Enrich your classes with Canadian coins
Doug De La Matter
Pity the poor coinage scientists who have to manage the costs Experiment 2 of inflation. As the Royal Canadian Mint has struggled to keep What other differences can be observed among these coins? the true cost of making a coin in line with the value of its constituent materials, the composition of low denomination coins Students may be able to suggest that nickel is a magnetic has changed markedly through the years. Fortunately for material. (Nickel, iron and cobalt are ferromagnetic. In this teachers, these changes lead to variations in properties that can article we’ll use “magnetic” to mean “attracted to a magnet”.) lead to interesting and convenient teaching opportunities and Have them test their nickel coin with a magnet. Have them classroom activities. correlate “attraction to a magnet” with mass and date.
This graph shows actual data from a variety of dated coins using One of the advantages to using coins as subjects of study is that an electronic centigram balance. Note that the Mint sometimes most people think a penny is a penny and a nickel is a nickel, so “cheated” in 2000 and 2001, by using coin blanks of the old variations in properties come as a surprise. Discrepant events (1982-1999) composition. Note also that the masses fall within are easy to produce as demonstrations, and by handing out the prescribed limits of tolerance given in the table. coins of several vintages, a variety of results show up in the classroom without the students being aware that there is “more than one correct answer”. Best of all, relative to supply-house Canadian Five-cent Coins materials, coins are cheap sources of interesting chemicals.1 Magnetic-old Not Magnetic Magnetic-new 4.8 A. Five-cent coins 4.6 This chart shows the history of the Canadian nickel.
4.4 Composition of the Canadian Five-cent Coina
From To Mass/g Composition Mass/g 4.2 1922 1942 4.54 99% nickel 4.0 1942 1943 4.54 88% nickel, 12% zinc
1944 1945 4.54 Chrome plated steel 3.8 1946 1951 4.54 99.9% nickel 1970 1980 1990 2000 2010 Year 1951 1954 4.54 Chrome plated steel 1955 1981 4.56 99.9% nickel Demonstration 1. Curie point 1982 1999 4.60b 75% copper, 25% nickel Both new and old nickels are magnetic, but they have different 2000 - 3.95b 94.5% steel, 3.5% copper, 2% nickel masses. Can we provide physical evidence to distinguish a. Source: Royal Canadian Mint (See Link 1) between the materials in b. The allowed tolerances on these masses are ±0.18 g for the 1982- 99 series, and ±0.12 g for the 2000 series.2 these coins? Clamp a ferrite (not neodym- Most students know two “facts”: ium) magnet in place and magnet 1. “Nickels” are made of nickel metal. suspend two nickels edge-to- coins 2. Nickel metal will be attracted by a magnet. edge from the magnet, using one pre-1980 and one post- Experiment 1 2000 coin. Position a can
“Can you weigh things accurately?” half-filled with water to catch metal can any hot coins that fall. Heat & water 1. Have students weigh ONE nickel each (from a variety of dates) the coins with a small flame and post their observations on a blackboard. Who is right? from a propane torch, 2. Then have them repost the data including the year of the avoiding the magnet as much coin and graph the results to see the change in composition; as possible. something is different — what??
