ave you ever stood infront of a vendingmachine, pumping inyour change, and themachine just keptspitting out one dime,until you realized that it was a

Canadian dime? Why can’t you use Canadiancoins in American vending machines, and viceversa? And how do vending machines distin-guish between real money and fake money?What are our coins made of? Are nickelsmade of nickel? These questions and manymore will be answered as we examine thecaptivating chemistry of coins!

Early cashMetallic money has been around for thou-sands of years, while paper money has onlybeen popular for a few hundred years. Thefirst coins were worth their face value of what-ever precious metal they were made from.Today, all coins are deliberately made to be

worthless than their facevalue, so as to prevent them from being melted down and themetals recovered and sold. All coins wereoriginally made from gold, silver, and copper,and these elements are still referred to as thecoinage metals. The drachma and denarius,which were widespread in Greek and Romantimes, were composed of silver. The aureus, agold coin, was also popular.

AlloysAlthough some ancient coins were

sometimes made from pure metals, today, allcoins intended for circulation are made fromalloys. An alloy is a homogeneous mixture oftwo or more elements, one of which must be

a metal. There are numerous advan-tages to using alloys. Alloys typically are

harder, more durable, and more corrosionresistant than the pure metals by themselves.Pure gold and silver, for example, are verysoft, and would not hold up to the wear andtear that circulated coins experience. Even theancients were well aware of the advantages ofalloys, as bronze was a common materialused to make coins. The development ofbronze was soimportant that anentire historicalera—the BronzeAge—was namedin its honor. Thebronze alloyused to makecoins today istypically com-posed of 95%copper, 4% tin,and 1% zinc.

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By Brian Rohrig

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A newer coin, the Sacagawea dollar,looks like a gold coin. It is actually made froman inner core of copper surrounded by anouter layer of manganese brass (an alloy ofcopper, zinc, manganese, and nickel). Brasswas chosen because of its gold color, sinceprevious dollar coins were disliked by con-sumers because they too closely resembledother silver coins such as quarters. Brass isan extremely durable metal with excellent cor-rosion resistance, as evidenced by its com-mon use in instruments and plumbingfixtures.

Even though the 1792 Mint Act man-dated that all American coins be made fromcopper, silver, or gold, few American coinstoday actually contain these metals. They havebecome far too expensive. These preciousmetals have been replaced with cheaper met-als, even though most coins do tend to retainthe appearance of the more valuable metals.

A pretty pennyThe ubiquitous penny used to be made

mostly of copper but is now mostly zinc. Zincis much less expensive than copper. Today’spenny is made up of 97.5% zinc, with a paper-

thin copper coating that only makes up 2.5%

of its total mass. This change came about in1982. From 1962 to 1982, the penny was95% copper and 5% zinc. This makeovercame about because the value of the copper ina penny began to approach one cent andlooked like it might rise higher. As a result,there were nationwide penny shortages due toincessant hoarding.

There are several ways to distinguishbetween old and new pennies. A post-1982penny has a mass of 2.5 grams, while the pre-1982 pennies have a mass of 3.1 grams. 1982pennies may have either mass. Since all pen-

nies have an identical volume, a greater massindicates a greater density. Copper is denserthan zinc. (The density of Cu is 8.96 g/cm3,while that of Zn is 7.13 g/cm3.)

Another big difference between old andnew pennies is their melting point. If heatedover a Bunsen burner, the new penny will bereduced to a silvery liquid blob in just a fewmoments. The older copper pennies can beheated over a Bunsen burner flame withoutmelting. Zinc melts at a much lower tempera-ture than copper. The melting point of zinc is420˚C, while that of copper is 1083˚C.

Don’t eat the changeBefore 1982, if a small child swallowed a

penny, doctors would generally advise to justlet it pass, since the hydrochloric acid (HCl) inthe gastric juices of the stomach will not reactwith copper. If you are familiar with the metalactivity series, then you will know that zinc ismore reactive than copper, and HCl will reactwith zinc.

