MeasuringMeasuring Surface TensionSurface Tension

Najee Quashie Najee Quashie

Orange Preparatory Orange Preparatory

AcademyAcademy

88thth Grade Grade

Ms. Brown Ms. Brown

ProblemProblemThe goal of this project is to construct The goal of this project is to construct and use a homemade single-beam and use a homemade single-beam balance to directly measure the surface balance to directly measure the surface tension of a liquid.tension of a liquid.

ResearchResearchThe cohesive (attractive) forces between The cohesive (attractive) forces between

liquid molecules are accountable for the liquid molecules are accountable for the event known as surface tension. The event known as surface tension. The molecules at the surface do not have other molecules at the surface do not have other like molecules on all sides of them and like molecules on all sides of them and therefore they join together more strongly to therefore they join together more strongly to those openly linked with them on the surface. those openly linked with them on the surface. This forms a surface “film” which makes it This forms a surface “film” which makes it more difficult to move an object through the more difficult to move an object through the surface than to move it when it is completely surface than to move it when it is completely covered. Surface tension is the reason why covered. Surface tension is the reason why some bugs (water striders) are able to some bugs (water striders) are able to “stand” on water.“stand” on water.

ResearchResearchMolecules can be grouped as polar or non-Molecules can be grouped as polar or non-

polar molecules. Some are also in between. The polar molecules. Some are also in between. The arrangement of the atoms in some molecules is arrangement of the atoms in some molecules is such that one end of the molecule has a positive such that one end of the molecule has a positive charge and the other side has a negative charge. If charge and the other side has a negative charge. If this is the case, the molecule is called a polar this is the case, the molecule is called a polar molecule, meaning that it has electrical poles. molecule, meaning that it has electrical poles. Otherwise, it is called a non-polar molecule. Otherwise, it is called a non-polar molecule. Whether a liquid is polar or non-polar helps to Whether a liquid is polar or non-polar helps to determine how strong the surface tension will be determine how strong the surface tension will be between its molecules. This is why water (a strong between its molecules. This is why water (a strong polar molecule) generally has a very high surface polar molecule) generally has a very high surface tension.tension.

Polar Water Molecule

HypothesisHypothesisThere will be more surface tension There will be more surface tension with the hand soap because it is a with the hand soap because it is a thicker substance.thicker substance.

MaterialsMaterials• a beam (drinking straw)a beam (drinking straw)• a fulcrum (nail)a fulcrum (nail)• 2 supports of equal 2 supports of equal

height (two wood blocks)height (two wood blocks)• pan for weights (made pan for weights (made

from aluminum foil)from aluminum foil)• 5 cm length of 5 cm length of

straightened paper clip straightened paper clip wire)wire)

• thread (for attaching pan thread (for attaching pan and needle to balance) and needle to balance)

• Petri dishPetri dish

• small bit of modeling small bit of modeling clay to counterbalance clay to counterbalance the empty panthe empty pan

• waterwater• Hand soapHand soap• Rubbing alcoholRubbing alcohol• Vegetable oil Vegetable oil • weights (common pins)weights (common pins)• a way to calibrate pins a way to calibrate pins

(scale)(scale)• Lab notebook (for Lab notebook (for

results).results).

ProcedureProcedure Background research. Background research. Gather materials and find a good place to work. Gather materials and find a good place to work. Construct the balance.Construct the balance.

Take your time and work carefully. You'll get better results. Take your time and work carefully. You'll get better results. First construct the beam. First construct the beam.

There are many choices for materials. You just need There are many choices for materials. You just need something stiff enough to support a few grams at each end. something stiff enough to support a few grams at each end.

Mark the center point for the fulcrum. The beam needs to Mark the center point for the fulcrum. The beam needs to rotate freely about the fulcrum. rotate freely about the fulcrum.

Make holes at each end of the beam, halfway from the Make holes at each end of the beam, halfway from the center. Attach loops of thread through the holes. center. Attach loops of thread through the holes.

Push the fulcrum through the center hole of the beam, and Push the fulcrum through the center hole of the beam, and place it on the supports. place it on the supports.

Next construct the pan. Next construct the pan. This can be a simple box or dish folded from aluminum foil. This can be a simple box or dish folded from aluminum foil. Tie a thread to the center of your paperclip wire. Adjust the Tie a thread to the center of your paperclip wire. Adjust the

thread so that the wire hangs horizontally. thread so that the wire hangs horizontally.

Measuring surface tension.Measuring surface tension. • Hang the pan from one end of the beam and the wire from the other. Use a Hang the pan from one end of the beam and the wire from the other. Use a

small piece of modeling clay as a counterbalance to balance the wire and small piece of modeling clay as a counterbalance to balance the wire and empty pan. empty pan.

Place your container of liquid to be tested so that the wire, still hanging Place your container of liquid to be tested so that the wire, still hanging horizontally, is submerged. horizontally, is submerged.

You will add small amounts of weight to the pan, and measure the force You will add small amounts of weight to the pan, and measure the force needed to pull the wire free from the surface of the liquid. needed to pull the wire free from the surface of the liquid.

It will not take much weight, so you need to add it in small increments.It will not take much weight, so you need to add it in small increments. Use common pins as your weights, adding them one at a time. Adjust them Use common pins as your weights, adding them one at a time. Adjust them

by weighing a bunch of pins on a scale, and dividing by the number of pins by weighing a bunch of pins on a scale, and dividing by the number of pins to get the weight per pin. to get the weight per pin.

Repeat the measurement (steps 1–3) Repeat the measurement (steps 1–3) at leastat least 2 times, to assure consistent 2 times, to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the liquid), repeat the trial from the beginning. the needle out of the liquid), repeat the trial from the beginning.

