Introduction: Surface tension refers to water’s ability to “stick to itself”, due to the forces of cohesion.  Surface tension can be measured and observed by dropping water (drop by drop) onto a penny. The number of water drops that can fit on a penny will surprise you.

Initial Observation:

  • Observe surface tension by seeing how many drops of water can fit on a penny.

Number of Drops ___________

2. Develop a hypothesis that answers the experimental question stated above, “How is surface tension of water affected by soap?” Write your hypothesis below, using the proper “If…then…” format.

3. Test your hypothesis by comparing the number of drops of tap water that can fit on a penny to the number of drops of soapy water that can fit on a penny. Because water drops may vary depending on how well you drop the water, it is best to run many trials and take an average. Record your data in the table below

Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Average
Tap Water
Soapy Water

4. Analyze the data and organize your results in a graph.

5.  Conclusion: Write a paragraph below (using complete sentences) that explains how soap affects the surface tension of water, using your data to help you answer the question. Suggest a reason for your observations (Why did it happen?)  Hint:  read in textbook about water’s cohesive properties. Include a brief description of your findings.  Be sure to re-state the hypothesis in your conclusions and tell if it was supported or not supported by this test.  Explain sources of error that could have affected your results and changes you could make in future experiments.

Post- Lab Analysis:

Answer the following questions in your lab notebook, after you completed the conclusions.

5. Explain what surface tension is.

6. Why were many trials taken and then the results averaged?

7. In this experiment, what was your control group?

8. Identify the independent variable in the experiment.

9. Identify the dependent variable in the experiment.

10. What if the experimental question was “How does sugar affect the surface tension of water?” Describe how you would answer this question using the scientific method.



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