Enrich your classes with Canadian coins Doug De La Matter <[email protected]> and Lew Brubacher, Chemistry Department <[email protected]> Box 655, Barry’s Bay ON K0J 1B0 University of Waterloo, Waterloo ON N2L 3G1 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?? 16 Chem 13 News/September 2006 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.