Composition of Pennies Lab Name ______

Date ______Period ______

Purpose:

1.  To measure mass and volume and determine the density of eight pennies.

2.  Interpret data to determine approximately when the composition of pennies changed.

Background:

U.S. pennies have been composed of copper and zinc since 1959, but the ratio of copper to zinc has changed over the years because of increases in the price of copper. Copper and zinc are both metallic elements and they share many physical properties, but they have different densities. Pure copper has a density of 9.0 g/mL, while pure zinc has a density of 7.1 g/mL. By measuring the density of pennies from different years, it’s possible to track changes in the composition of the penny.

Procedure:

1.  Line the pennies up in chronological order.

2.  Record the mint date of each penny in Table 1.

3.  Weigh each penny and record the mass in Table 1 to the nearest 0.01 g.

4.  Fill the graduated cylinder approximately half-full of tap water and record the volume to the nearest 0.1 mL in Table 2.

5.  Carefully add the eight pennies to the cylinder so that no water splashes out. Jiggle the cylinder to dislodge any trapped air bubbles. Read and record the total volume of the water and the eight pennies to the nearest 0.1 mL in Table 2.

6.  Dry pennies off before putting them back in the canister.

Data and Observations CANISTER # ______

Table 1

Mint Date / Mass (g) / Volume (mL) / Density (g/mL) / Zinc or Copper

Table 2

Volume of water + 8 pennies (mL)
Volume of water (mL)
Volume of 8 pennies (mL)
Average volume of a penny (mL)

Analyze and Conclude (Show work below.)

1.  Subtract the initial volume of water from the volume of the water plus pennies to calculate the volume of the eight pennies. Divide the volume by 8 to calculate the average volume of a penny. Record this volume is Table 2.

2.  Calculate the density of each penny by dividing its mass by the average volume. Record this density in Table 1.

3.  Classify each of the pennies you examined as mostly copper (density around 9 g/mL) or mostly zinc (density around 7 g/mL) and put results in Table 1.

4.  Make a bar graph of your results. Plot density on the y-axis and the mint date on the x-axis. Make sure to label your graph and give it a title.

5.  Look at your data table and determine what year (or between what years) the composition of the penny changed. Write answer here. ______

Follow-up Question

1.  In 1943, all pennies issued in by the U.S. mint were made of zinc-plated steel, not copper. Why were steel pennies used in 1943? (review your history)