This is not so much a lesson but rather a demo. This is kind of a “Wow!” demo. The chemical composition of the penny has varied greatly through the years. From 1793 to 1837 it was actually pure carbon. It went through many changes since then. From 1864 to 1962 it was bronze (95 percent copper, 5 percent zinc and tin). In 1962 the penny’s tin content was removed, which made it 95 percent copper and 5 percent zinc. In 1982, the composition changed to 97.5 zinc and 2.5 percent copper (copper plated zinc). In this demo you need pre 1982 pennies (The US Mint has a good video on how coins are made). The purpose of this demo is to give the penny a zinc plating, then heat it up to form a brass alloy so that it will look like gold. You can discuss in this demo the various properties of metals. Also you can discuss the technique of wet chemical plating, a technique first discovered by alchemist centuries ago, to try to fool people into believing they had the ability to change common metals into precious metals. This demo was taken from the Chemistry Demo a Day book.
Procedure:
1. Get the materials as shown in the first slide.
2. Mix about 24 g of sodium hydroxide into 200 ml of water. Use a 400 ml beaker. Heat the mixture on a medium low setting. Do not boil.
3. Mix in 5 g of granular zinc.
4. Place penny into beaker and stir for about 4 minutes.
5. Use tongs to take out penny and place in water. This stops the process. Rub penny with cloth to create a sheen. The penny is now zinc plated. It kind of looks like a silver penny.
6. Use tongs to slowly heat penny over a Bunsen burner flame. Slowly move the penny back and forth across the flame. The heat makes the zinc atoms mix with the copper atoms to form a brass alloy. It looks kind of like gold.
Tips: Have a student assistant help you with the procedure. They will have fun. Have pennies that have already been done, to pass out to students during the demo so they can have an up close look.
Safety: Make sure students wear goggles, and know how to properly use a Bunsen burner safely. Make sure room is well ventilated.
This album is powered by
BubbleShare
- Add to my blog
I’ve found you site to be a great resource. I haven’t used this as a demonstration yet as we don’t get to alloys until later in the semester, but I have tried it myself.
Not having any granular zinc on hand, I electroplated the zinc onto the penny, using the procedure listed on this site: http://www.finishing.com/faqs/howworks.html.
Neither my zinc nor my brass pennies turned out looking quite as nice as yours, though. Thanks again for a great site.
Thanks for the link on electroplating–I might try it. Sometimes my pennies don’t turn out too nice as well. I seem to get a 60% success rate. Definitely order some granular zinc, it’s probably better than the battery source.
this is not in response to this expirement but one of yours i saw on google video where a penney was heated and then suspened over acetone to make it glow. i heated a penney over the stove and dipped it into 90% rubbing alcahol (i was careful that it didn’t catch fire) on my first attempt the penny was left with a large air bubble on my second attemt the penny turnesd a beautiful blueish with green red and yellow tinges. just thought i would let you know you should try it or explai n how it works if it has any relavancy.
Very interesting. Maybe the heat made the zinc/copper burn a little, thus changing color (total guess). I might try it.
stunk
I had been using a water-based metal polish to polish the pennies before plating them and then before heating them, but I found that these polishes leave a coating on the penny which interferes with the plating process. The polish also tends to remove some of the zinc. A thick paste of baking soda and water applied with a toothbrush does a pretty good job of polishing them up, but isn’t a strong enough reagent to remove the zinc. My students love this demonstration, now that I have gotten better at it.
That’s interesting. It’s kind of funny, my 6 year old nephew showed me how to clean a penny using Taco Bell hot sauce. The vinegar in the sauce was the cleaning agent.