Science Buddies: "Ask an Expert"

I'm doing a project to find out which liquid will tarnish a coin the fastest. Can you please tell me what kind of US coin I can use to have a good result on my project? Thanks.

From,
Van 7th Grade

Hi vanj510,

Any result is a good result and the more information you are able to gather the better conclusions you are able to draw.

As for what coin will show the greatest oxidation in solution I believe that all different coins minted from around the 1980s to now will be mostly composed of copper with an external layer of some nickel alloy. This excludes pennies which are mostly zinc with a copper based exterior.

In other words you won't get much difference between all the different silver coins.

Also be sure you define "tarnish" carefully in your problem statement before you start out on any real testing.

Depending on what grade you are in (Have you had chemistry yet or is there somebody who you can talk to about your project that knows chemistry?), you might want to do some research on cathodic protection and electro plating. Not that you necessarily want to cause these things to happen; however, you may want to make sure they don't happen and contaminate your experimental results.

The US mint uses alloys that won't corrode or oxidize quickly on their own or even in various weak salt solutions; however, place them in a solution with other metals and interesting (in terms of chemistry) things happen relatively quickly and can be sped up by the introduction of a voltage. Penneys mixed with any other US coins in a solution can cause cross plating of metals.

Putting oxidized coins in some solutions will actually cause a reduction reaction and reverse the oxidation.