The Gold Penny Lab – A Cautionary Tale

When I was a boy, my father demonstrated the “gold penny” lab for me, in which one takes a copper penny, coats it with zinc, then heats this under a flame to make an alloy of the copper and zinc (brass, which has a gold color).  Or so we thought.  My father, professor of physics, is friends with a professor of chemistry, Bassam Shakashiri, who literally wrote the book on chemistry demonstrations.  However, it turns out the common explanation of what was happening (the one my father taught me) was…not quite right.

I learned this the hard way when I was preparing to lead a series of labs with students I was teaching in the Seattle area.  The lab goes like this:  heat a mixture of zinc powder and zinc chloride (originally NaOH, stay tuned) to a gentle boil, then drop in a penny.  Leave the penny to sit as still as possible for 10 minutes, then remove and cool.  The penny appears silver in color due to the electroless plating of zinc.  Then heat the penny under a flame and the penny turns a gold color due to the formation of the brass alloy.  Imagine my surprise, then, when I sat down and looked at the activity series. I saw the following:

Activity Series

Activity series for common metals. Note the position of zinc and copper relative to one another.

For those of you who know chemistry, you will notice that in order for the zinc to plate on the copper, some or all of the following half-reactions should be occurring:

Zinc half reaction 1

Zinc half reaction 2

Copper half reactionIn other words, the zinc should be reduced and the copper should be oxidized.  But this is opposite what should happen energetically according to the activity series!  I was befuddled to say the least.  Luckily, so were much better chemists than I.  I looked up a paper written in 1995 by Szczeoankiewicz and coworkers from Canisius College in New York State.  They did a lot of work to show that the zinc is actually the reducing agent for itself.  That is, there is a driving force for zinc deposited on zinc (a la the zinc powder) to become zinc deposited on copper (a la the penny), without needing the copper to act as a reducing agent.  Wacky, indeed.  This allows the use of zinc chloride instead of the nasty 3M sodium hydroxide that was historically used.  The end result of all this is that the silver color you see is actually a brass alloy already.  Why then does it look silver?  Because the percentage of zinc in the copper is higher than that needed for the color to appear gold.  When the penny is heated, the zinc penetrates more into the copper, and the percentage of zinc in the copper at the surface layers decreases, giving the familiar gold color.

So why is this important?  Because the demonstration was meant to show simple oxidation and reduction in a visually appealing and intuitive format.  Yet, any high school chemistry student who compared the lab to his or her textbook would discover a very confusing paradox, one that until 1995 was not resolved.  Not so intuitive.  One must always be careful that the simplification of scientific principles in the form of demonstrations does not actually lead to less clarity, not more.


4 responses to “The Gold Penny Lab – A Cautionary Tale

  1. Very interesting and I remember doing science projects with my father. It is true when you attempt to over simplify it can lead to less clarity rather than resolving your questions. This was surely a tricky one.

    • Yes indeed it was! I remember motor demonstrations where the forces pointed strange directions when I tried them. I also remember a refraction demo I attempted to assemble as a grad student, but I used round beakers for some of the liquids and square containers for the others. The lensing effects destroyed the results. I’m sure there are lots of other examples as well 🙂

  2. I just became aware of this demo last year and was looking up some info for a summer school class I’m teaching. Thank you for finding the reference for the best explanation. I’m going to use the safer ZnCl2 in my lab now!

    • I’m glad you found the paper! In my experience, the ZnCl2 works more slowly than the hot acid, so you will need to be patient. Also, it’s vitally important to make good contact between the solid Zn powder and the penny, not to move the penny once it’s seated in the powder, and to keep the boil as low as possible so that the penny doesn’t move. Any of these will prevent the reaction from proceeding. I usually have to wait 15-20 minutes to get a good coating. Good luck and have fun!

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