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- Colorful Copper Chemistry around the Campfire
Tom Kuntzleman | Tue, 07/28/2020 - 13:09
Deanna Cullen, Scott Milam, Doug Ragan, and I recently published an article, Rapid Formation of Copper Patinas: A Simple Chemical Demonstration of Why the Statue of Liberty Is Green,in the Journal of Chemical Education1 that describes how to create a blue-green patina on the surface of copper coins. This experiment provides students with hands-on access to experiments that illustrate how the green color forms on the Statue of Liberty. You can learn a little more about these experiments inVideo 1 below:
Video 1:The Chemistry Behind Why the Statue of Liberty is Green,Tommy TechnetiumYouTube Channel, accessed (7/28/20)
As you can see in the video, copper patinas are comprised of copper salts. Because many copper salts generate a green color when placed in a flame, I thought it might be fun to place some coated pennies in my campfire. Watch Video 2 below to see what I learned:
Video 2:How to make a green flame in a campfire,Tommy TechnetiumYouTube Channel, accessed (7/28/20)
Isn’t that cool? You can create green flames in your campfire with copper salts that are formed using only pennies, household ammonia, salt, aluminum foil, paper towel, and a 9V battery. I used two different paper towel-soaking "recipes" to formpatinas for the campfire experiments:a) a 6% solution of baking soda or b) a 6% solution of table salt dissolved in household ammonia. Interestingly, it was only the pennies treated with the ammonia-salt mixture that displayed the green flame when placed in the fire. I’m not sure why this is, but I have some guesses. I was able to track down several papers2 which demonstrate that emission from transient Cu(OH) (g) is responsible for the green color associated with flame emission from copper salts. Could it be that the patinas formed via treatment with ammonia form more basic copper salts on the penny due to the greater basicity of ammonia as compared tobaking soda? If so, might more basic copper salts allow for more facile generation of Cu(OH) (g) as the campfire flame interacts with the penny patinas? I will definitely be posing these questions to my students as potential questions forexploratory projects. Maybe you’ll invite your students to investigate the same questions. I’d certainly be interested if anyone discovers that green flames can be produced from patinas generated with something other than the ammonia-salt mixture.
Figure 1:Comparison of two pennies placed in a campfire and retrieved from the ash the following day.
One more thing I’d like to share. I would often retrieve the pennies from the campfire ash the day after doing penny experiments. Upon doing so, I found the following (Figure 1).
Figure 2
Figure 2:Another penny retrieved from the campfire ash
Hmmmm…..can you explain the differences observed in the two pennies above? Why is one penny melted and the other is not? Why does the melted penny have a slight grey color? Why did the un-melted pennies have red, orange, and brown hues? Why were some of the un-melted pennies black in color (Figure 2)? I’ll leave the reader to answer these questions. For a few hints as to what might be going on, consider reading our recent article1 and a blog post3, Melting Pennies, I wrote several years ago.
Happy Experimenting!
Editor's Note: Teachers may find the "Oxidation and Reduction of Copper" video*from our Chemistry Comes Alivecollectionto be helpful when using the ideas that Tom has shared here.
Copper oxidizes slowly in air, corroding to produce a brown or green patina. At higher temperatures the process is much faster and produces mainly black copper oxide. The oxide can be reduced by hydrogen gas, which is a moderately strong reducing agent, producing a shiny, clean copper surface. This provides a striking illustration of oxidation and reduction of a metal.
Equations for the reactions are
2 Cu(s) + O2(g) → 2 CuO(s)
CuO(s) + H2(g) → Cu(s) + H2O(g)
*Oxidation and Reduction of Copper from ChemEd Xchange on Vimeo.
References:
1. Kuntzleman, T. S.; Cullen, D. M.; Milam, S.; and Ragan, R., Rapid Formation of Copper Patinas: A Simple Chemical Demonstration of Why the Statue of Liberty Is Green,J. Chem. Educ. 2020
2. Trkula, M.; Harris, D. O. J., Laser spectroscopy of the1A′′–X̃ 1A′ system of CuOH and CuOD,Chem. Phys. 1983, 79, 1138-1144, and references therein.
3. Kuntzleman, T. S., Melting Pennies, Chemical Education Xchange, Accessed July, 2020.
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Comments 7
The why behind blue and green flame colors
I really enjoyed your Colorful Copper Chemistry around the Campfire article in ChemEdX! I noted that you mentioned “Interestingly, it was only the pennies treated with the ammonia-salt mixture that displayed the green flame when placed in the fire. I’m not sure why this is…” Your observations make excellent sense, for two reasons:
First,a blue flame color is due to emission from low temperature emission, at less than 1200 C, from CuCl. Chlorine is important in pyrotechnic compositions to produce volatile compounds that can emit in the gas phase and also for the formation of the emitting substance during the combustion of the mixture. For example, SrCl2 and CuCl2 give beautiful red andblue flame colors in a Bunsen burner flame but are totally unsuitable for pyrotechnic mixtures because they’re deliquescent. This is one reason why chlorates and perchlorates are used in colored fire compositions, and additional chlorine donors such as PVC are often included as well. For references, see John A Conkling in C&E News, 1981, June 29 issue, page 24 – 32). A more detailed discussion can be found in his book “The Chemistry of Pyrotechnics” (Marcel Dekker, 1985). Interestingly, oxygen rich flames, or too high flame temperatures, or a shortage of chlorine, give CuOH instead which emits green and starts to wash out the blue color. So the green flame color that you’re seeing is likely primarily due to CuOH, formed as a result of the inclusion of ammonia.
Second, the ammonia produces a better flame color because ammonia can solubilize the CuO and Cu2O present on the copper surface, facilitating formation of CuOH or reaction with chloride ion. As an aside, CuO emits red and can often be seen at the edge of a blue copper flame where air is being drawn into the flame.
What is surprising is that the yellow sodium flame color doesn’t overwhelm the blue copper flame color. I never would have imagined that NaCl could be used in this procedure, because the sodium ion flame color is so persistent. Traces of sodium ion are fatal to colored fire formulations, to the point that yellow fire compositions are often prepared in separate structures at fireworks factories.
WOW!
Steve:
Thank you so much for your insights. It is so nice to have you provide more information and context on this particular experiment. Looks like I have a few more ideas to try out around the campfire. Given that school starts soon, these experiments might have to wait until next summer <sigh>.
I greatly appreciate the input, Steve!
copper flames
The reason that pennies don't produce the green copper flame color is that pennies don't contain that much copper anymore. When my kids were small and we were camping I had accumulated some copper filings, probably from a lab, and I'd thow a few in the campfire. Great green color which lasted a while since the filings were fairly long.
I'm going to try that!
Copper filings are a great idea! I look forward to adding some to my campfire next summer...maybe this fall if I get a chance. Thanks, Robert!
Insightful reply!
Hi Steve,
What an insightful reply. Thank you so much and thanks to Tom of course. The association of different flame colours with different copper species have always fascinated me.
Thanks for sharing.
Andres
age of penny?
fun. Curious if you have penny age requirements since the new ones just have a coating of copper on zinc. And zinc is easy to melt in a fire. And the zinc could act differently if exposed in the battery step.
Or does it work equally as well with new and old pennies?
age of penny
Hi Jarral! In our JChemEd article that Tom referenced pre-1982 pennies are required when placing the penny in a bunsen burner flame because the newer pennies will melt because they are only coated with copper while the inside is zinc. In the case of adding copper salts to the surface of the pennies using the battery method he shares in that article and in this blog post, any age copper penny can be used. I hope you try it out!
