Colorful Copper Chemistry around the Campfire

green flame

Deanna Cullen, Scott Milam, Doug Ragan, and I recently published an article, , 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 in Video 1 below:

Video 1:The Chemistry Behind Why the Statue of Liberty is Green,  YouTube 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,  YouTube 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 form patinas 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 to baking 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 for exploratory 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: 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, , I wrote several years ago.

Happy Experimenting!

 

Editor's Note: Teachers may find the "Oxidation and Reduction of Copper" video* from our Chemistry Comes Alive collection to 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)

 

* from on .

 

References:

1. Kuntzleman, T. S.; Cullen, D. M.; Milam, S.; and Ragan, R., , J. Chem. Educ. 2020

2. Trkula, M.; Harris, D. O. J.,, Chem. Phys. 1983, 79, 1138-1144, and references therein. 

3. Kuntzleman, T. S., , Chemical Education Xchange, Accessed July, 2020.

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

NGSS

Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.

Summary:

Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.

questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.

Assessment Boundary:
Clarification:

Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.

Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?

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:

Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas.

*More information about this category of NGSS can be found at 

Summary:

"Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas."

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.

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Comments 4

Stephen Wright's picture
Stephen Wright | Tue, 08/25/2020 - 17:22

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 and blue 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.

Tom Kuntzleman's picture
Tom Kuntzleman | Fri, 08/28/2020 - 12:25

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!

Robert Buntrock's picture
Robert Buntrock | Fri, 08/28/2020 - 20:42

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.

Tom Kuntzleman's picture
Tom Kuntzleman | Mon, 08/31/2020 - 19:20

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!