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:
  1. Split your pennies into 3 groups.
    1. It’s fine for you to have one more penny in one or two groups than the other(s).
  2. Pour each group of pennies into its own cup.
  3. Pour some water into one of the cups. Make sure that all of the pennies are submerged in the water.
  4. Pour some lemon juice into the second cup, making sure that all of the pennies are submerged in the lemon juice.
  5. Pour some coffee into the third cup, making sure that all of the pennies are submerged completely.
  6. Wait about 10 minutes.
  7. Take the pennies out of the cup filled with water. Have they changed at all?
  8. Take the pennies out of the cup filled with coffee. Have they changed at all?
  9. 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:
  1. What exactly makes the penny rusty?
    1. 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. 
  2. When copper reacts with air, is that a chemical or physical reaction?
    1. 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). 
    2. A chemical reaction occurs because the copper and oxygen combine to form a different substance entirely, copper oxide.
  3. Why does lemon juice clean pennies?
    1. Lemon juice contains a compound called citric acid. 
    2. Citric acid chemically reacts with the copper oxide and can dissolve it, revealing copper underneath that has not been exposed to air. 
  4. What is an acid and what is pH?
    1. Substances with a pH less than 7 are acids, while substances with a pH greater than 7 are bases. 
    2. 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.
    3. 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.
    4. Coffee is considered a weak acid. It has a pH of around 5 (depending on the variant of coffee). 
    5. Lemon juice is a very strong acid. It has a pH of about 2.

Additional Experimentation:
  1. 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? 
  2. What happens when we try to clean other coins, like nickels or dimes? Does lemon juice work just as well as with pennies? ​
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​Published 10.29.2020

Author: Alex Zhang