I'm guessing that the process you saw was a
"wet-chemical plating" technique. The
probably a mixture of Zn in a sodium hydroxide
solution. When the penny is added to this
solution, the Zn will deposit on the copper and
make it appear silver (I think the solution needs
to be heated for this to happen, but I can't
remember for sure). As I recall,there is
additional trick with this experiment where if you
heat that "silver" penny, the Zn and Cu can form an
alloy (brass) with a "golden"color.
Sorry but I couldn't come up with a good answer for
you. I found something that says the solution
should be 3M of NaOH added to powdered zinc. The
solution should also be kept near boiling. But I
don't know much else.
I don't know if I'd
suggest you try this for yourself as I believe you
should do the experiment under a fume hood. If
you do it, make sure you are in a spot with a lot
of fresh air.
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