Penny Lab
Mrs. Johnston
Science Class A, B, C & D
Mrs. Johnston
9/25/13
I) Introduction
The purpose of this Penny Lab is to determine how soap on a penny affects the number of
drops that it can hold, compared with a penny with no soap. Water molecules are attracted to
each other, so they tend to try to stick together (cohesion). This cohesion causes surface tension,
which allows for the penny to hold more water drops then one would anticipate.
II) Experimental Design
The experimental problem is: How does soap affect the number of water drops that the
surface of a penny can hold, compared with the number of drops that a penny without soap can
hold? The independent variable in the experiment is the soap and the dependent variable in the
experiment is the number of water drops on the surface of the penny. The control is the penny
without soap.
III) Hypothesis
If a penny is covered with soap, then it will not hold as many water drops because the soap does
not allow for adequate surface tension to occur.
IV) Test/Experiment
Materials
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1 penny
2 eye droppers (1 for water and 1 for soap)
3 beakers (1 for drops of water, 1 for soap and 1 for rinsing)
Data sheet
Paper towels for clean up
Procedures
1.
2.
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4.
Gather all materials. Make sure the two water beakers are filled with water.
Begin your control experiment.
Place the penny (tails side up) on a flat surface.
Fill the eyedropper with water from the water beaker and carefully count the number of
drops that fit on the penny until the water spills over the edge. Record this number on
your data table under trial 1.
5. Rinse and dry the penny. Repeat step 4 two more times.
6. Calculate the average for the clean penny.
7. Place one drop of soap on the penny (tails side up) and rub on so that the soap covers the
entire surface.
8. Fill the eyedropper with water from the water beaker and carefully count the number of
drops that fit on the penny until the water spills over the edge. Record this number on
your data table under trial 1.
9. Rinse and dry the penny. Repeat steps 7 & 8 two more times.
10. Calculate the average for the soapy penny.
V) Observations
See attached data table and graph.
VI) Discussion
The results of this experiment showed that the clean penny held more water drops than
the penny with soap on it. Based on class data, the penny without soap held a range of 16 – 28
drops of water, compared with the soapy penny that had a range of on 7 – 16 drops. This
observation was consistent with all groups and seems to show that the surface tension is
compromised when the soap is placed on the penny. The soap does not allow for enough
cohesion between the water molecules to allow for a large number of drops. More accurate
measurement of the amount of soap is needed to make sure that each trial is consistent and this
may have caused water to prematurely fall off due to too much soap. A determination of how far
away the dropper needed to be held from the penny should also have been made, as a dropper too
far could cause the water to prematurely fall off and a dropper held too close could cause a larger
number of drops.
VII Conclusion
How does soap affect the number of water drops that the surface of a penny can hold,
compared with the number of drops that a penny without soap can hold? In this experiment, the
data collected did support the hypothesis that if a penny is covered with soap, then it will not
hold as many water drops because the soap does not allow for adequate surface tension to occur.
Class data showed that, on average, the clean penny held 10 more drops of water than the penny
with soap on it. This indicates that there is a relationship between surface tension and an added
substance on a surface. Multiple people in groups completed the drops, which could have
affected the number of drops that stayed on the pennies. Time did not allow for more trials but
more trials could further demonstrate the acceptance of the hypothesis. It would be interesting to
complete additional experiments using substances other than soap, or dropping something other
than water.