Money Trouble
Graphing the Mass of Pennies

Sharon Price
Dollarway High School, Pine Bluff, AR

Grades    8 to 12
Subjects    Mathematics, Science, Process Skills, Chemistry, General Science


Activity Description
Students use triple beam balances to graph the mass of pennies, only to discover that the graph is not a straight line and all pennies don't have the same mass. The frustration soon leads to inquiry about why some pennies have more mass than others.

Objectives
In this activity, students will:
  1. Develop and implement a workable scientific inquiry.
  2. Use a triple beam balance and a graduated cylinder correctly.
  3. Be able to explain the differences in pennies in relation to our economy.
  4. Be able to calculate density
  5. Perform error analysis on collected data.

Materials
This activity requires the following resources: triple beam balances
100 ml graduated cylinders
Microsoft® Encarta® or other reference materials to determine the density of metals
Reference materials to explore the minting of coins.

Procedure

  1. On Day One, have students determine the mass of standard paper clips.
    Put data into a data chart showing 5,10,15,20,30,40 clips and have them graph the data.
    Discuss results (which should be a nice line graph).
  2. On Day Two, have students do the same activity using assorted pennies they have brought in.
    They will see that their results seem to be "wrong" and the better students will keep working to make a straight line.
    Gradually, they will become very frustrated and want to know why this isn't working.
  3. By Day Three, students are ready to form a hypothesis about why a straight line was not achieved.
    Some good ones are color, date, political party in office during minting, location of mint, etc.
    Students collect data to prove or disprove their hypothesis. They may need to form a new hypothesis and work from there.
    Eventually they will all come to the conclusion that the mass of the penny changed in 1982.
  4. Next, I have them determine the density of the pre 82 pennies and the post 82 pennies. The best way to do this is determine the mass of 40 pennies and then determine the volume by water displacement in a graduated cylinder. I provide them with some reading material to discover the differences in makeup of the pennies. They are able to look up the metals zinc and copper in the Microsoft reference materials and find the expected density of their pennies. Lastly, they can compare their calculated pennies to the accepted values for the densities of the corresponding metals and calculate a percent error on the data.
  5. For an extension, students can insert their data into a Microsoft spreadsheet and have the computer graph the data.
 

Background
All kids love money and they love to do activities in the classroom with money.

In 1982, due to the rising cost of copper, the US changed the content of the penny. Before 1982, pennies were 95% copper and 5% zinc. After 1982, pennies were made of over 97% zinc and simply coated with a layer of copper. This is a real-world application to chemistry and the kids really get involved.

 

References
I found the background material for this lab in Modern Chemistry, published by Holt, Rinehart, and Winston. Chemcom—Chemistry in the Community also contains some background information. Both books present the information in a textbook kind of way, but I changed the presentation to make the learning inquiry based and I believe the students will remember the activity much longer this way.