Why does tarnished penny weigh less than shiny one?

A. Hello Claire,

Ask yourself a few simple questions about what you are doing.

How many old coins are shiny?

How many new coins are tarnished?

What is the tarnish composed of.

How does the weight of an old coin compare with the weight of a new coin.

If you think about it for a while you will probably conclude that the tarnish is an indicator of the age of a coin. Old coins will have more wear than new coins and so they will weigh less.

As a matter of interest if you have two identical weight untarnished coins. You then tarnish one of the coins it will become heavier and will weigh MORE than the untarnished coin. The difference is so small that you are unlikely to have access to scales that will be able to measure the difference.

Hope this helps.

A. I applaud you, Claire, for doing the experiment before asking us what the result should be; you are doing science!

However, I would expect the tarnishing process itself to add to the weight of a penny, not reduce it. That is because the copper on the surface takes oxygen out of the air to form copper oxide tarnishes on the surface of the penny. So for each atom of copper previously on the surface, you know have a molecule of CuO or Cu₂O.

I suspect that this tarnish has worn off and rebuilt itself on the tarnished pennies and that you are seeing a reduced weight due to wear not the tarnishing action. Your experiment was nice work but you seem to have at least one uncontrolled variable, wear, that is affecting the weight.

A. One might also wonder if she is comparing older copper pennies to the newer plated ones..I've never done a comparison in weight, but now I'm wondering if they both weigh the same in new condition.

A. The composition of pennies was changed in 1982. It is possible that your tarnished pennies are before 1982 and your shiny pennies are post 1982. If that is true read on: From 1962-1982 the composition of pennies was 95% zinc and 5% copper. In 1982, the gov't decided that copper costs too much, the result of this is the current penny with a composition of 97.5% zinc and only 2.5% copper. If you check the periodic table, the atomic weight of copper is 63.546, while zinc is 65.38. You do the math...

 Hi, Margot. I don't think there is any way to get from atomic weight to density without the almost impenetrable math of nuclear physics and quantum dynamics, but the density of copper is 8.92 g/cm³ and the density of zinc is 7.1 g/cm³, so the 1983 and newed plated pennies are significantly lighter than pre-1982 pennies.

Regards and thanks,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. According to the United States Mint "The Composition of the Cent

Following is a brief chronology of the metal composition of the cent coin (penny):

* The composition was pure copper from 1793 to 1837.
* From 1837 to 1857, the cent was made of bronze (95 percent copper, and five percent tin and zinc).
* From 1857, the cent was 88 percent copper and 12 percent nickel, giving the coin a whitish appearance.
* The cent was again bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962.
(Note: In 1943, the coin's composition was changed to zinc-coated steel. This change was only for the year 1943 and was due to the critical use of copper for the war effort. However, a limited number of copper pennies were minted that year.)

* In 1962, the cent's tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.
* The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year.

www.usmint.gov/about_the_mint/fun_facts/index.cfm?action=fun_facts2

A. Weigh the bottle with the pennies, Mindy, then weigh it or a similar bottle without the pennies to get the net weight of the pennies. Divide by the weight of a single penny. Good luck.