Pennies in tap water and salt water

[Carl H.]

My hypothesis was that if one penny was placed in tap water and one in salt water, the penny in the salt water would corrode quicker than the one in fresh water. After 7 days, the one in fresh water started to turn green, but the salt water penny got shiny and clean. I can't find anything to explain why the salt water penny didn't corrode. One penny was 1969 and one was 1972 so they are of the same composition. I need to explain why the salt water penny did not corrode.

[Louise]

My hypothesis was that if one penny was placed in tap water and one in salt water, the penny in the salt water would corrode quicker than the one in fresh water. After 7 days, the one in fresh water started to turn green, but the salt water penny got shiny and clean. I can't find anything to explain why the salt water penny didn't corrode. One penny was 1969 and one was 1972 so they are of the same composition. I need to explain why the salt water penny did not corrode.

Hello!

It is usually a good idea to repeat your experiment multiple times. I don't know how much time you have, but if you have time, I would repeat this with multiple pennies. For all you know, there was something strange about that 1972 (or 1969) penny that created an abnormal result. And with only only trial, you really don't know if these results are representative of penny chemistry.

But, to answer your question, I will suggest some things that may be happening in the salt case.

There is a common trick to clean pennies where you use pennies, vinegar (acid) and salt.

See this link:
http://chemistry.about.com/cs/demonstra ... 22204a.htm

Now, we recently had a discussion on this board about the pH of water, and it can be somewhat acidic. So, it is possible that your water was slightly acidic and so this reaction happened over a long time.

There is also an electrochemical method to clean silver where you use aluminum foil and salt water and baking soda.

http://www.darylscience.com/Demos/Silver.html

So, I would look at every part of your setup and make sure you don't have some other chemistry going on. Was your container glass or plastic (probably unreactive) or metal (maybe a problem). Can you test the pH of the water?


Louise

[Craig_Bridge]

If the penny in your salt water solution got shiney, it obviously underwent a chemical reduction reaction. The copper oxide was reduced to shiney copper. These types of chemical reactions are called oxidation reduction reactions. Something gives up oxygen (or something similar) and is reduced. Something else oxidizes (takes up oxygen or similar). The real question is what oxidized in your case.

The composition of these pennies were a bronze (95% copper, 5% zinc).

Look up the galvanic series to see what other metals might be oxidized preferentially. Zinc would definitely oxidize preferentially; however, if the surface copper was already oxidized, the surface zinc in the penny would have been as well, so something else is involved.

Look up the mineral content in your water supply. iron, magnesium, manganese, calcium for other possibilities. If you were using an aluminum container, did it turn black?

As for the penny in tap water turning green, again check your water supply content. It obviously underwent a chemical reaction The oxidized copper gave up oxygen for something else. Look up Cu(OH)2, CuCl2, CuCO3, CuCl for some possibilities.

[davidkallman]

Just to check that nothing funny was going on, the U.S. mint reports in: The Composition of the Cent:

http://www.usmint.gov/about_the_mint/fu ... fun_facts2

Composition changes happened most recently in 1962 and 1982, well within your experiment.

Does anything change if you use a post 1981 penny?