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How Does Soap Affect Surface Tension?

written by: Terrie Schultz • edited by: Donna Cosmato • updated: 1/5/2012

Students will learn about surface tension and apply the scientific method by observing the number of drops of water that stick to the surface of a penny, and determining whether adding soap to the water has an effect on the surface tension.

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    Preparation

    Ask students if they have ever seen a water strider sitting on the surface of a pond. The water molecules stick together and form a "skin" on the surface strong enough to hold up the insect. This is an example of surface tension, the ability of a liquid to stick to itself. Surface tension may be affected by adding soap to the water, and the effect can be measured by comparing the number of drops of plain tap water versus the number of drops of soapy water that stick to a penny. If soap increases the surface tension, more drops of soapy water than tap water will stick to the penny, whereas if soap decreases the surface tension, fewer drops of soapy water will stick. Students will employ the scientific method by developing a hypothesis, testing the hypothesis by collecting and analyzing data, and drawing a conclusion.

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    Materials

    • Eye droppers
    • Liquid dish detergent
    • Plastic cups for water
    • Pennies
    • Paper towels

    Add some dishwashing detergent to 2 or 3 liters of water ahead of time and mix thoroughly, so that there is enough for all the groups to use the same batch. Label the plastic cups "Tap" and "Soapy" so that students don't get them mixed up. These cups can be reused for multiple classes. Have separate eye droppers for the tap and soapy water so that no soap gets into the tap water.

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    Procedure

    Divide students into groups. Each group will place a penny on top of a paper towel on the table or lab bench, making sure that it is flat. One student will carefully drop tap water onto the surface of the penny, while the others in the group help to count the drops and record the data. It is surprising how many drops the penny will hold before the water begins to run off! They will repeat this four times with tap water, and four times with soapy water, drying the penny off between trials. All of the students should have a turn at dropping water onto the penny. They will then average the numbers for each, and compare the results.

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    Data Sheet

    Question: Record the question you are trying to answer by doing this experiment. (Does soap affect the surface tension of water?)

    Hypothesis: Develop a hypothesis that answers the experimental question. (Students' hypotheses will be that soap will increase, decrease, or have no effect on the surface tension as measured by how many drops stick to the surface of the penny.)

    Test the Hypothesis: Compare the number of drops of tap water that stick to the penny with the number of drops of soapy water that stick to the penny. Because water drops may vary in size according to how you drop the water, it is best to run several trials and take an average. Record your data below.

    Drops of Tap Water Trial 1 ___ Trial 2 ___ Trial 3 ___ Trial 4 ___ Average ____

    Drops of Soapy Water Trial 1 ___ Trial 2 ___ Trial 3 ___ Trial 4 ___ Average ____

    Analyze the Results: Look at your data and compare the difference between the average number of drops of tap water and soapy water that stuck to the penny. (Which was more, on average.)

    Draw Conclusions: Write a paragraph that explains how soap affects the surface tension of water using your data to help you answer your question. Does your data support your hypothesis or not? (Answers will depend on results and hypothesis.)

    Further questions:

    Why were so many trials taken and averaged? (Variations in size of drops)

    Suggest a reason for the results you observed. (Soap interfered with the way water molecules stick together.)

    If the experimental question was, "How does salt affect the surface tension of water?" describe how you would answer this question using the scientific method. (Do an experiment similar to the one we just did, but add salt to the water instead of soap, and compare the number of drops of salty water to the number of drops of tap water that stick to the penny.)