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Science experiment help!

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We were supposed to put a new penny in the oven for 30 minutes and compare it to both an old penny and a new penny. It was supposed to look dull and dark like the old penny. However, it ended up even shinier than the "control" new penny but without the copper color. Why? (Heated new penny on top, control new penny in middle, old penny on bottom)



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Ok, I could be TOTALLY wrong, but I'm reading that pennies made after 1982 only have 2.6% copper. They are now made mostly of zinc. 
When you heat zinc, it can diffuse with the copper forming brass.  

(Or so says what I'm reading and trying to understand on my very quick lunch break. I may need to hide my head in shame for being incredibly inaccurate. haha)

Was it supposed to be something like this?

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From what I can find the old pennies (pre 1982) form copper oxide and blacken-forming a bronze alloy.


1983 on should produce an alloy of zinc and copper, a brass alloy-which should be shiny.


So, to me, penny on top looks like it should. The bottom one just looks dirty and should go through the same process as the new penny.


Were you supposed to clean them first?


Here's another link that may or may not help


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As this book was written a while ago, maybe the even newer pennies have changed "formulas".  I would use either an older penny if the formulations have changed (they can be quickly deoxidized with vinegar for 5 min if necessary before oxidizing).  And I assume you had a high temp and used aluminum foil?  I'm not sure if the aluminum acts as a catalyst for the experiment or not.  I'll try to research a little but that's my first guess.


ETA -- it appears the composition has been the same since they made the last change so modern pennies are the same as when the book was written so that should not be the problem.


ETA2 -- The only thing I can think was the aluminum foil and the temperature. 

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We did the experiment exactly as written. We tried again with two more pennies. One was the 2012 in the middle on the picture. It also turned light and shiny. The other was 1993. It did what the book said it would.

Maybe the government has changed the formulation of pennies but didn't put it on the mint website I checked.? Strange.

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