Alex Bishop
Surface Tension
1/7/14
Surface Tension
TEACHER’S INSTRUCTIONS
DESCRIPTION: The lesson uses both demonstrations and activities to help students learn about
the structure of water and the importance of cohesion and surface tension.
LEARNING GOALS:
Students will testA) The amount of small rocks fit in a filled glass of water before it overflows
B) The number of drops of water that fit on the surface of a penny
C) How soap or excess pressure can break surface tension
Students will observeA) The structure of a water molecule (including polarity) by creating a Mickey Mouse model
B) That surface tension can be stronger than gravity (Demonstration: No Spill Screen)
Students will understandA) Unique properties of water (based on structure and polarity)
B) Cohesion (5th-8th), and surface tension
C) Air pressure (brief introduction)
RECOMMENDED GRADE LEVELS: K-8th grade
ESTIMATED TIME REQUIRED: 45 minutes - 1 hour, prep time 5 minutes
KEY CONCEPTS AND DEFINITIONS OF TERMS:
Structure of Water: H2O, diameter: 2.75 angstroms, H-O-H bond angle: 104.5o (angle smaller than
expected due to the force of the two lone pairs of oxygen)
Surface Tension: Water molecules on the surface of an object cling so tightly to one another that a
“skin” forms.
Polarity: A separation of electric charge due to a lack of electrons, excess of electrons (in atoms) or
an unequal sharing of electrons (between atoms of a molecule)
Cohesion: The “clinging” property that causes surface tension, occurs between like atoms or
molecules (ex: water molecules are attracted to each other)
Adhesion: The strong attraction between dissimilar particles or surfaces (ex: water molecules
“cling” to string)
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Air pressure: The force exerted on the earth by air particles. Air pressure is similar to the pressure
we feel when underwater; however, we do not “feel” air pressure because we never are without it.
MATERIALS:
1. Cup (1), water, rocks (~60-70)
2. K-4th grade version: Colored pencils, Mickey Mouse water molecule design (1 per
student—see page 13), 5th-8th grade version: plastic grocery bags (a strip of plastic per
student), scissors
3. Pennies (1 per student), plastic droppers (1 per student), cups (1 per student), water
4. K-4th grade version: Plastic cups (1 per student), duct tape, string, water bottle with thin
straw opening, water, 5th-8th grade version: plastic cups (2 per student), duct tape, string,
water
5. Pepper (1 shaker), pie pan (1), water, dish soap, toothpicks (1 per student), small cups (1
per student), Styrofoam bowls (1 per student)
6. Glass bottle with narrow neck (ex: 125 mL Erlenmeyer flask, 1 needed), metal screen
that is large enough to cover the mouth of the Erlenmeyer flask, rubber bands (to secure
metal screen), toothpicks (1 per student), water
7. Small weigh boats or plastic cups (1 per student), small buttons (1-6 per student), water
SAFETY INFORMATION:
Instructors and students should wear safety goggles at all times.
GENERAL OUTLINE OF PROCEDURES:
NOTE: Before beginning any demo or activity, ask that students that have previously seen the
experiment not answer pre-lab questions.
1. Demonstration: How Many Rocks Fit in a Glass of Water? (Omit if older students)
A. Fill a glass to the brim with water. Display all the rocks.
B. Pre-lab Question: Ask the students how many rocks they think will fit in the glass before
it overflows, and have them record the prediction on their observations chart.
C. Present a challenge. Ask students to line up and put a rock into the glass (one at a
time) without making the water overflow.
D. Result: A dome of water about ¼ inches high will form over the mouth of the glass
before it overflows.
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E. How it works: When the water rises above the rim of the glass, it forms a surface tension
“skin”. The water molecules “cling” to one another, preventing the water from spilling over.
The water does not spill until the strength of the surface tension cannot withstand the force
of gravity any longer.
Variations to Try: Small metal objects (nails, paperclips, coins, etc.) may be used in place of the
rocks. Additionally, other liquids can be used to teach students about surface tension.
2. Class Activity: Mickey/Minnie Mouse Water Molecule (K-4th)
A. Have students flip to the Mickey/Minnie Mouse water molecule on their worksheet and
explain that Mickey/Minnie’s chin is negative (-) while the ears are positive (+).
Mickey/Minnie’s ears are attracted to the chin like the positive end of a water molecule is
attracted to the negative end.
B. Play “It’s a Small World” and allow students time to color the Mickey/Minnie Mouse
model. Ask students to indicate positive and negative charge on the drawing.
Class Activity: Explanation of the Structure of Water Molecule (5th-8th)
A.Explain that the structure of a water molecule looks like Mickey Mouse (brief reminder).
B. Have students rub the plastic bag against their head. Relate the static formed to the
attraction of the positive and negative ends of a water molecule.
C. Atoms are made up of protons (positive charge), neutrons (neutral charge), and electrons
(negative charge).
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D. Protons and neutrons are in the middle of the atom (nucleus) and electrons surround the
nucleus.
E. Molecules are made up of two or more atoms chemically bonded to one another. If atoms
in a molecule share electrons unequally, the molecule is polar.
