Celebrating the 80s - 50th Celebration of Chem 13 News
United States pennies have been minted in the past few years with the less expensive metal zinc as a center filler and the more expensive copper metal as an overall cover. By using a file and some hydrochloric acid one can do a simple experiment that has redeeming educational value for only a few pennies in cost.
Collect from 5 to 10 late issue pennies (1982-83) and weigh them. Hold a flat or triangular file firmly, then grip a penny and stroke it on the file at a 45o angle to expose a tiny bit of the shiny zinc filler under the rim. Then flip the coin and expose the opposite edge on the opposite side of the coin (see figure). A single exposed spot will not work as effectively as opposite spots will.
Drop these coins into a graduated cylinder or beaker containing hydrochloric acid. 6 M HCl seems to work well. Let them stand for several hours. The HCl will eat out the interior of each coin and leave the copper covering. You should have at least 12 mL of 6 M HCl per penny. When the zinc has completely dissolved, as recognized by a lack of bubbling (about seven or so hours later), rinse the foil-like copper envelopes, dry and weigh them. Find the percentage of copper in the pennies. The speed of the entire reaction depends upon the molarity of the HCl and the size of the exposed areas.
The interesting part of the experiment is evident while watching the pennies when they are nearly disembowelled. At that time some will rise to the surface and fall again in cycles as the trapped hydrogen alters the buoyancy of each coin. Some will stay floating, and some will lie on the bottom. To complete the lesson, the copper envelope can easily be teased apart into the head, tail, and clamping rim sections.