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Wednesday
Jun172009

Demo: Gold Penny

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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 slideshow 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 slide 1.
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. (slide 2)
4. Place penny into beaker and stir for about 4 minutes (slide 3).
5. Use tongs to take out penny and place in water (slide 4). 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 (slide 6)
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 (slide 6).
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.

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 you and the student wear goggles, and know how to properly use a Bunsen burner safely. Make sure room is well ventilated.

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