F. Water is an example of a polar molecule: Oxygen is negative, Hydrogens are positive.
G. Since water molecules are polar, they are very attracted to one another (cohesion).
Variations to Try: If the students are old enough, explain the difference between ionic and covalent
bonds and polar ionic and covalent ionic molecules.
3. Class Activity: How Many Drops of Water Fit on the Surface of a Penny?
A. Pass a penny, pipette, and glass of water out to every student.
B. Pre-Lab Question: Ask students to predict how many drops of water will fit on the penny
before it spills off. Have students record predictions on their observations charts and then
ask the students to raise their hand if they think: at least 5 drops fit, 10 drops, 15 drops, etc
will fit. Group the predictions this way and write on the board.
C. Result: Several drops fit on the penny (~25-35); the better the technique (i.e. gently
dropping the water onto the dome at the correct angle), the more drops fit.
D. How it works: This activity uses the same principles as How Many Rocks Fit in a Glass
of Water? The water droplets “cling” to one another (cohesion) to form the surface tension
“dome”. The water spills off when the force of gravity becomes too great for the surface
tension of the water to withstand.
E. Relate the dome on the penny to that seen in How Many Rocks Fit in a Glass of Water?
Help students recognize the connection between the two examples.
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Variations to Try: Other round, flat objects may be used in place of a penny. For example, the top of
a gallon of milk could be used.
4. Demonstration: Walking Water
NOTE: Instructions C-E (5th-8th grade) assume that the demonstrator is right-handed
Set-Up (K-8th grade)
A. Pre-Lab Question: Ask students how to move the water to the empty cup (without
spilling) using only the string.
B. Have students dampen string, and tape to the bottom of one of the cups (cup 1).
K-4th grade:
C. Have students put the tip of the nozzle against the string and lightly squeeze the bottle.
D. Instruct students that once they get the hang of it, they can squeeze the bottle harder to
get a steadier stream of water.
5th-8th grade:
C. Have students fill the other cup (cup 2) with water and hold the free end of the string into
cup 2 left index finger.
D. Have students hold cup 2 in their left hand and cup 1 in their right hand. Ask students to
stretch the string taut and slant it almost vertically towards cup 1.
E. Have students tip cup 2 and slowly pour onto the string using their left index finger to
guide the water to the string.
F. Result: the water travels down the string into the empty cup with little or no spilling.
G. How it works: The water molecules “cling” to one another (cohesion) and to the string
(adhesion). The cohesion of the water molecules prevents the water from spraying in all
directions. The adhesion of the water molecules to the string creates the directed stream.
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5. Class Activity: Soap and Surface Tension
A. Pass a Styrofoam bowl, a small amount of soap in a Dixie cup, and a toothpick to each
student. Prepare a pie pan with water for your demonstration.
B. Walk around and pour a layer of pepper in each of the bowls and your pie pan so that the
bottoms are not visible.
C. Pre-Lab Question: Ask students to predict what will happen when a drop of dish soap is
added to the pepper.
D. Ask students to add a drop of dish soap using the provided toothpick and cup of soap.
E. Result: The pepper is “pulled back” in all directions when the soap hits the surface.
F. How it works: Soap decreases surface tension. The pepper is “pulled back” because the
surface tension is strong in the areas where the soap has not reached.
Variations to Try: Baby powder may be substituted for pepper to demonstrate the same principles of
surface tension.
6. Demonstration: No Spill Screen
NOTE: Directions D-E assume the demonstrator is right-handed
A. Cover the opening of a narrow-neck glass bottle with a small amount of metal screen.
Secure with a rubber band.
B. Pour water through the screen into the bottle to show that it does not obstruct the flow of
the water.
C. Pre-Lab Question: Ask students to predict how to prevent the water from spilling out
when the Erlenmeyer flask is tipped upside down.
D. Cover the top of the Erlenmeyer flask with your left palm and flip it over with your right
hand.
E. Remove your left hand. The water does not spill.
G. Ask students to try lightly poking the toothpick in the holes. The water remains in the
Erlenmeyer flask.
H. How it works: No water spills because the strength of the surface tension and air pressure
are greater than the force of gravity (working in the opposite direction). The cohesion of the
water molecules forms a “skin” and the air pressure pushes up against the screen (from the
bottom), keeping the water in the Erlenmeyer flask.
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7. Extra Class Activity: Floating Buttons
A. Pass out a weigh boat (filled to the brim with water) to each student. Also give each
student a small button. Prepare a larger cup filled with water and a button to use for your
demonstration.
B. Drop the button into the cup and show students that it sinks. Add water so that the cup is
filled to the brim.
C. Pre-Lab Question: Ask students how to float a button on the surface. Have students
record predictions on their observations charts.
D. Show that carefully sliding the button from the rim of the glass allows it to float (surface
tension). The button must be slid onto the top layer of the surface tension “dome.”
E. Allow students to try floating their buttons.
F. Challenge students to move the button to the opposite side of the weigh boat as quickly as
possible.
