Kids, chemistry is so common that it can even be found in money. Here we'll learn some science about coins and bills. Let's talk about coins first. Pennies obviously look different by their color while all the rest appear to be the same silvery color, until the new 2000 "golden" Sacagawea dollar coin came along. Have you ever thought about what metals are used to mint these coins? All the silvery-looking coins are actually made out of copper with small amounts of nickel. This nickel amount can be as low as 8% (dimes, quarters, and half dollars) to as much as 25% (nickels). Makes sense that the most is used for the nickel, right? The new gold dollar is really made of "manganese-brass", which is 88% copper, 6% zinc, 4% manganese, and 2% nickel. The penny has seen quite a few changes over the years. It was pure copper way back in 1793-1837. For the next 20 years, it was bronze (95% copper, 5% tin+zinc). Then even this small amount of tin was removed in 1962. In 1982, pennies became copper-plated zinc coins, with a thin coating of Cu (2.5%) over a pure zinc core.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
Have you noticed that some coins have grooved edges? Dimes, quarters, half-dollars, and dollars used to contain precious metals like gold and silver. Grooved edges helped stop counterfeiting. It also stopped the filing down of edges by people who were collecting (actually stealing) the precious metals. Even though coins no longer have such metals, grooved edges are kept because they help the visually impaired to identify them. The chemical element symbols for all of these coinage metals are: copper (Cu), nickel (Ni), manganese (Mn), zinc (Zn), tin (Sn), gold (Au), silver (Ag).
Did you know that "paper" money is actually made of 1/4 linen and 3/4 cotton? This makes it more like fabric than paper and explains why it's washable. There are many counterfeiting-fighting features used, especially on the new "big head" bills, but we'll mention only a few here. You notice of course that the portraits are enlarged and off-center. This allows for a watermark, which is another portrait visible when held up to bright light. The watermark is formed by varying paper density in a small area during the papermaking process, and does not copy on color copiers or scanners. A security thread appears in a different location on each new denomination. When held to a light you can see "USA TEN" or "USA TWENTY", etc., and flags in this thin thread. When viewed under ultraviolet light, the thread glows different colors. For $5 it is blue/purple, $10 is orange, $20 is a bright green, $50 is yellow, and $100 is pink-red. These colors arise from various fluorescent dye molecules used in the inks. Finally, for every new bill except the $5, a color-shifting ink feature is used. The number in the lower right corner changes from green to black as the bill is moved. The change in color is the result of multi-layered metallic flakes added to the ink. When the bill is tilted, light reflects off these flakes at different wavelengths and changes colors. This is called color diffraction, which is also responsible for the color variations found on the wings of some butterflies.
Kids, a fairly addictive thing to try on the internet is the site www.wheresgeorge.com. This tracks dollar bills by serial number as they float around the country. Have fun!
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
October 2000
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References: The Bureau of Engraving & Printing's website at www.moneyfactory.com, and the U.S. Mint's website at www.usmint.gov (both have great kids sections). Also www.pbs.org/wgbh/nova/moolah/anatomyprinting.htm