If a newer penny is swallowed and thecopper coating has worn thin or has devel-oped even a tiny crack, the HCl in the stomachcan react with the zinc within the penny. If thepenny remains in the stomach long enough, itcan develop some jagged edges as the stom-ach acid eats away at it. These jagged edgescan potentially perforate the intestine as thepenny passes through the digestive system.However, most of the time these swallowedpennies pass with little harm done.

Changingchange

The penny is not the only coin toundergo a makeover in recent years. TheCoinage Act, passed by Congress in 1965,mandated that silver be either removed oreliminated from dimes, quarters, and half dol-lars. Silver was completely removed fromdimes and quarters in 1965 and replaced withan outer layer of a copper-nickel alloy bondedto an inner core of pure copper. In 1971, thecomposition of all half dollar and “silver” dol-lar coins were changed to that of the dime andthe quarter. With coins nowadays, little is as itappears. Not only are our “copper” penniesmostly zinc, but our “silver” coins are mostlycopper!

Dimes and quarters minted before 1965were composed of an alloy of 90% silver and10% copper, and they are considered some-what valuable by collectors. You can easilytest for the presence of silver with a simpleexperiment. Rub a little mustard on a silvercoin and also on a nonsilver coin and let themstand overnight. In the morning, rub off the

ChemMatters, APRIL 2007 15

Thin copper outer layer

Zinc core

Cross-section showing the structure andcomposition of a post-1982 penny.

2 HCl(aq) + Zn(s) Þ H2(g) + ZnCl2(aq)

Mustard mystery? If you suspect you have a“silver” coin, apply a generous portion ofmustard to it and let it sit overnight.

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In the morning, remove the mustard. If silver isindeed present, there will be a tell-tale blackspot, indicating Ag2S.

The mass of a penny tells you whether it wasmade before or after 1982.

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mustard. A black spot will remain on the silvercoin, but not on the nonsilver coin. Mustardnaturally contains sulfur compounds, and sul-fur reacts with silver to form a black precipi-tate of silver sulfide (Ag2S).

Coin magnetismEver wonder why Canadian coins cannot

be used in American vending machines, andvice versa? To answer this question, test anumber of American coins with a magnet. It isdoubtful you will find any that areattracted. The only exception isthe 1943 zinc-plated steelpenny, which was manu-factured during WorldWar II to conservecopper for the wareffort. There areonly three ele-ments that areferromagnetic(stronglyattracted to amagnet) at roomtemperature.These are iron,nickel, and cobalt.Even the nickel coin con-tains only 25% nickel. Therest is copper. The concentration of

nickel is not greatenough to make the coinferromagnetic.

When you place acoin in a vendingmachine, up to a dozentests may be performedto verify whether a coinis genuine. Sensorswithin the vendingmachine measure thecoin’s weight, diameter,

and rollingspeed. An electrical current

passes through eachcoin deposited to

determine its rate ofconductivity, thusdetermining itsmetallic content.The coin alsopasses throughthe poles of amagnet. If a

coin is magnetic,it is rejected by

the machine.In Canada, the

vending machines mustbe arrayed differently, since

most Canadian coins are mag-

netic. However, Canadian nickels have beenmade mostly from copper since 1982. ButCanadian dimes, quarters, and one and twodollar coins are still mostly made from nickeland therefore are strongly attracted to a magnet.

One bus driver in Alberta, Canada, usedthe magnetic properties of Canadian coins tohis advantage. Every day after work for 13years, he would collect a number of coinsfrom the coin collection unit on his bus using

Experiments With Pennies:1. See how many drops of water you can place on a penny, using an eyedropper.

Because of the incredible surface tension of water, you can fit anastonishing number of drops on a penny before they will tumble off.

2. Determine which household substance is the best copper cleaner.Immerse discolored pennies in various household solutions, suchas pop, juice, milk, vinegar, ketchup, lemon juice, and detergent.Leave the pennies immersed for 1 week. Remove the pennies andexamine them. Which substance did the best job at cleaning thepennies? Which did the worst job?