G. Explain that touching the button breaks the surface tension. In order to efficiently move
the button, the student must use the surface tension from his/her finger to move the water
(therefore moving the button).
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H. Explain that the button floats because the surface tension “dome” does not break to let the
button through.
Variations to Try: Paperclips may be used in place of buttons. Additionally, if students put soap on
their index finger and run it along the water behind the button, the soap will break the surface
tension and propel the button.
CLEANUP: All discarded materials may go in the trash. No chemical cleanup necessary.
1. Discard water and collect the rocks.
2. Allow students to keep Mickey Mouse water molecules. Collect colored pencils.
3. Collect all materials. Give each student a penny at the end of the lesson.
4. Discard water, tape and string. Reuse cups.
5. Discard water. Collect and reuse Styrofoam bowls, toothpicks, cups, and pie tin.
6. Rinse bottle. Metal screen and rubber band may be reused.
7. Remove buttons. Discard water. Reuse weigh boats and buttons.
NOTES: See page 12 for student instructions for the class activities: How Many Drops of Water Fit
on the Surface of a Penny?, Walking Water, Soap and Surface Tension, and Floating Buttons. 5th-8th
grade worksheet may be found on page 14.
REFERENCES:
Bridgewater, Khrystian. "Enzymes Food Chemistry Project." Enzymes Food Chemistry Project.
N.p., n.d. Web. 08 Jan. 2014.
https://sites.google.com/a/kent.k12.oh.us/rhschem8kbridgewater/ (Beaker image)
Carag, Arelene. "Mickey Mouse Turns 84." Annewsments. N.p., 19 Nov. 2012. Web. 08 Jan. 2014.
http://www.inquisitr.com/403582/mickey-mouse-turns-84-year-old/ (Mickey Mouse
image)
Herbert, Don, and Hyman Ruchlis. Mr. Wizard's 400 Experiments in Science,. Brooklyn, NY:
Book-Lab, 1968. Print. (How Many Rocks Fit in a Glass of Water, No Spill Screen, How
Many Drops of Water Fit on the Surface of a Penny, Soap and Surface Tension, penny
image)
"Liquid Soap." Photo Dictionary. N.p., n.d. Web. 12 Jan. 2014. http://www.photodictionary.com/phrase/8913/liquid-soap.html (Soap image)
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"Mickey Mouse Face Outline." N.p., n.d. Web. 9 Jan. 2014.
http://printncolor.coolphotos.in/color/mickeyearsoutline-mickey-mouse-face-coloringpages.html (Mickey Mouse Model)
“Polarity." Merriam-Webster. Merriam-Webster, n.d. Web. 09 Jan. 2014. http://www.merriamwebster.com/dictionary/polarity (Definition)
"Protons, Neutrons, and Electrons." Middle School Chemistry. ACS, n.d. Web. 14 Jan. 2014.
http://www.middleschoolchemistry.com/lessonplans/chapter4/lesson1 (Polarity explanation,
polarity class activity)
"Water on a String." Zoom Science. PBS Television, n.d. Web. 7 Jan. 14.
http://pbskids.org/zoom/activities/sci/wateronastring.html (Walking Water)
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Fun Sheet
(K-3rd grade)
1. How is Mickey Mouse like a water molecule?
2. What causes surface tension?
3. Why happens when you add soap to water? (Hint: What happens to the surface
tension?)
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Predictions and Observations
How Many Rocks
How Many Drops of Water
Fit in a Glass of
Fit on the Surface of a
Water?
Penny?
Floating Buttons
How to make buttons float:
Prediction
__________ rocks
__________ drops will fit.
will fit.
How we made buttons float:
Results
_________ rocks fit.
__________ drops fit.
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Student Instructions
How Many Drops of Water Fit on the Surface of a Penny?
A. Write down your prediction of how many drops of water will fit on the penny.
B. Add a drop of water to the penny at a time use the pipette.
C. Using the pipette: Squeeze the top part when you put it in the water and then let go.
D. Add drops to the sides of the penny first and then the middle.
Walking Water
A. Get the string wet and tape it to the bottom of a cup (cup 1).
C. Fill the other cup (cup 2) with water. Hold the other end of the string in the empty cup
using your left pointer finger.
E. Hold the filled cup in your right hand and stretch the string so that it is tight. Slant the
string so that it is almost straight up from cup 2.
F. Tip the cup of water and slowly pour onto the string using your left pointer finger to
guide the water to the string.
Soap and Surface Tension
A. When you are instructed, put the toothpick in the soap.
B. Touch the toothpick the middle of the pepper in the bowl.
C. Ask students to add a drop of dish soap using the provided toothpick and cup of soap.
Floating Buttons
A. Drop the button into the cup.
B. Carefully slide the button onto the water from the edge of the weigh boat.
C. Once the button is floating, move it around using the surface tension of your finger.
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Student Worksheet (5 -8
th
th
grade)
1. Draw an atom and show where the proton, neutrons, electrons, and nucleus are located.
2. What is cohesion?
3. Why does a paperclip float if it is gently pushed from the side of a glass of water instead of
dropped in?
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