As a kid, I was quite the numismatist (I collected coins). Because I had a paper route I was always getting coins to pay for the newspapers and every once in a while I might get a wheat penny or a silver-clad quarter or half-dollar. On a rare occasion, I might see an Indianhead penny or even a silver certificate dollar bill! It turns out that coins were really quite boring for quite a while, as they didn’t really change much from the late 1960’s until almost 2000 with the advent of the States and Territories quarters. The Lincoln penny came out in 1909 and had an obvious change in 1943 due to the shortage of copper for the war and so the change to steel was necessary. It quickly changed back to the same copper wheat penny until 1959 and then changed to having the Lincoln Memorial on the reverse. It stayed this way until 2009 – or did it?
Materials:
Pre-1982 penny
Post-1982 penny
A file or coarse sand paper
4 cups of vinegar or lemon juice
2 clear containers that will hold at least two cups of solution
salt
Experiment:
Using the file or the sandpaper, scrape the edge of the pennies down on four sides removing some of the metal. I find that a file works a little better. Put one penny into each of the containers and add 2 cups of vinegar or lemon juice to each. Watch to see if anything happens and let it go for up to a week. After a week, add a little salt to the solution with the old penny and see if there are any changes occurring with this new solution.
What’s happening?
Hopefully on the newer penny you saw some bubbles emanating from the penny. You probably didn’t notice much with the old penny until you added the salt and then you should have seen the penny get much shinier. It turns out that copper was getting too expensive for our government to make pennies that were 95% copper so in 1983 they switched to a zinc penny that was copper coated. You can see how thick that copper coating is when the acid in the vinegar or lemon juice reacted with the zinc in the middle of the penny to form hydrogen gas (causing the bubbles) and Zn2+ that was dissolved in the solution. We can’t make coins out of pure zinc because zinc is too reactive and it would get destroyed in a short period of time. Copper however is quite unreactive and can coat the zinc penny so that it well last for a very long time. Copper does react a little bit as you can tell the difference between a new and old penny. The old penny turns brown as gasses in the atmosphere tarnish, corrode, or patina, the copper. We can clean that up by reacting it with a little salt and vinegar (or try a little taco sauce!). The chloride ions in the salt are able to react with the patina on the surface of the coin and the vinegar helps to wipe away the coating. Salt has the same effect on other metals causing them to tarnish as well and we see this on our cars in the wintertime here in the Midwest.
References:
http://www.terrificscience.org/lessonpdfs/PennySandwich.pdf
To view all past “ChemShorts for Kids”, go to:
http://chicagoacs.org/articles/article_category/1
