Penny Diffusion Lab
In February 2003, my two chemistry classes sorted over 3,000 pennies by date and mint mark (city where coins were minted). The pennies were collected in November 2002 as part of our schools fundraising for our yearly community holiday meal. Our goal was to determine if pennies distribution could be a model for gas molecules.
While gas molecules clearly have different means of transportation (it is their own internal kinetic energy that moves them, not people’s pockets and purses), the measure of entropy or disorder was seen to increase in the pennies just like in a gas.
Students in groups of two recorded the date and mint mark of about 150 pennies per group. Data was entered into a spreadsheet and graphs were created (graphs can be seen below).
Resources and results
- Data collection sheet if you’d like to try this lab in your class.
- Spreadsheet in Quattro or Excel you can use to aggregate data.
- US Mint’s mint mark pages.
- Here’s a spreadsheet showing my classes’ aggregated data.
Upon examining their data of 150 pennies, students could clearly see a decrease in the number of pennies over time.
But when the class data was combined, a ‘diffusion’ trend becomes clear.
Graphs showing data trends
The San Francisco Mint produced pennies up through 1975, but we only found a few of those (mostly due to how few pennies we found from that era). The graph above does not include the SF pennies; the graph below shows them.
Of the 3,113 pennies we sorted, more than half were from the previous ten years.