Average your results. Average your results. The force you will be measuring can be expressed by the equation:The force you will be measuring can be expressed by the equation:

FF = 2 = 2sdsd, where , where FF is the force, in newtons (N), is the force, in newtons (N), the factor of 2 is because the film of the liquid being tested pulled up by the factor of 2 is because the film of the liquid being tested pulled up by

the needle (or wire) has 2 surfaces, the needle (or wire) has 2 surfaces, ss is the surface tension per unit length, in units of newtons/meter (N/m), is the surface tension per unit length, in units of newtons/meter (N/m),

and and dd is the length of the needle (or wire), in units of meters (m). is the length of the needle (or wire), in units of meters (m).

ProcedureProcedure

Procedure To convert grams to the force, To convert grams to the force, FF, you have to account for , you have to account for

gravity pulling down on the mass in the pan. Do this by gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10-3 N/g. multiplying the mass (in grams) by 9.83×10-3 N/g.

You can rearrange the equation above to solve for You can rearrange the equation above to solve for ss, the , the surface tension of the liquid. Measure the length of the surface tension of the liquid. Measure the length of the wire, and you'll have all the information you need.wire, and you'll have all the information you need.

ResultsResults

Surface Tension Date Table (Trial 1)

Liquid Substance

# of pins needed to pull wire from

surface

Calculated Surface Tension (S = F / 2d)

Water 11 pins = 0.583 gS = 0.0057

2 x 0.055 ↓ 0.11

= 0.052 N/M

Rubbing Alcohol 5 pins = 0.265gS = 0.0026

2 x 0.055 ↓ 0.11

= 0.024 N/M

Vegetable Oil 7 pins = 0.371gS = 0.0036

2 x 0.055 ↓ 0.11

= 0.033 N/M

Hand Soap 7 pins = 0.371gS = 0.0031

2 x 0.055↓ 0.11

= 0.028 N/M

ResultsResultsSurface Tension Data Table (Trial 2)

Liquid Substance

# of pins needed to

pull wire from surface

Calculated Surface Tension (S = F / 2d)

Water 7 pins = 0.371g S = 0.0036

2 x 0.055 ↓ 0.11

= 0.033 N/M

Rubbing Alcohol 5 pins = 0.265g S = 0.0026

2 x 0.055 ↓ 0.11

= 0.024 N/M

Vegetable Oil 7 pins = 0.371 g

S = 0.0036 2 x 0.055

↓ 0.11 = 0.033 N/M

Hand Soap 5 pins = 0.265gS = 0.0026

2 x 0.055 ↓ 0.11

= 0.024 N/M

ResultsResultsAverage Surface Tension Data Table

Liquid Substance

# of pins needed to

pull wire from surface

Calculated Surface Tension (S = F / 2d)

Water 9 pins = 0.0.477g

S = 0.043 N/M

Rubbing Alcohol 5 pins = 0.265g S = 0.024 N/M

Vegetable Oil 7 pins = 0.371 g

S = 0.033 N/M

Hand Soap 6 pins = 0.318 g

S = 0.026 N/M

ResultsResults

Results

ConclusionConclusionI found that water had the greatest surface tension of 0.043 I found that water had the greatest surface tension of 0.043

N/M, vegetable oil had the second greatest surface tension calculated N/M, vegetable oil had the second greatest surface tension calculated to be 0.033 N/M, hand soap had the third greatest surface tension to be 0.033 N/M, hand soap had the third greatest surface tension (0.026 N/M), and rubbing alcohol came in last with a calculated (0.026 N/M), and rubbing alcohol came in last with a calculated surface tension of 0.024 N/M.surface tension of 0.024 N/M.

This data disproved my hypothesis because I believed that This data disproved my hypothesis because I believed that hand soap would have had the greatest surface tension. In fact, the hand soap would have had the greatest surface tension. In fact, the surface tension of water was 0.017 N/m greater than that of the hand surface tension of water was 0.017 N/m greater than that of the hand soap. soap.

I also questioned how I knew that I was measuring surface I also questioned how I knew that I was measuring surface tension , and not an attractive (adhesive) force between the wire and tension , and not an attractive (adhesive) force between the wire and the tested liquid. In order to answer this, I observed the wire carefully the tested liquid. In order to answer this, I observed the wire carefully after it was pulled out of the liquid. It remained wet, which means after it was pulled out of the liquid. It remained wet, which means that it must be the liquid that pulled apart, and this is the force that it must be the liquid that pulled apart, and this is the force (surface tension) that was measured. If the wire was dry, then it (surface tension) that was measured. If the wire was dry, then it would be the attractive (adhesive) force only that I was measuring.would be the attractive (adhesive) force only that I was measuring.

ReferencesReferences Science Buddies:Science Buddies:

http://www.sciencebuddies.org/science-fair http://www.sciencebuddies.org/science-fair projects/project_ideas/Phys_p012.shtmlprojects/project_ideas/Phys_p012.shtml

http://hyperphysics.phy-astr.gsu.edu/hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/surten.html

WWLPT Biology Institute: WWLPT Biology Institute:  http://www.woodrow.org/teachers/bi/1998/waterstrider/http://www.woodrow.org/teachers/bi/1998/waterstrider/

student_lab.htmlstudent_lab.html

Teach Engineering:Teach Engineering: http://www.teachengineering.org/view_lesson.php?http://www.teachengineering.org/view_lesson.php?

url=collection/duk_/lessons/duk_drops_mary_less/url=collection/duk_/lessons/duk_drops_mary_less/duk_drops_mary_less.xmlduk_drops_mary_less.xml

Ron Kurtus’ School For Champions:Ron Kurtus’ School For Champions: http://www.school-for-champions.com/chemistry/polar_molecules.htm http://www.school-for-champions.com/chemistry/polar_molecules.htm