie slices and put some into Coke and some into vinegar ⇦in bulk on eBay or Amazon [affil links] . My first check, after 30 hours, and second check after 4 days, 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 about 30 days and that the Coke won't do it even in months. Hopefully time will tell.

I am confident that you could leave an old (pre-1982) copper penny in Coke for the better part of a lifetime without it dissolving. 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!

Take a lesson, students. Do the darned experiment! How hard is it to drop a penny into a glass of coke? -- and then you won't be as ignorant as your classmates, teachers, administrators, textbook authors, politicians and the rest of us who keep lecturing you about WHY coke dissolves pennies when it doesn't :-)

Otherwise, the same people who spread this misinformation will then go on to explain and demand that you accept their Gordian sociological, geopolitical, and complex earth science theories -- all "proven" of course by similar "research" :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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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 [Ed. note: 1982] when pennies previously minted were of 90% copper content.
I do know that a 1974 penny (copper variety) will slowly but surely dissolve 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, Florida, U.S.A.
November 21, 2008

Q. I mixed lemon juice and salt water in a approx. 60% (salt water) to 40% (lemon) and poured it onto gold plated computer parts. It promptly started bubbling slightly, and after it stopped, I looked at some of the parts, and they did not appear to have gold on them anymore. I don't know what happened, but if you do please let me know. Also, it was mixed in an old soup can, so it would have been exposed to zinc and steel. Thanks!

A. Hi Jacob. Your project is not ready for the science fair yet. You've conducted an uncontrolled experiment: you don't know what materials were in the parts, you don't know what the results were (what materials are dissolved and precipitated), and you conducted the experiment in an environment that you didn't control -- an old soup can which you feel participated in the reaction, but aren't sure, and which isn't made of what you think it was (there is no zinc in food cans). Instead try this ...

Take one homogeneous part (not a mix of stuff), wash and rinse it. Do Archimedes experiment to determine it's volume, from which you can determine its density. Dry it, and weigh it.

Then put it in a plastic or china dish (not reactive) with your measured amount of lemon juice, water, and salt. Capture the gas that evolves in a glass bottle. After the reaction, filter the solution and retain the solution, the filter, the part. Rinse, dry and weigh the part.

Now, between the weights, the gas collected, the filtered precipitates, the color of the filtered solution, etc., you have a little bit of data from which you can begin to try to figure out what happened. Good luck!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 7, 2013

A. A novice in science, I put a penny in Coca-Cola when I was a kid. Now in India, I have resorted to Thumbs -Up. Does a great job, better than coke! Took care of a lot of the work but you probably have to go to the next strongest chemical. Cheers, sincerely done it!

A. Hi Tenley. First off, do you have a clear understanding of what you mean by "cleans a penny the best"? Secondly, do you have any reason to believe your hypothesis that diet soda will clean a penny best? Thirdly, who would care and why? Nobody uses soda to cleans pennies, and Darwin himself urged us not to (paraphrasing) 'waste our lives going down to a gravel pit to sort pebbles by color and shape with no larger aim'.

I think I'd rather cut pennies into quarters/pie slices so both the copper and zinc are exposed, and try to make them corrode. Rather than using different sodas, I'd pick one like Coke, and try such things as:

Regards,

A. Yes, James, that project will indeed resolve your confusion over the conflicting statements.

However, when you've learned which of the four sodas corrodes a penny the fastest (assuming that any do), what will you have learned that is actually of any value? I think that's the bigger question. Seriously, who cares which of four sodas corrodes a penny most, and why should they care? And what extensible lesson will you learn from the project?

Personally, I think the lesson to be learned is how to conduct a science experiment, rather than which of four sodas is most corrosive to a penny. Good luck.

Regards,

A. I honestly don't know what people are saying about how soda won't corrode a penny when placed in soda for prolonged periods of time since I recently did my science fair project on which soda corrodes a penny the most, containing the following: coke, diet coke, coke life, sprite, and mountain dew. It seemed that mountain dew and coke did the most corrosion so therefore, I have no clue where the people who said it will do nothing heard that from.

Hi Andrew. Thanks for the feedback. Your results were significantly different from mine. While there was some discoloration, I saw no substantial dissolution in Coke after 4 days, even though I had cut the penny into pie slices to make sure the zinc inside was well exposed.

Regards,

A. Hi Trey. Andrew just said it does, I just said it doesn't -- how do you suppose your science project could resolve this impasse?

Regards,

A. I was surprised to see that people online were claiming that coke dissolves copper pennies. The reason why I was surprised is not so straight forward as one might expect. Copper is a noble metal, so we don't expect it to be attacked really by any low pH. Even boiling, full concentration (33%) hydrochloric acid won't attack copper. Yes, I have tried this. However, any acid that includes additional oxidizing potential above that of H+ ions is likely to attack copper. Phosphoric acid is just such an acid because it includes the polyatomic phosphate ion, which has 4 oxygen atoms attached. That oxygen would love to have the copper even though the hydrogen ions don't care about it.

But there's an additional surprise that a good metallurgical chemist should know: Phosphates tend to be very insoluble. This means that what we expect to happen if we immerse copper even into a very strong phosphoric acid is for the acid to attack the surface of the copper which then quickly builds a very thin copper phosphate passivation layer (same deal with iron and iron phosphate). The passivation layer will be immune to all acids.

Now, there is the possibility that boiling or super-heated coke could de-stabilize the passivation layer and re-enable reaction. Also, if you tumble a bunch of copper pennies for example such that they scratch each others passivation layers off in abrasion, we may expect to see a very slow chemical attack by coke.

Even zinc phosphate is insoluble in water, so new zinc pennies should behave the same way.

Sodas without phosphoric acid should attack iron and zinc, but these reactions are likely to be slow unless you raise the temperature quite high. Such sodas won't attack a penny though because they won't get past the copper coating.

Have tried hundreds of dissolve metals in acid experiments, so this info. should be quite reliable.

A. Thanks Matt. I've only done it three times that I recall, but my answer was: coke dissolving pennies is very easily disproved urban legend. I personally have not disproved the possibility that Mountain Dew, Diet Coke, etc., can dissolve pennies ... I tend to doubt it but haven't demonstrated it -- so what I doubt or don't doubt means nothing :-)

Regards,

  Nice answer Matt. We use zinc phosphate a lot as a high end treatment on hot dip galvanized for corrosion resistance.