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If the older coin is on the bottom, it will fall off (and into the can). well. We can detect these changes with a centigram scale If the older coin is on top, both coins will fall. weighing actual pennies. Again, a classroom experiment can be designed to have students organize data and find the changes CAUTION: Hot coins look the same as cold coins and they in composition. Some typical results are shown in the graph in roll away just as easily. Warn students not to try to pick up the next column. All masses fall within the allowed tolerances. an “escapee coin”. Have an ice-water bath handy for First Aid just in case. Most pennies dated 2005 and 2006 are not magnetic and weigh about 2.25 g. In our collections of several dozen pennies dated Explanation — The Curie temperature for nickel is 358oC. For 2005 and 2006, only one coin (dated 2005) stuck to a magnet iron, it is 774oC. At the Curie temperature, thermal motion and weighed 2.35 g. All the other coins for these two years overcomes the internal organization of the metal that allows it to have the properties of the zinc core model. So it appears that maintain its magnetic properties. The old coins are almost pure the Mint has returned to, at least mainly, the zinc core model for nickel and so, once the temperature exceeds 358oC, the coin 2005-2006. Either that, or the rising price of steel may have will fall from the magnet.3 The propane flame can heat the coin created artefacts that make easy-to-find collectibles for those who above 358oC, but not over 774oC, and so the newer coins, know how to look — and collectors are removing steel core pen- which have a steel (mainly iron) core remain below their Curie nies of these two years from circulation! Also, we find very few temperature and their attraction is not affected. The change in zinc core coins dated 2003 — perhaps another collectors' item. the pre-1982 coins is not permanent however. As they cool to below their Curie temperature, they will again be attracted to a magnet. (Incidentally, this provides an interesting argument to Canadian One-cent Coins say that the earth’s magnetic field is from an electromagnetic origin, not from a solid lump of magnetized iron. The earth’s core Not Magnetic-old Not Magnetic-new Magnetic-new temperature is above the Curie temperature of iron, so the iron 3.4 in the core cannot have a residual magnetic domain.) 3.2 B. One-cent coins 3.0 But copper is not attracted to a magnet, so pennies are much 2.8 less confusing, right? Not so fast!
2.6 Demonstration 2. Magnetic pennies Mass/g Sweep a magnet over some pennies. Some will stick to the 2.4 magnet. Is this evidence of magnetic copper?? 2.2 (If your class can understand the joke, use two ferrite magnets to pick up pennies. Wow! Since we are using a pair o’ magnets, 2.0 have we shown that copper is pair-o-magnetic??) 1965 1975 1985 1995 2005 Over the years, as the Royal Canadian Mint struggled to keep Year the cost of the penny under one cent, they have tried different innovations as you can see in the following table. Demonstration 3. “Peel a penny”
a Composition of the Canadian One-cent Coin 1. Prepare an intact “core” From To Mass/g Composition First, stretch a pair of tweezers so that the jaw stretches across 1942 1979 3.24 98% Cu, 1.75% Sn, 0.25% Zn the diameter of a zinc core penny (1997-1999). With this arrange- ment, the coin should stay firmly in the tweezers’ jaw without 1980 1981 2.8 98% Cu, 1.75% Sn, 0.25% Zn any pressure being applied to the instrument. This is important, b 1982 1996 2.5 98% Cu, 1.75% Sn, 0.25% Zn because you do not want the coin to slip into the acid beaker. 1997 1999 2.25b 98.4% Zn, 1.6% Cu plating IN A FUME HOOD, put only enough concentrated nitric acid in a 2000 - 2.35b 94% steel 1.5% Ni, 4.5% Cu plating 100-mL beaker so that the coin can be immersed in the acid just a. Source: Royal Canadian Mint (See Link 1) by lowering the tweezers into the beaker. b. The allowed tolerances on these masses are ±0.11 g for the 1982- Prepare about 300 mL of a concentrated (~1 M) solution of 96 series, and ±0.06 g for the two later series.2 NaHCO3 in a 400-mL beaker and place it beside the acid Note that in 1997 they tried the US coinage formula (zinc core) beaker. This will be needed to stop the reaction on the coin. for a penny, but quickly moved on to the steel core model — and (This procedure should be done only by the teacher or a then back again. Indeed, even while using the steel core in qualified technician! Do not allow students to perform this 2002-2004, the Mint continued to make zinc core pennies as reaction with nitric acid.) September 2006/Chem 13 News 17
