Penny, Vinegar, and Blue Leftover
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!
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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Update: June 2012