# Surface Tension with soap film

#### Summary

In this Chemistry experiment, students explore surface tension using toothpicks, clay, water, and dishwashing liquid. They discover that soap film has a unique surface tension that behaves in a different way than water's surface tension. Students create geometric shapes with toothpicks and clay and dip them in soapy water. They prepare a lab report describing their observations along with the variable they changed.

## Learning Goals

This activity is designed for students to observe the effect that soap has on water's surface tension. Students will use critical thinking skills to compare and contrast the results while changing only one variable. They will need to observe closely during the experiment and use their questioning skills to determine why the soap film behaves as it does. Students will write and draw what they observe and come to conclusions about soap's surface tension. New vocabulary words to be introduced include: surface tension, cohesion, and atoms. One key concept that students will discover is that when bubbles meet, they will merge and share a common wall. Another concept they will discover is that soap decreases the pull of surface tension to about a third that of plain water.

## Context for Use

This lesson is appropriate for the intermediate grades 3 through 6. It would be a good extension lesson after students had learned about and experimented with surface tension in water. I would conduct this experiment in pairs or small groups of four with each student making at least one shape. It is a 45 – 50 minute activity. Equipment for the experiment is found in most households.

**Subject**: Chemistry:General Chemistry:Properties of Matter, Physics:Fluid Mechanics:Surface Tension,

**Resource Type**: Activities:Classroom Activity

**Grade Level**: Intermediate (3-5)

## Description and Teaching Materials

Materials can be prepared in advance and placed in a small tub or sandwich bag for each group. Each child needs: a piece of thread (6 – 12 inches long), a ball of clay (the size of a golf ball), and 12 toothpicks. For the class you need a small tub with dishwashing liquid and water (at least 4 inches deep).

Steps:

1. In small groups each student creates a three dimensional shape out of toothpicks and clay. Students can make a pyramid (4 faces with 6 toothpicks) or a cube (6 faces with 12 toothpicks). Toothpicks are used for each edge and a piece of clay secures each vertex.

2. Hang shapes from a thread and dip completely in soapy water.

3. Draw the results. Write down your observations.

I would introduce this activity by reviewing how surface tension works in water. Then I would hold up a toothpick pyramid and ask students to predict what would happen if I dipped it in soapy water. Students could write and draw their predictions and then share them with the class. Materials would then be distributed to small groups and students would assemble their pyramids or cubes. After submerging their shapes in soapy water, they will compare and contrast the results on the 2 different shapes. I would ask them to hypothesize about why the geo-panes formed the way they did. Questions to consider: How many surfaces come together to form a line? (3) Estimate the angle at which these surfaces join. (120 degrees) As students share observations, I would ask them to use our new vocabulary words. For closure, I would review key concepts we observed.

*Soap decreases the pull of surface tension in water.

*The surface tension in the bubble skin tries to shrink the bubble into the smallest possible surface area.

*When bubbles meet – they merge and share a common wall.

Steps:

1. In small groups each student creates a three dimensional shape out of toothpicks and clay. Students can make a pyramid (4 faces with 6 toothpicks) or a cube (6 faces with 12 toothpicks). Toothpicks are used for each edge and a piece of clay secures each vertex.

2. Hang shapes from a thread and dip completely in soapy water.

3. Draw the results. Write down your observations.

I would introduce this activity by reviewing how surface tension works in water. Then I would hold up a toothpick pyramid and ask students to predict what would happen if I dipped it in soapy water. Students could write and draw their predictions and then share them with the class. Materials would then be distributed to small groups and students would assemble their pyramids or cubes. After submerging their shapes in soapy water, they will compare and contrast the results on the 2 different shapes. I would ask them to hypothesize about why the geo-panes formed the way they did. Questions to consider: How many surfaces come together to form a line? (3) Estimate the angle at which these surfaces join. (120 degrees) As students share observations, I would ask them to use our new vocabulary words. For closure, I would review key concepts we observed.

*Soap decreases the pull of surface tension in water.

*The surface tension in the bubble skin tries to shrink the bubble into the smallest possible surface area.

*When bubbles meet – they merge and share a common wall.

## Teaching Notes and Tips

One safety guideline would be to wear goggles to prevent soap in eyes. This activity could be extended to include more geometric shapes. No matter what geometric shapes students form, all the vertices where three walls meet will have 120 degree angles. I have done this activity with teachers and noted that the larger shapes are more fragile and need extra clay to stay together. In working with students, allow enough time for students to compare results with different shapes. To understand the reason why soap breaks the surface tension in water you need to look at its structure. Soap molecules are made up of a long chain of carbon and hydrogen atoms. One end of the chain likes to be in water (hydrophilic) and the other end does not (hydrophobic). The end of the molecule that shuns water makes its way to the surface and squeezes between the water molecules breaking the water's surface tension.

## Assessment

For individual assessment, students will hand in their lab sheets with their observations and conclusions. New vocabulary should be used appropriately.

## Standards

5.1.1.2.2 Identify and collect relevant evidence, make systematic observations and accurate measurements, and identify variables in a scientific investigation.