Using rubber gloves and goggles, with the fans running in the (Note that all US pennies since 1982 have a copper-plated zinc fume hood and the hood door down as far as is practical, lower core and can be “peeled”. The advantage of the Canadian coins the coin into the concentrated acid. Brown, toxic NO2 gas will is that a variety of years can produce a variety of properties for evolve rapidly. After a few seconds, as the gas evolves, lift the your students to encounter.) tweezers and coin out of the acid and stop the reaction by immersing the coin in the bicarbonate solution. If copper Some teachers might wonder whether it’s legal to alter and/or remains on the surface of the coin, repeat the reaction with acid. partly destroy coins as suggested here. At ChemEd 99 the late Cliff Schrader presented a written opinion he had received from The zinc core will appear dark, but can be cleaned with any the US Mint that “damaging or defacing” a small number of coins commercial metal cleaner until it has a bright grey appearance. for educational purposes was acceptable practice. What the US If the reaction has been quenched promptly, the details of the Mint considers illegal is the practice of modifying any coin to stamped images will still be clearly visible on the coin. misrepresent it as a coin of higher value. We assume that coins modified in demonstrations or in student experiments will be 2. Prepare an intact “shell” removed from circulation.4 A zinc-core penny bearing the same date as the core prepared above can be nicked with a file at one or two places along the C. Related activities rim of the coin and the coin can then be immersed in 250 mL of 2 M HCl overnight. The beaker should be left uncovered and Brass pennies (copper-to-gold) away from any source of flame since hydrogen gas will evolve Pennies from both Canada and the US can be brass-coated slowly. After 24 h or so, the coin will have no zinc left in the safely in a simple modified copper-to-gold experiment. For core, but the copper plated “skin” will remain. This should be detailed instructions, look at the recipe by Cathy Sarisky at immersed carefully in a sodium bicarbonate solution and then in http://labs.timistry.net/labdb/get.tcl?experiment_id=228. pure water to prevent reaction between copper and the chemical residues of the process. In older methods, the clean penny is immersed in a mixture of granular zinc and hot 6 M NaOH.5 (Avoid powdered zinc for 3. How did you do it?! safety reasons.) Ms. Sarisky’s experiment also requires care, but seems to be safer than the older method. Another, safer Now you have a hollow “penny skin” and the solid zinc core method, which uses cold (not hot) 6 M NaOH over two days, has which will still bear the markings of the coin face. Present this to 6 your classes as evidence that you can “peel a penny”! Let them also been suggested. try to figure out how you did it. That leads into the next part. In the step where you heat the penny, you must do it carefully, particularly if the coin has a zinc core. If a zinc-core penny gets 4. Student activity hot enough, the zinc melts and forms an entertaining blob inside Provide the class with a series of pennies covering a variety of a small copper sack. However, with mild heating the copper and dates. Have students score the edges in two or three places zinc overlay will blend the metals into a brass mixture — gold! with a file. Place the scored coins in 2 M HCl solution (using at least 0.3 moles of HCl per coin). (This should be done by a Loonies and $2 coins safety-equipped teacher to avoid splashing and the related The Canadian $1 coin, popularly called the Loonie, was safety hazards.) supposed to be a “normal” dollar coin. For many years, a silver The next day, the teacher should collect the coins, check to see and then a nickel dollar coin bore the image of fur-traders and 7 that Zn cores have been dissolved away, and then WASH the natives in a cargo canoe. As the story goes, the stamping dies coins in 1 M sodium bicarbonate solution and then water before bearing this image for the new coin were “misplaced” in transit returning them to students. In our experience the steel cores from Ottawa to the mint in Winnipeg. Rather than run the risk of are not dissolved significantly by 2 M HCl in one day. having counterfeit coins produced, the Mint held up production until another design was implemented. This design had the Not all coins will have lost their cores ! image of a loon, symbol of the Northern Lakes. The coin is a • Correlate these results with the dates on the coins. 