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This activity explores the
concept of surface tension. Using common, inexpensive materials,
your students perform a simple experiment that allows them to
see surface tension at work. They will also learn to recognize
examples of surface tension in the natural world, such as water
drops on spider webs or dewdrops on grass. This activity is
designed for students in grades 3–8 and addresses the following
National Standards for physical science: K–4 Properties of
Objects and Materials, and 5–8 Properties of and Changes of
Properties in Matter.
Background
Why do water drops gather on a spider's
web? Why are dewdrops found glistening on blades of grass in the
bright morning sun? Why don't
water striders
break through the water's surface? All of the answers to these
questions are in some way related to surface tension.
At the heart of surface tension is the
attractive force between the surface molecules and inner
molecules of a liquid. The surface molecules of a liquid are
pulled toward the center of a mass of liquid by the inner
molecules, causing the surface area of the liquid to contract
and become as small as possible. The energy needed to break
through the surface of a liquid or disrupt a drop is the
liquid's surface tension. Water has a greater surface tension
than most other common liquids. That is why water striders don't
break the water's surface, dewdrops collect on blades of grass,
and water drops collect on spider webs, as seen in this month's
calendar photo.
Materials (For each group of 2–3
students)
| • |
Dropper or pipet |
| • |
Hand lens |
| • |
Paper towels |
| • |
Penny, nickel,
dime, quarter (1 of each for each group) |
| • |
Small beaker
or cup |
| • |
Water |
Preparation (Teacher)
| 1. |
Gather
materials needed for each group and place them in a central
location. |
| 2. |
Fill a small
beaker or cup about half full with water. |
Procedure
| 1. |
Divide your class into groups
of 2–3 students each. |
| 2. |
Have the students in each
group discuss and predict how many drops of water can be
placed on top of a penny lying flat on a table. Each student
should record their prediction and design a data table to
record their observations. |
| 3. |
Designate one student from
each group to collect the materials for their group,
including filling the dropper or pipet with water. |
| 4. |
Instruct each group to place
one drop of water at a time on their penny using their
dropper or pipet until the water rolls of the penny. Each group
counts the number of drops placed on their penny until the
water rolls off and each student records the count in his or
her data table. |
| 5. |
Have your students observe the
water drops on their pennies using a hand lens and then
sketch their observations on a sheet of paper. |
| 6. |
Direct the groups to perform
step 4 at least 3 times and average their results. |
| 7. |
Tell the groups to dry their
coins and work area with the paper towels and return all
materials to the central location. |
| 8. |
Optional: Have your students
create a class chart or graph to display the averaged data
from each group. |
| 9. |
Ask your students to consider
some of the following questions: |
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• |
How did their predictions compare with their results? |
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What did the water drops look like on the penny? |
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• |
What happened when too many water drops were placed on the
penny? |
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Did the water drops spread out over a large area or form a
puddle? |
| |
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Why do you think water behaves like this? |
| 10. |
Define surface tension for
your students and discuss with them how it relates to this
experiment. |
| 11. |
Have students describe
examples of surface tension they may have seen in nature,
e.g., a water strider gliding across a pond. |
Extension activities
| 1. |
Instruct each
group to repeat the experiment with 2 other coins, e.g., a
dime and a quarter. |
| 2. |
Have the
groups create a class chart or graph showing the average
number of drops that can fit on each type of coin. |
| 3. |
Discuss the
relationship between the size of the coin and the number of
drops that can fit on the coin. |
Related products
Guide your class through an extended
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