CHEMICAL REACTIONS: HOLLOW PENNY
Reaction of penny with hydrochloric acid
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Zinc will react with the hydrochloric acid, but copper will not. Zinc is more reactive than hydrogen.
Copper is not more reactive than hydrogen. So only zinc can replace hydrogen and bond with chlorine.
Reaction: Zinc + Hydrochloric Acid  Zinc Chloride and Hydrogen
Zn + HCl  ZnCl2 + H2
The balanced equation looks like: Zn + 2HCl  ZnCl2 + H2
Procedure:
1. Give each student or group a pre-1982 penny and a post-1982 penny. Ask them to examine them talk
about any differences they notice. There really shouldn’t be any noticeable difference. Some pennies
may be “dirtier” looking than others, but that’s about it. Ask the students if the pennies function any
differently. If they tried to buy something using those pennies, would a store treat them any
differently? Obviously, the answers should be “no”. Then explain that the pennies are completely
different substances.
2. Have each student or group scratch away at the surface of a post-1982 penny. An iron nail typically
works well for this. They need to scratch pretty hard. Some of the copper coating needs to be
scratched away to reveal the zinc underneath. Just a few good scratches on each side should do fine.
3. Give each group or student a small beaker fill about a third full with hydrochloric acid. Then they will
carefully drop the penny into the beaker. Be very careful with hydrochloric acid. It is a strong acid. It
can be purchased at most hardware stores. It may be found as a pool cleaner or called ‘muriatic acid’.
4. After a few minutes have students observe what is happening to the penny. They should see small
bubbles coming off the penny. This is the hydrogen gas being produced. If you’re using glass beakers,
the students can hold it up their ears and should be able to hear the penny rattling against beaker.
5. Leave the pennies in the beakers of hydrochloric acid overnight. By the following day the zinc should
have fully reacted and you should have a hollow penny. Carefully pour out the acid, remove the penny
and rinse it with water before drying it with paper towels.
6. Students examine the hollow pennies. They’ll feel almost like a candy wrapper.
Lab Questions:
1. Zinc reacts with hydrochloric acid. Write the chemical formulas for the reactants in this reaction.
2. Predict what the products will be and write the full equation for this reaction.
3. Balance the equation.
4. What type of reaction is this?
5. Critical thinking extension. Why do you think copper is now more expensive than zinc?