91.5% nickel core plated with 8.5% bronze.8 • Check the coins that resisted the acid using a magnet. • Can the results be correlated with the published composition The $2 coin, introduced in 1996, has an outer ring of 99% Ni of the coins according to their year? and an inner disc made from 92% Cu, 6% Al and 2% Ni. 9 If the demonstrations are done in this sequence, you can It has the embarrassing, simple moniker “Toonie”. Although provide discrepant events that can be resolved with a small the Curie temperature experiments work for both coins, we amount of investigation or testing on the part of the students. would advise against it since you can have a fairly large, hot You will have the opportunity to point out that “knowledge” that coin rolling around after it falls off the magnet. And although the they take for granted might not be true (i.e., “a penny is like all hot coins look identical to the cold ones, they don’t feel the other pennies”), but with simple evidence and organized same! Early on, people used to go to great lengths to separate thinking, the truth can be determined. the ring from the core on the Toonie, using heat, hammers, and 18 Chem 13 News/September 2006 occasionally liquid nitrogen. The novelty apparently wore off — 8. See http://www.cheminst.ca/ncw/articles/1991_loonie_e.html for an or the cost became prohibitive at $2/try! interesting description of how the bronze metal is prepared for use in this coin. Links 9. One of us (LJB) is on a low-key campaign to call the $2 coin “Nanuk”, the Inuit name for the polar bear on the coin, a name 1. The Royal Canadian Mint has a “front door only” website, so follow proposed in an editorial in The Globe and Mail when the coin was these directions to obtain technical information about the coins. introduced. Canadian readers, can you help?! g Go to www.mint.ca/royalcanadianmintpublic/ Click on Enter Click on The Passion Benzene in soft drinks then on The Museum then on Circulation currency Earlier this year several sources in Europe and North America then on Technical Specifications reported that some soft drinks contained benzene at levels up to Alternatively, use: http://en.wikipedia.org/wiki/Penny_%28Ca five times higher than is allowed in drinking water. Guidelines nadian_coin%29 for technical data on the one-cent coin. vary, but the limit is 5 parts per billion in Canada and the US, 1 2. For the 5-cent coin and for a portal to other Canadian coin ppb in the European Union. information look at http://en.wikipedia.org/wiki/Nickel_%28Cana dian_coin%29. This page has links to information about coins of A report in the Journal of Agricultural and Food Chemistry many other denominations at the bottom. (volume 41, pages 693-695, 1993) shows that benzene is 3. Properties of US coins can be found at http://www.usmint.gov/abo formed via a free radical decarboxylation of the benzoate ion ut_the_mint/index.cfm?action=coin_specifications. under conditions similar to those found in soft drinks. Hydroxyl Or: Go to www.usmint.gov free radicals are formed from ascorbic acid and dissolved Search composition penny Click on Circulating coins oxygen, especially in the presence of trace amounts of transition metal ions such as Cu2+ and Fe3+. Benzene formation was 4. Much information about the US penny can be found at fastest at pH 2 and occurs at decreasing rates up to pH 7. (Have http://en.wikipedia.org/wiki/Penny_(U.S._coin). Again, there are your students discover that soft drinks have a pH of about 3!). links to information about coins of other denominations.
5. The Royal Canadian Mint Act 1985 (updated March 2006), which is It has been known since the early 1990s that benzene forms in available at http://lois.justice.gc.ca/en/R-9/text.html, gives the mass soft drinks that contain sodium benzoate and Vitamin C and composition of all circulation coins. (See also reference 2.) (ascorbic acid). At that time, the soft drink manufacturers quietly
References discontinued the use of Vitamin C in their products. Meanwhile, new manufacturers, who are not aware of this problem, began 1. L.J. Brubacher, “Fun with pennies”, What on Earth, Summer 2003, reintroducing Vitamin C and the problem has reappeared. [LJB] University of Waterloo, pages 16-17; also in Crucible, September
2004, pages 25-26. 2. The Royal Canadian Mint Act 1985 (see Link 5), on pages 7-9, includes the mass tolerances on circulation coins. For the 1982-99 5-cent coin, for example, item 6.1 on page 8 says, “the margin of tolerance with respect to weight is 39.77 g per kilogram of 217 pieces.” We have converted this to ±0.18 g per coin — and similarly for the other coins.
3. F.W. Huss, Jr., Chem 13 News, December 1989, page 6, gives the original procedure for demonstrating the Curie temperature using Canadian nickels dated 1955-1981. 4. One of us (DDLM) attended Cliff’s session. And what about Canada? We’re not aware of any written opinion. But we can’t imagine the Royal Canadian Mint taking a teacher to court for altering a few coins, which are then taken out of circulation, in an educational setting. 5. The copper-to-silver-to-gold demonstration was originally introduced by Paul D. McCormack, Chem 13 News, January 1974, page 8; it was reprinted in Journal of Chemical Education, February 1975, page 102, and in Chem 13 News, May 2002, page 14. It is also given in L. B. Summerlin and J.L. Ealy, Jr., Chemical Demonstrations, A Sourcebook for Teachers, volume 1, 2nd edition, American Chemical Society, 1988, pages 137-138. 6. C. Chamberlain, Chem 13 News, May 2002, page 15. 7. See http://www.snopes.com/business/money/loonie.asp for the Loonie design story.
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