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Copper and nickel plating from a vinegar solution

You specify that electricity is required to plate with copper, nickel, and a vinegar [in bulk on eBay or Amazon] solution. Could you please explain why I have observed what appears to be plating in the following experiment.

One pre 1982 penny and a nickel are placed into a small china cup and covered in about 1/4 cup of vinegar 5%. This cup is left on the counter for up to 2 weeks. At the end of 2 weeks the penny appears to have a silver spot on it and the nickel appears to have a copper spot on it. This is all done without electricity. Why does this appear to work?

Angela S
student - Lincoln Park, Michigan
2007

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Good observation, Angela! But please remember to set the parking brake if you park your car at the top of a hill, even if the manual says it will run only on premium gas.

Cars need gas to run properly and plating solutions need electricity to electroplate properly ... but just as gravity can make a car roll, galvanic action without external electricity applied can cause copper to dissolve from a penny (exposing the zinc core) and deposit upon a nickel given enough time. Metals which are more "noble" will plate out onto more active metals without electricity applied because they provide their own electricity. Sketch out how a battery works and how an electroplating cell works and you'll see that they are actually the same thing.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007