by Eric Chyo | Lakeshore Elementary Product Development Manager
If your students have ever done an accidental belly flop into a swimming pool, they know firsthand what surface tension is. The flat surface of the water pushes back on their skin, causing it to sting. But when you try to explain surface tension to students in terms of molecules, you may notice that a few students can’t quite conceptualize what you mean. This simple experiment makes it easier for students to visualize how water molecules stick together to create surface tension—and keeps them actively involved in the investigation.
Note: This experiment is designed for 3rd – 5th graders. While this lesson plan was created for the classroom, it’s also a fun activity to try at home!
Download the lesson plan.
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• Exploring the concept of surface tension
• Making and testing a hypothesis by estimating the number of water drops that can fit on a penny and testing their predictions
• Analyzing data by averaging and comparing figures
You will need:
Ask students if they have ever seen a mosquito or other small bug sitting on the surface of a body of water. Explain that the surface of the water acts as “skin.” It tends to hold together because the water molecules are attracted to each other. This is called surface tension. Tell students that they are going to conduct an experiment that demonstrates how water molecules stick together to create surface tension.
- Give each student a penny. Ask students to predict how many drops of water will fit on the surface without overflowing.
- Encourage volunteers to share their predictions and explain their reasoning. Did they base their predictions on the size of the drops? The size of the penny? Something else?
- Distribute copies of the Water Drop Record Sheet, and instruct students to record their predictions in the correct box on their sheet.
- Divide the class into groups of two or three. Provide each group with an eyedropper and a cup of water.
- Instruct students to place the penny tails up and hold the eyedropper one inch above the surface. Have students count the drops as they add them one at a time to the surface of the penny.
- Prompt students to record the number of drops the penny held before any amount of water overflowed. Have them calculate the difference between their predictions and the results.
- Have students complete the experiment two more times and record the results in the spaces provided.
- Ask students to average the results by adding the total number of drops from all three trials and dividing that total by 3. Instruct them to record the average on their sheet.
- For further investigation, have students repeat the experiment with the variables listed on the chart at the bottom of their sheet. Encourage them to record any observations in the space provided.
Explain to students that the surface tension causes the drops of water to stick together on the penny instead of rolling off. The molecules of water on the surface of the penny are attracted to each other, so they tend to combine into one large drop rather than overflowing. Finally, discuss how the results changed when students changed each variable. Invite students to share their thoughts and observations.