|MadSci Network: Chemistry|
Density of a solid (esp. a crystal) depends greatly on the structure. One should be clear that penny is a piece of zinc electroplated with copper! Electrochemical baths are basically solution-solid interfaces. when Cu comes to the surface of zinc, it actually induces oxidation of Zn to its oxide because of the large difference between the electronegativity of Zn and Cu! This oxygen interference would be little in terms of mass but would affect the density to a significant extent. Thus the apparent density of the coin seems reduced than the parent ingredients. Please mail to email@example.com if you still have doubts!
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