The Copper-to-Silver-to-Gold Alchemy Demo

September 21st, 2012

As part of the outreach effort for our NSF Solar Fuels center at Caltech, we run a program that enables high-school students to conduct research using a Caltech-built kit to screen the activity of metal oxides in the photoelectrolysis of water (into molecular hydrogen and oxygen). During the summer, we host a select group of students to work on projects aimed at improving the kit and method. I briefly talked about my participation in the program before, and you can read more about it here.

On the last day of work this summer, aside from cleaning up the lab, we let the students select a few experiments from Bassam Shakhashiri’s fabulous Chemical Demonstrations series. The students I helped supervise selected the “copper-to-silver-to-gold” demonstration, in which a copper penny is plated with zinc (to appear silver) and then heated to yield a top layer of brass (which appears gold). The London Olympics could not have been better timed:

To my relief, the demonstration was remarkably cheap and easy to set up. All you have to do is grab a stock solution of NaOH, a bottle of zinc granules, and some pennies coined prior to 1983. (U.S. pennies dated before 1983 are composed primarily of copper, whereas more recent coins are copper-plated zinc.)

When you drop a penny into a beaker of zinc granules immersed in near-boiling 0.5 M NaOH, zinc will plate onto the copper such that the coin appears silver within two or three minutes. When the penny is washed and heated on a hot plate set to ~200-300 °C, the zinc and copper blend to form a brass alloy that is golden in color. This transformation happens quickly, within 20 seconds or so.

The zinc suspension can be used indefinitely; I think we plated around 15 pennies. The students had an assembly line going:

So, that’s it. The copper-to-silver-to-gold alchemy demo gets my full endorsement. It is cheap, easy, and a lot of fun. The students got to learn a little electrochemistry and materials chemistry, plus they all wound up with a cute conversation piece as a souvenir.

25 Responses to “The Copper-to-Silver-to-Gold Alchemy Demo”

  1. See Arr Oh Says:

    Tried this in my lab today – totally verified. Only thing I would change is to indicate in your prep that you have to HEAT the initial Zn / base mixture (guess who sat there watching it for 10 minutes at RT?)

    Also, the initial plating proceeds in about 30 seconds in hot 1M NaOH with Zn dust. Just sayin’

  2. Chemjobber Says:

    In some circles, the fact that modern coins are copper-coated zinc is a sign of the impending doom of American civilization. #OnlyPartiallyKidding

  3. Paul Says:

    Thanks for the good point, SAO. I’ve added a note on temperature for the plating.

  4. Trent Wallis Says:

    This also works with tin in place of zinc, to make bronze. I like the idea of heating the coins on a hotplate, elegantly gets around the overheating that can happen with a Bunsen burner.

  5. Paul Says:

    @Trent: Yeah, the Shakhashiri book gave instructions that a Bunsen burner should be used. I didn’t want the kids spending too much time around an open flame, so I thought I’d give a hot plate a chance. It worked like a charm.

  6. Oliver Says:

    Paul, We did this today in lab and it works with newer pennies as well. Just saying.

  7. wolfie Says:

    Regard the commodity prices. They all tend to go parallel, recently. That means, when I speculate on gold, I could evenly do it on silver, or even on copper ?

    I prefer the German company Leoni (ticker symbol : LEO). They have been dealing with wires all the time, mostly copper, and i hope, they still know, how.

    Their orginal name was : Leonische Drahtwerke. Like : Lion’s wire factory.

  8. azmanam Says:

    Love this demo. I do it all the time :)

  9. Paul Says:

    @Oliver: Good to note that newer pennies work, too.

    In a personal communication, Oliver also notes that the pennies turn out much nicer/shinier if you cool them quickly by immersion in a water bath (instead of letting them sit out and cool). That’s how Shakhashiri likes to do it, and how we did it with the students as well.

    One note about using newer pennies…if you are using the post-1982 US cents, which are copper-plated zinc, then you will not want to use a Bunsen burner as your heat source. Copper melts at over 1000 C, while zinc melts at a little over 400 C. If you’re using a flame to heat your coins—as Shakhashiri suggests—my guess is that you’re going to have a misshapen product. I think the hot plate is the best option regardless of the coins you choose.

  10. azmanam Says:

    I’ve never looked at the year of the penny, but I’ve always used a Bunsen burner for making the alloy. As long as you remove the penny from the flame AS SOON as the ‘silver’ color changes to ‘gold’ and immerse in rt water, all is ok. Leave it in the flame too long, it turns back to looking like a regular copper penny.

  11. Cwarren Says:

    Since the pennies already have zinc on the inside of the copper plating couldn’t you just heat the penny to a high temp and get the same result? Or is there another reaction here I am not getting?

  12. Robert F Says:

    Is the silver coloring on the penny permanent? Could it rub off easily? Is the gold coloring permanent? could it rub off easily? Could this change have occurred with metals other than copper?

  13. Paul Says:

    @Robert F: I have not run the experiment myself, but I am told the zinc slowly fades to a silvery-copper color over time, presumably due to oxidation and loss of the zinc metal. The layer of zinc that is deposited under these conditions is presumably very thin. If you left the penny in the bath longer, perhaps the silver color would persist longer.

    As far as I know, the brass (golden) coating is permanent. It is formed by mixing of the zinc and copper atoms to form a new alloy that is more resistant to oxidation than the zinc.

    You need the copper to form the brass, so this demo only works with copper coins.

  14. Gach Says:

    I like it

  15. Gach Says:

    I live in South sudan I well do something

  16. laura Says:

    It is not actually gold. It just has the color of gold but it is surly fun to have an unusual penny. but we need to be careful about the sodium-hydroxide because it is very corrosive, wash your hand if you feel it slippery. Have fun.

  17. a 4th grade student Says:

    what we need to remember is that in a chemical reaction only the electrons are exchanged not the protons. The only way to form gold is by nuclear means(by bombarding neutrons). But it is surely fun to have an unusual penny.

  18. michelle Says:

    Will a solution of zinc nitrate work?
    Also, what are the amounts of reagents used?


  19. Liz Says:

    What would you recommend for disposal of the zinc dust/NaOH solution?

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  24. Dr.Bob Says:

    As for disposal of the zinc/NaOH slurry, do not dump in in your waste basket. Once dry, it can generate enough heat to ignite paper. If it gets on paper towels, soak them with water before disposal.
    We do this every year in my high school chemistry classes. Typically, the bright brass penny will fade to a more coppery color.

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