This document describes an experiment to investigate how soap affects the surface tension of water using the "drops on a penny" method. Students are asked to make initial predictions about whether soap will increase or decrease the number of water drops that can fit on a penny. They then perform trials counting the number of drops of tap water and soapy water that can fit. The results are analyzed to determine if the hypothesis that soap decreases surface tension was supported. Repeating the trials and calculating an average helps account for variability between drops. The control is the tap water trials and the independent variable is the type of liquid (tap water or soapy water), while the dependent variable is the number of drops that will fit.

1. PENNY LAB
Introduction: Surface tension refers to water’s ability to “stick to itself”. Surface tension can be measured
and observed by dropping water (drop by drop) onto a penny. The number of water drops that can fit on a
penny will surprise you.
Part A: Perform a CONTROL test for comparison with later results.
1. Make a guess about how many you think will fit on a penny. _________________
2. Hypothesis: I think a penny will hold ________________ drops of water.
3. Materials: Penny, Eyedropper , tap Water, soapy water, paper towel, forceps
4. Procedure:
1. Put the penny on a flat surface.
2. Use the eye dropper to drop water on the penny, one drop at a time.
3. Count the number of drops until the water spills over the edge of the penny.
4. Record your data.
5. Repeat steps 1 -4 for a total of 5 trials.
Sometimes a steady hand can make more drops fit on the penny. A bump of the table can reduce the
drops. We should do a few tests to get an average to see how many drops fit on a penny.
Test 1 Test 2 Test 3 Test 4 Test 5 AVERAGE
Tap water
Part B: Perform tests with the TESTING LIQUID.
Question: How does soap affect the water’s surface tension?
5. Develop a hypothesis. (underline the word that you think is correct)
Soap will [ increase /decrease ] the surface tension of the water.
6. Make a prediction. Choose the one below that matches your hypothesis
a. Soap will increase the surface tension, therefore I will be able to place more drops on the
penny
b. Soap will decrease the surface tension, therefore I will be able to place less drops on the
penny
Names _____________________________________________________ DATE:________________________________

2. 7. Test your hypothesis by comparing the number of drops of tap water to the number of drops of soapy
water that can fit on a penny. Because water drops may vary depending on how well you drop the water,
it is best to run many trials and take an average. Record your data in the table below
Step 1: Start with a “clean” penny. Rinse the penny in tap water and dry completely. Be sure to remove as
much residue as possible -
Step 2: Place penny on dry spot on a paper towel. Place drops of SOAPY WATER on the penny (one at a
time) until ANY amount of water runs over the edge of the penny.
Step 4: Record your observations and the number of drops for that trial in the table.
Repeat Steps 1 - 5 three more times before calculating the average.
Test 1 Test 2 Test 3 Test 4 Test 5 Average
Soapy Water
8. Draw Conclusions: Was your hypothesis correct? ______________
Complete this statement now that you know the facts
Soapy water will [increase / decrease] the surface tension of water. You should be able to place
[ more / less ] drops of soapy water than you can tap water.
9. Communication: An important part of science is comparing your results to others. Check the groups
near yours. Did they get the same results? __________
10• Why did we perform more than one trial?
11. What benefits are there to repeated trials?
12. • What could we have done to make sure all the groups ended up with similar results?
13. • What is the control for this experiment?
14. What is the independent variable?
15. What is the dependent variable?

3. Drops On A Penny Lab
Cohesion
Water molecules are attracted to other water molecules. The oxygen
end of water has a negative charge and the hydrogen end has a
positive charge. The hydrogens of one water molecule are attracted to
the oxygen from other water molecules. This attractive force is what
gives water its cohesive properties.
Surface Tension Surface tension is the name we give to the cohesion
of water molecules at the surface of a body of water. The cohesion of
water molecules forms a surface "film" or “skin.” Some substances
may reduce the cohesive force of water, which will reduce the strength
of the surface “skin” of the water.
Use this information to help you answer the questions on
the lab sheet after you have completed the experiment!

4. Drops On A Penny Lab
Fill in with the missing words.
Cohesion – Water molecules are _______________ to other water molecules. The
_____________ end of water has a _____________ charge and the _____________ end
Has a _____________ charge. The hydrogens of one water ______________ are attracted
to the oxygen from other water molecules. This attractive __________ is what gives water
its_____________ properties. Surface Tension - Surface tension is the name we give to
the ______________ of water molecules at the ___________ of a body of ___________.
The cohesion of water molecules forms a surface "_________" or “_________.” Some
substances may ____________ the cohesive force of water, which will reduce the
_______________ of the surface “skin” of the water.

5. 1. Draw the water molecule
2. Illustrate how the molecules are arranged in each State of water
Solid Liquid Gas

6. 3. Why are water molecules polar?
A water molecule is polar because there is an
uneven distribution of electrons between the
oxygen and hydrogen atoms.
The unequal sharing of electrons causes the
water molecule to be polar.
4. How are cohesion and adhesion similar? Different?

7. How is the Surface Tension of Water Affected By Soap? (LAB)
http://www.uni.edu/~iowawet/H2OProperties.html#states (reading)
http://www.biologycorner.com/worksheets/pennylab.html (lab)
http://www.science-class.net/NOS/penny_drops.pdf http://www.aquadyntech.com/watermolecule.html
http://www.chem1.com/acad/sci/aboutwater.html