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Name:  Jordan
Status: educator
Grade: 4-5
Country: N/A
Date: 1/13/2006

One of my students mixed vinegar and five copper pennies together as a science experiment for several weeks. When we returned from winter break the vinegar had evaporated leaving behind blue-green pennies and what appears to be crystals. Why did this happen? What chemical reaction occurred?

You had a reaction between the copper in the penny and the acetate portion of the vinegar (vinegar is a dilute acetic acid). There might be a little sulfur from the air in there, as well. You are probably looking at green copper acetate and blue copper sulfate.

If you do a search on the Internet--maybe something like "pennies and vinegar," you may find more information---this is a common experiment for science classes. Sounds like you are doing some fun things in your classes!

Patricia Rowe


Many of the soluble copper(2+) binary compounds (e.g. copper(II) nitrate, copper(II) sulfate, copper(II) acetate) are blue or blue-green in color when they are hydrated. I suspect that the crystals formed are hydrates of some copper(II) compound. However, copper itself is not soluble in most acids (not even hydrochloric acid). Thus, it can not be that the copper in the penny reacted with the acetic acid in the vinegar. I think the most likely occurrence is that the pennies used were highly oxidized and so the CuO (which is black) dissolved in the vinegar which then formed some other blue-green ionic hydrate (possibly copper(II) acetate), then when the vinegar evaporated, the crystals formed out of that solution. You could test this hypothesis by using very clean pennies (either leave the pennies in vinegar - in a sealed jar - until the pennies become very shiny or apply sandpaper to the pennies until they are shiny), then you can replicate the experiment conditions as before and see if you get much blue-green crystals. Alternatively you can set-up three containers, one with just vinegar, two with vinegar that has had pennies sitting in them for a while (until the pennies are reasonably shiny) and then with the pennies fished out, and a third with the pennies left in them. Allow all three to evaporate as before and see what you get - the idea being that you and your students can use this seemingly strange occurrence as a jumping point for more investigations.

Greg (Roberto Gregorius)

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