Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: My class conducted an experiment with pennies suspended over 
cotton balls so soaked in white vinegar.  This was all sealed in a plastic 
cylinder.  As time went by, the pennies began to change.  They turned black, 
then green.  Being the procrastinator that I am, I left these pennies alone 
for about one month.  After this time, the paper clips used to suspend the 
pennies had fallen apart.  Those cotton balls touching the pennies turned 
orange.  But the really BIG deal was that the pennies from 1990 and 1992 had 
begun to fall apart too.  The edges crumbled, and the surface became pitted.  
The pennies from before 1983 were tarnished, but not really damaged.  We 
already discovered that the copper content has changed over the years.  Our 
question now is:  What is the material that covers the surface of the new 
pennies?  Is it easily damaged by vinegar?  Can we learn anything special from 
this?  This is for the fourth grade.
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I wish I really knew, but to be honest I would be guessing if I 
told you anything.  If I were you, I would have a look in the library for new 
books on coin collecting.  Another possibility is to get on the phone to local 
coin collectors/dealers and see if you can find out.  I would guess that the 
new coins are coated with copper but that the insides are some cheaper, 
lighter metal (maybe zinc).  Have you considered seeing if the older coins are 
heaver or lighter than the newer ones?  That might rule out some 
possibilities.  Otherwise, I would consider soaking the new coins in an 
ammonium solution and doing a bunch of qualitative-analysis tests to find out 
what is in it.  You might enlist the aid of a local high school chemistry 
teacher, who could enlist her/his class to "solve the mystery" for your class, 
and then come in and present the results of their study to your class.  I hope 
these suggestions are helpful.  I really like the thrust of this study; 
empirical observation, followed by rationalization, perhaps leading to a 
hypothesis which could be tested, etc.
Topper
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Pennies now contain a core of zinc that is basically covered with 
a foil of copper although the copper is relatively unreactive to the acetic 
acid in vinegar the zinc is highly reactive zinc cores were introduced around 
1982 if you file one corner of the penny you can have the zinc inside react 
while the copper on the outside is left relatively untouched they also weigh 
differently due to the difference in zinc's density vs. a solid copper penny.
Gerald Bokowy
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