3rd–5th Grade
Objectives
• 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
Materials Needed
• Water Drop Record Sheet
• Pennies (one for each student)
• Eyedroppers
• Cups of water
Introduction
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.
Procedure 1. Give each student a penny. Ask students to predict how many drops of water will fit on the surface without
overflowing.
2.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?
3.Distribute copies of the Water Drop Record Sheet, and instruct students to record their predictions in the correct box on their sheet.
Guided/Independent Practice
1. Divide the class into groups of two or three. Provide each group with an eyedropper and a cup of water.
2.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.
3.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.
4.Have students complete the experiment two more times and record the results in the spaces provided.
5.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.
6.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.
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Conclusion
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.
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Name: ________________________
How many drops of water will fit on the surface of a penny without overflowing?
Prediction
Trial
(with eyedropper one inch
above tails side of penny)
Actual Result
Difference
(between the prediction
and actual result)
1
2
3
Find the average of your results using this formula.
( _____ + ______ + _____ ) ÷ 3 = _______
Trial 1 Trial 2 Trial 3 Average
Repeat the experiment with the following variables. How did your results change?
Trial
Result
With the penny heads up
With the eyedropper three inches above the penny
With ice water
Observations: ____________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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