kristima

Nitric acid is one of the few acids that dissolved copper (HCl and H2SO4 wont). Using concentrated nitric acid, the copper in the penny begins to react immediately. The color of the formerly clear nitric acids becomes green with lots of bubbles. The bubbles are toxic NO2 gas which has a brown color.

The demonstration is done in a fume hood to remove any of the NO2 gas generated by the reaction of the nitric acid with the copper in the penny (note this is a pre-1982 penny and made entirely of copper).

The reaction (since the Nitric acid is concentrated) is:

Cu + 4HNO3 = Cu(NO3)2 + 2NO2 + 2H2O

You can see that the penny is eaten away but still retains the raised pattern of Lincoln.

After the brown gas has stopped being produced the remaining solution (not blue) is added to baking soda to neutralize any remaining acid.