Chemistry 110
Pennies: An Exercise in Thinking Scientifically
Experiment A
Albert Szent-Gyorgyi, the Hungarian-born biochemist who was awarded the 1937 Nobel Prize in
Medicine and Physiology for his discovery of vitamin C, once observed that discovery is seeing
what everyone else has seen, and thinking what no one else has thought. Anyone who has done
research has had this experience. The goal of this experiment is to introduce you to the
methodology of scientific research: observation, data acquisition using basic laboratory techniques,
mathematical data analysis, graphical representation of data, interpretation of results, construction of
hypotheses based on your interpretation, and the design of experimental tests of your hypotheses. In
this experiment you will make a study of some of the physical properties of U.S. pennies.
Color, shape, volume, mass, density, and hardness are examples of physical properties. We often
identify objects by their physical properties. Chemists also use physical properties to identify
chemicals.
Procedure:
The experiment consists of two parts. The first part is spelled out in detail. The second part will
require you to design and perform experiments to test hypotheses you developed to explain the
results of Part I. Part II will hopefully lead you to frame still other hypotheses. Since almost all
contemporary research involves several people working together, you will work in teams of two
students.
Safety First!
You must wear safety glasses.
Do all experiments in your hood.
Part I
Data Collection
Obtain a numbered beaker, which should contain forty pennies. This is the sample you will work
with. Record the beaker number in your laboratory notebook.
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A. Mass of Pennies
Weigh the entire sample of pennies.
Weigh each of the pennies individually. Record all decimal places. Enter each mass and
year in a table in your notebook.
B. Volume of Pennies
We want to measure the total volume of the 40 pennies. Finding the volume of a solid can be
difficult. The Greek mathematician, Archimedes, was faced with the problem of measuring the
volume of a crown. He devised a method while relaxing in the bath. He was so pleased that he
jumped out of his bath and ran through the streets shouting, "I found it." In his excitement he
even forgot to dress.
Place about 50 mL of water in a 100 mL graduated cylinder. Record the volume of water. Get
your partner to take a reading too, and compare numbers only after each of you has written down
your own reading. (This is because it is easy to misread the volume.) With care you should be
able to measure the water levels to the nearest 0.5 mL.
Carefully add the 40 pennies to the water in the graduated cylinder. What is the total volume in
the graduated cylinder now? Again, you and your partner should take independent, “blind”,
readings. What is the volume of the 40 pennies?
Data Analysis
We want to find out if the pennies are pure copper. We will do this using density. Frequently
people confuse density with weight. They say that gold is heavy while aluminum is light. This
is not always true. A gold ring is much lighter than a Boeing 747 airplane's body. What they
mean to say is that a piece of gold is heavier than the same size piece of aluminum. This is
density. The density of an object can be found by dividing its mass by its volume.
The density of a pure substance does not depend on where the substance is from or its size or its
shape. Archimedes knew this. He knew that the density of gold is 19 g/mL. He knew that if the
crown were pure gold its density would be 19 g/mL. If there were other metals in the crown its
density would be different.
Calculate the average density of your pennies using the total mass of the 40 pennies and
their volume. Compare this to the value for the density of copper as given in the Handbook
of Chemistry and Physics.
What conclusion can you make about your pennies?
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Construct a graph of your data. One type of graph you might use is a histogram of the mass of
the pennies. Use Figure 1, a mass distribution for nickels, as a guide. (A good size for the mass
ranges is 0.03 g, i.e., 2.441 g - 2.470 g, 2.471 g - 2.500 g, etc.) What conclusions can you draw
from this graph? (It might help to compare the general form of your graph to that of Figure 1.)
Write two different hypotheses, either of which can explain your conclusions about the
sample of pennies. In other words, come up with two possible causes for your findings.
What measurements can you make, using no additional equipment, to test the two
hypotheses above? The outcome of your planned measurements should be capable of
eliminating one of the hypotheses. Describe what you plan to do, and how the possible
outcomes of the measurements would affect each hypothesis.
Part II
You now have two hypotheses about the pennies in your sample and plans for testing them. In Part
II, you will be given half an hour to test one or both of your hypotheses, using your plans. After you
make the measurements you proposed, do any necessary calculations, and decide which hypothesis,
if either, could still be correct.
On the basis of your observations, do you still believe pennies are made of copper, and if not,
how could the data you have amassed thus far help you to decide what they are made of?
Safety First!
Always wash your hands before you leave the
chemistry lab.
This lab was developed by D.J. Sardella, Boston College. Used with permission.
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Mass Distribution of U.S. Nickels:
An Example
A more-or-less random sample of 30 nickels was collected from members of the Chemistry
Department. The masses of the nickels ranged from 4.86 to 5.12 g, and the distribution is shown
in Figure 1.
Number of nickels
Figure 1. Mass distribution of U.S.
nickels
12
10
8
6
4
2
5.16-5.20
5.12-5.16
5.08-5.12
5.04-5.08
5.00-5.04
4.96-5.00
4.92-4.96
4.88-4.92
4.84-4.88
0
Mass range (g)
The average mass of a nickel was found to be 5.01 g. A sample of 8 nickels was taken and its volume
determined by water displacement using a 100 mL graduated cylinder. The average volume of a
nickel was found to be 0.54 mL, making the average density 9.3 g/mL. According to the Handbook
of Chemistry and Physics (61st Edition), the density of elemental nickel is 8.90 g/mL, in reasonable
agreement with the experimental result.
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