Surface Tension: An Attractive Force
 

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 38 and addresses the following National Standards for physical science: K4 Properties of Objects and Materials, and 58 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 23 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 23 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:
  How did their predictions compare with their results?
  What did the water drops look like on the penny?
  What happened when too many water drops were placed on the penny?
  Did the water drops spread out over a large area or form a puddle?
  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 exploration of surface tension with these great Carolina products.
A Closer Look at Soaps and Detergents Kit

 

     
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