Chemical Tests on Coins

chemical tests on coins

I recently shared some simple experiments using magnets and coins that connect to the 2019 National Chemistry Week (NCW) theme, “Marvelous Metals!”1 Because coins are familiar items, made of metal, and so very easy to obtain, I think doing experiments with coins is a great idea for NCW 2019! Because of this, I did some further investigating in the lab to come up with some simple chemical tests that can be done on coins. These chemical tests can be used to indicate (but not prove) that a metal sample or coin contains one of the following metals: aluminum, iron, nickel, copper, zinc, or silver. You can learn about these tests in the video below.

Video 1: Chemical Tests on Coins, Tommy Technetium YouTube Channel, 9/14/19. 

There is quite a bit of chemistry involved in these tests. First, ammonia can be used to detect copper. This is because Cu2+ reacts with ammonia to form a dark blue colored complex:2

Cu2+(aq) + 4 NH3(aq) à Cu(NH3)42+     (Equation 1)

Thus, when a cotton swab soaked in 15 M ammonia is rubbed over the surface of a copper-containing coin, any Cu2+ on the surface causes the cotton swab to turn a blue color as a result of Equation 1.

Also, when coins that contain silver are rubbed with an ammonia-soaked cotton swab, a dark color often appears on the cotton. This color change is due to the formation of a black-brown colored silver complex on account of following reaction:3

Ag+(aq) + 2 NH3(aq) à Ag(NH3)2+                  (Equation 2)

The video above shows the use of the reaction between dimethylglyoxime and nickel ions to detect nickel in coins. The chemistry of this reaction has previously been discussed on ChemEdX, 4 and therefore will not be covered here. The reader is referred to this article for further information about this test.

In addition to these tests, solutions of Cu2+ can be used to detect iron, zinc, and aluminum. Iron reacts with dissolved copper, causing solid copper to plate out on the iron surface:

Fe(s) + Cu2+(aq) à Cu(s) + Fe2+(aq)             (Equation 3)

The solid copper formed on the iron surface produces an orange color. It should be noted that while some coins do contain iron, iron-containing coins are treated in a manner to avoid the oxidation of iron. Therefore, testing coins for iron as indicated in Equation 3 will not always yield a positive result. It is better to detect iron in coins by showing that a coin is strongly attracted to a magnet,5 but will not show a positive result upon reaction with dimethylglyoxime.

Zinc also will react with Cu2+, forming a black color on the surface of the zinc metal. This observation can be explained by the formation of black-colored copper oxide on the surface of the zinc:

Zn(s) + Cu2+(aq) + ½ O2(g) à Zn2+(aq) + CuO                    (Equation 4)

Dissolved copper can also be used as a test for aluminum, as long as chloride ion is present. Samples of aluminum metal generally have a thin, surface layer of Al2O3, which prevents reaction with dissolved copper. However, chloride ions allow Cu2+(aq) to “breach” this so-called passivation layer, which allows for the following reaction to occur:6

 2 Al(s) + 3 Cu2+(aq) à 3 Cu(s) + 2 Al3+(aq)            (Equation 5)

Thus, black or orange flecks of solid copper will form on samples of aluminum that are treated with solutions of Cu2+ that contain chloride ions.

Use caution when adding solutions of Cu2+ to coins. In my experiments, I often noticed that several coins were irreversibly tainted by such treatments.

There is another simple method for the detection of silver that I did not mention in the video above. The chemistry of this detection method is very interesting, but unfortunately the method tarnishes silver in a manner that is not easy to remove. Therefore, I did not feature this test in the main video above.

(accessed 9/17/19)In this second method, household bleach, which contains hypochlorite ion (OCl-), is added to a sample of metal. If the metal contains silver, it quickly tarnishes. You can watch this test in the video:

 

Video 2: Bleach test for silver metal, Tommy Technetium YouTube Channel, 9/15/19.  

This tarnishing effect can be described by a series of reactions. First, addition of bleach causes silver to be oxidized to Ag+, while OCl- in is reduced to Cl-:

H2O(l) + OCl-(aq) + 2 Ag(s) à  Cl-(aq) + 2 Ag+(aq) + 2 OH-(aq)    (Equation 6)

The Ag+ thus produced has the potential to form a precipitate with the OH- and Cl- ions formed. Given the lower Ksp of AgCl (1.8 x 10-10)3 as compared to AgOH (4.3 x 10-8),3 it is AgCl that is likely produced:

Ag+(aq) + Cl-(aq) à AgCl(s)             (Equation 7)

When light strikes the AgCl(s) that is formed, deposits of Ag(s), which are dark in color, form in the AgCl precipitate. This causes a darkening effect:7

2 AgCl(s) + light à 2 Ag(s) + Cl2(g)             (Equation 8)

Reactions 6-8 are consistent with the tarnishing of silver observed upon addition of bleach:

Like the Cu2+(aq) test, the bleach test has the potential to irreversibly disfigure any silver-containing coins that you test. I happened to find the tarnish-removing experiment described previously on ChemEdX8 to help remove some (but not all) of the chemical stains that formed on some silver coins I tested during my investigations.

If you happen to try out some of these experiments with your students, please let me know in the comments how things went. Also, please share any suggestions you have for other ways to test for various common metals in coins. Finally, if you have any insight into the chemistry that might have occurred in the experiments reported here, please let me know. While I’m pretty sure that ZnO forms on the surface of Zn(s) treated with Cu+2+(aq), I’m not entirely convinced that the reaction described in Equation 4 accounts for how the zinc oxide forms. I’d love to hear other ideas on how this might occur.

Happy experimenting!  


Acknowledgement:

I wish to thank Andres Tretiakov for inspiration. The experiments reported here were inspired by Andres Tretiakov’s article on ChemEdX,4 in which he describes how solutions of dimethylglyoxime (DMG) can be used to test for nickel in coins.


References:

1. Kuntzleman, T., , ChemEd X, 9/4/19. (accessed 9/17/19)

2. Chemical DemonstrationsA Handbook for Teachers of Chemistry; Gilbert, G.L., Williams, L.G., Shakhashiri, B.Z., Dirreen, G.E.K Juergens, F.H., Eds.; University of Wisconsin Press, Madison, WI, 1983; Vol. 1, pp. 307 – 313.

3. Chemical DemonstrationsA Handbook for Teachers of Chemistry; Gilbert, G.L., Williams, L.G., Shakhashiri, B.Z., Dirreen, G.E.K Juergens, F.H., Eds.; University of Wisconsin Press, Madison, WI, 1983; Vol. 1, pp. 318 – 323.

4. Tretiakov, A., , ChemEd X, 5/7/19. (accessed 9/17/19)

5. Kuntzleman, T., , ChemEd X, 9/4/19. (accessed 9/17/19)

6. Sobel, S.G. & Cohen, S., J. Chem. Educ. 2010 87 616-618.

7. J. Chem. Educ., https://pubs.acs.org/doi/pdf/10.1021/ed046pA310.4 (accessed 9/17/19)

8. Kuntzleman, T., , ChemEd X, 3/3/16 (accessed 9/17/19) 

NGSS

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Summary:

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Assessment Boundary:
Clarification:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Summary:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Assessment Boundary:
Clarification:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.