The Change in the Composition
of Pennies
Problem: When did the composition of pennies change?
Hypothesis: If the penny was made in the 1980's, then the density would have decreased, showing that
the composition had changed.
Introduction:
The very first pennies, or cents, were made in the United States as early as 1793. (World Book) . Since
then, pennies as well as other forms of change have undergone severe modifications in their identity.
For pennies, especially, the journey has been great, which have, for many years, been completely made
of copper. Yet recently, their composition changed, causing the standard cent to become less dense. In
the years between 1978 and 1985, the U.S. mint made a change in the composition of pennies, adding
to their core a layer of zinc. This was done to adapt to rising prices for copper. ( Coin World Almanac)
The following lab determines in which year this occurred. By making calculations with mass and
volume, the density found therefore proves when the composition of a penny changed.
Materials:
-One set of five pennies for each year between 1978-1985
-A 100 ml graduated cylinder
-An electronic balance
-beaker and water
-A cloth to dry pennies
Procedure:
A pile of pennies was obtained and sorted into groups of five from years 1978-1985, and dried using a
towel. In this experiment, the independent variable was the year of the penny, which determined the
dependent variable, the density. The groups were then massed on an electronic scale one at a time, and
the data was recorded. Afterward, 30 ml of water were added to a 100 ml graduated cylinder, and
volume was recorded. Then, groups of five pennies were placed gently into the cylinder, one group at a
time, and the new volume was recorded with the pennies in it. When the group from the first year was
completed, water and pennies were removed from the cylinder, and water was once again added,
volume was recorded, and a different group of pennies was added. This continued until all the groups
of pennies from the assigned years were completed.
Data:
Year
1978
1979
1980
1981
1982
1983
1984
Mass of 5 pennies (g)
15.5
15.5
15.5
15.4
13.3
12.6
12.5
Volume of water without pennies (ml) Volume of water with 5 pennies (ml)
30.0
32.0
30.0
32.0
30.0
32.0
30.0
32.0
30.0
32.0
30.0
32.0
30.0
32.0
Calculations:
Year
Volume of 5 pennies (ml)
Density (g/ml)
1978
1979
1980
1981
1982
1983
1984
1985
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
7.8
7.8
7.8
7.7
6.7
6.3
6.3
6.4
Volume of Pennies=Volume with 5 pennies-volume
without 5 pennies
32.0 ml-30.0 ml=2.0 ml
Density=Mass/Volume
7.8 g/ml =15.5 g /2.0 ml
Density g/ml
Change in Penny Density
8
7.5
7
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Density (g/ml)
1978
1979
1980
1981
1982
1983
1984
1985
Year
Analysis and Conclusion:
Having reviewed the data of this experiment, I feel it can be concluded that the year that the
density of the penny changed drastically, which was in 1982, also proved to be the year that the
composition changed. This also states the fact that the change took place all at once, rather than
gradually. More precisely, sources say the change came in mid-1982, when the prices of copper began
to rise, and the U.S. Mint decided to compensate by making the new cents have a core of pure zinc with
only a copper-plated surface. (Coin World Almanac) However, though the density and mass did change,
the volume did not, which will not change, no matter the mass.
These conclusions were made clear by the changes in data. Regarding density, its average
before the year 1982 was 7.8 g/ml, and the year of the change was 6.7 g/l. The density afterward also
did fluctuate a little, but the average of the years after the change has been about 6.3 g/ml. This seemed
to indicate that the more the mass of the penny decreased, the density also was less as well. Thus, some
change must have taken place in the year 1982 to alter these densities. Density is made by calculation,
though by examining the properties of the elements that the post 1982 pennies contained, one can see
that the element zinc (Zn) has a smaller density of 7.14 g/ml, where as copper (Cu) has a density of
8.92 g/ml. The two elements remain very close in this, but the difference is enough to cause the pennies
to have a smaller density after the composition changed.
My hypothesis proved to be somewhat correct. Having visited the U.S. Mint in Washington
D.C., I had some background knowledge based on what I had learned about coins there. In terms of
accountability of data, I feel that it can be relied on only somewhat. I did find that other groups in the
class who did the same experiment also came out with 1982 as the year that the change in the
composition of pennies took place. Yet, to make the experiment more reliable, it would have been
prudent to do more than one trial. In the experiment, the pennies were only massed once, and their
volume and densities were only calculated once as well.
This experiment also contained many sources of error. The most prominent was perhaps the fact
that the pennies were wet when massed, and though a cloth was provided to dry some of them off, it
does not seem likely that all were. In addition, another source of error could have been from the way
the pennies were put into the graduated cylinder. In my group in particularly, the four members
switched roles, which could have led to some inconsistencies. Some slid the pennies gently down the
side, (the ideal method,) while others plopped the pennies in from the top, causing water to splash up
the sides of the graduated cylinder and become displaced. This most definitely could have effected the
results of the data in measurement of volume. Still, another source of error could have been one that,
for the most part, was uncontrollable in the experiment. Due to the lack of pennies, groups had to use
what they were given. While most of the pennies remained unaltered, some had dirt and grime caked
on their surface from many years of use, and this could have caused them to weigh more when massed.
In addition, the experiment did have two other final sources of error. Another important one
was the fact that the cylinder was also wet before use, which also could have altered the true volume it
was holding, making it more. Lastly, the common error that shows in most science experiments was the
precision of the measuring instruments. To start with, the 100 ml cylinder used is much less accurate
then, say, a 25 ml graduated cylinder. Also, the scale and meniscus could also have been imprecise,
which decreases the accuracy of true measurement. To improve this experiment, I would suggest more
trials for the mass and volume of pennies, as well as thorough drying of the pennies and graduated
cylinder before use to allow for most accurate results. Also, to be even more precise, a larger number of
pennies from the selected years, as well as more from later years would also be helpful in proving the
result.
Works Cited:
1. Deisher, Beth. Coin World Almanac: 5th edition. Ares press, Inc., New York: 1987.
2. Doty, R.G. “Cent”. World Book Encyclopedia, 2004 ed. World Book Inc., Chicago: 2004.