[collusion]

I put 2 pennies with notches filed into them in 40 mL of 6M HCl. One was a 1981 penny (95% Copper, 5% Zinc), and the other was a 2001 penny (97.5% Zinc, 2.5% Copper).  After 1 day, the zinc had reacted with the HCl, but not the copper, leaving a hollow penny. Some of my classmates left their pennies in for 4 days, and the HCl had turned green. Also, the new pennies in their solutions were completely gone.


 Why did the HCl turn green, an why did the new penny completely dissolve only after 4 days?

[jdurg]

The green HCl solution is due to the presence of some copper chloride in solution.  While copper is said to 'not react' with HCl, in reality it does react a tiny bit.  New pennies have a very thin plating of copper on the outside of the relatively pure zinc core.  What happened is that over the four days, the very tiny reaction between copper and HCl was able to dissolve the thin copper shell on the new penny.