Gold Pennies Learn about the science of alloys and make a gold penny. email to friend
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There’s just something strange about a gold penny. First of all, gold pennies don’t exist… unless you’re a 20th-century alchemist who knows something about the science of alloys. Modern day alchemists, cleverly disguised as chemistry teachers, often share the secret of making gold pennies. Students take home an unforgettable lesson in alloys and a keepsake penny that can never be put back into circulation – even though it’s a “gold” penny, the US Treasury has a little problem with people buying something with gold pennies.

In 1983, the US Treasury started making pennies with a zinc core with a copper coating... turns out they weigh less and are cost less to make than their pre-1983 counterparts, which were made with an alloy of 95% copper and 5% zinc. In this activity, you'll dip the penny in the special zinc solution and some of the zinc will dissolve into the copper coating on the penny. At this point, your penny will change color because of the presence of zinc on the copper. Now, it’s time to turn up the heat! As you heat your newly “colored” penny, the copper atoms will start to move around quickly making a new mixture of copper and zinc on the surface of the penny. This mixing of metals is an alloy called “yellow brass.”

Materials

20 mL of 6 M NaOH
0.1 g zinc dust
evaporating dish
hot plate
tongs
200 mL beaker of water
penny
Bunsen burner or a propane torch

Experiment
Video
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SAFETY NOTE: This activity requires adult supervision. NaOH should be handled with great care. It is corrosive and can burn skin. Zinc dust should not be inhaled. Safety goggles should be worn the entire time.

Let’s Try It!

Place zinc dust in evaporating dish.
Add 20 mL of NaOH solution to the dish, on top of the zinc.
Set hot plate to medium heat and place the evaporating dish on top.
Heat for 5 minutes. Do not boil. When dish is hot, place a penny in it. Heat for two minutes or until the penny is coated and becomes silver in appearance.
Remove the penny from the dish with tongs and drop into water. When cool, wipe the penny clean with a cloth to remove excess zinc.
Using tongs, hold the penny in the flame of a Bunsen burner and gently heat. The penny should turn "gold" (brass). (Do not overheat the penny.)
Dip the penny in the beaker of water and cool to touch.

What Happened?

Artists often work with alloys like bronze, steel or brass because of their durability and color. Bronze is a mixture of copper and tin. Brass, made in the activity above, is a mixture of zinc and copper. Steel is made of iron and carbon. Alloys are used to make coins, jewelry, sculptures and other items.

Soaking the penny in the zinc solution actually coated the surface of the penny with zinc atoms. When the zinc covered penny was heated, the copper atoms of the penny and the zinc atoms coating the penny mixed and turned gold in color, but actually formed the alloy brass.