3. Place several shiny pennies between the layers of a paper towelthat has been soaked in vinegar. Sprinkle some salt on the penniesas well. The next day, observe what happens. The pennies will havedeveloped a green coating of verdigris, or copper (II) acetate. SeeFeb. 2003 issue of ChemMatters for more details.

http://chemistry.org/education/chemmatters.html16 ChemMatters, APRIL 2007

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a long pole with a magnet attached to the end.He collected a fortune, amassing over $2.3million. But he was eventually arrested, arous-ing suspicions when he purchased an$800,000 house on an annual salary of only$38,000!

An interesting experiment can be con-ducted with a pre-1982 Canadian nickel, amagnet, and a Bunsen burner. Place thenickel on the magnet, and holding the mag-net with tongs, heat the nickel over the Bun-sen burner flame. After a short while, thenickel will fall off! When heated, ferromag-netic substances lose their magnetic proper-ties. Substances are magnetic because tinyregions within the material known asdomains are all aligned in the same direction.When heated, these domains becomeunaligned, causing the substance to lose itsmagnetic attraction. After the nickel cools,the domains realign and it will once again beattracted to a magnet. The temperature atwhich a substance loses its magnetic proper-ties is known as its Curie point. For nickel,

the Curie point is 375 ˚C, easily obtainablewith a Bunsen burner.

If you have some foreign coins at home,test them with a magnet. Some will be mag-netic, but most will not. British pennies aremagnetic, as they are made of copper-platedsteel. They are even part of a popular chil-dren’s toy where a magnetic pyramid is constructed.

Loonie tooniesThe two-dollar Canadian coin—

affectionately known as thetoonie—is a fascinating amal-gam of art and chemistry. It iscomposed of an outer ring ofmostly nickel, with a gold-colored inner disk of 92%copper, 6% aluminum, and2% nickel. The outer ringis strongly attracted to amagnet, but the inner ringis not. If this coin is heatedstrongly over a Bunsenburner flame and then quicklysubmerged in cold water, thesmaller inner coin can be made topop out!

All metals expand when heated, but notat the same rate. The amount of expansion amaterial experiences when heated is known asits coefficient of linear expansion. Copper hasa higher rate of expansion when heated thannickel, which also means that copper shrinksmore rapidly when cooled. When plunged intocool water directly after heating, the inner coinwill shrink at a greater rate than the outer ring,causing the inner coin to fall out.

When the toonie was first introduced in1996, some defective coins would separate ifgiven a hard blow or frozen. For many Cana-dians, it was great sport to see if the twoparts of the toonie could be separated. Wear-ing the smaller inner coin as a necklace waseven considered a fashion statement. Thisflaw in the toonie was corrected not long afterits debut. It is currently against the law inCanada to deliberately separate the two partsof a toonie.

Well, hopefully some of the questions atthe beginning of the story have now beenanswered. Whether jingling in your pocket orslung around your neck on a chain, coins provide you with yet another opportunity todiscover chemistry in everyday items.

Theone-dollar

Canadian coin is knownas a “loonie,” for the

picture of the loon on its face.The “toonie” is the slang for thetwo-dollar Canadian coin (which

alludes to both “two” byanalogy with the

loonie, and to“Looney Tunes,”again paired with

the loonie).

ChemMatters, APRIL 2007 17

Brian Rohrig teaches at Jonathan Alder HighSchool in Plain City, OH. His most recentChemMatters article, “NASCAR: Chemistry on theFast Track,” appeared in the February 2007 issue.

REFERENCESHagenbaugh, B. A penny saved could

become a penny spurned. USA Today.July 7, 2006, pp. 1B–2B.

Huss, F. Ferromagnetism and the CuriePoint. Chem 13 News. Dec, 1989, p. 6.

McClure, M. Chemical Counterfeit Catcher.ChemMatters, Oct. 1997, pp. 13-15.

Yeoman, R.S. The Official Red Book: AGuide Book of United States Coins, 60thEdition. Whitman Publishing: Atlanta, GA,2006.

Wikipedia. http://en.wikipedia.org/wiki/ToonieScience Learning Center. http://www.fi.edu/

pieces/knox/mustardtrick.htm