Cleaning Pennies!
Key Concepts
Materials:
10 - 15 Pennies (try to find some rusty ones as well)
3 Cups (preferably disposable)
Water
Lemon Juice
Coffee
Activity Directions:
Discussion Questions & Science Explanations:
Additional Experimentation:
- Split your pennies into 3 groups.
- It’s fine for you to have one more penny in one or two groups than the other(s).
- Pour each group of pennies into its own cup.
- Pour some water into one of the cups. Make sure that all of the pennies are submerged in the water.
- Pour some lemon juice into the second cup, making sure that all of the pennies are submerged in the lemon juice.
- Pour some coffee into the third cup, making sure that all of the pennies are submerged completely.
- Wait about 10 minutes.
- Take the pennies out of the cup filled with water. Have they changed at all?
- Take the pennies out of the cup filled with coffee. Have they changed at all?
- Then, take the pennies out of the cup filled with lemon juice (it’s fine to reach in and grab them). Wash these pennies with some water—have they changed?
Discussion Questions & Science Explanations:
- What exactly makes the penny rusty?
- Pennies are made up of mostly zinc with a thin copper coating on the outside. This copper reacts with air to produce copper oxide, which coats the pennies and makes them dull.
- When copper reacts with air, is that a chemical or physical reaction?
- A chemical reaction occurs when the molecular bonds of a substance change. Changing the physical properties of a substance, such as state of matter, isn’t a chemical reaction because the substance is still the same. Some common examples of each include: burning wood (chemical), cooking an egg (chemical), iron rusting (chemical), ice melting (physical), cutting paper(physical), mixing play-doh (physical).
- A chemical reaction occurs because the copper and oxygen combine to form a different substance entirely, copper oxide.
- Why does lemon juice clean pennies?
- Lemon juice contains a compound called citric acid.
- Citric acid chemically reacts with the copper oxide and can dissolve it, revealing copper underneath that has not been exposed to air.
- What is an acid and what is pH?
- Substances with a pH less than 7 are acids, while substances with a pH greater than 7 are bases.
- pH is a measurement of how strongly acidic or basic a substance is. Unlike the number line, which starts at 0, the pH scale starts at 7, and substances with a pH of 7 are neutral (not acids and not bases), like water. In fact, the further away a substance’s pH is away from 7, the stronger it is.
- For instance, the pH of milk is about 6.8. On the other hand, the pH of stomach acid is 2.5. Both of these substances would be acids; however, because 2.5 is much further away from 7 than 6.8, stomach acid is a much stronger acid.
- Coffee is considered a weak acid. It has a pH of around 5 (depending on the variant of coffee).
- Lemon juice is a very strong acid. It has a pH of about 2.
Additional Experimentation:
- What if we use the same liquid in each cup, but vary the number of pennies? Is there a maximum number of pennies that each liquid can clean?
- What happens when we try to clean other coins, like nickels or dimes? Does lemon juice work just as well as with pennies?
Published 10.29.2020