# How Does a Water Strider Stay on Top of the Water?

Introduction:  The water strider can manage to slide along the surface of water while searching for other insects to eat.  Watch this video: “Why Water Striders Make Terrible Lifeguards.”

Surface tension refers to water's ability to "stick to itself".   Water is sticky, but some things can affect the surface tension, like pollutants in the water.  What do you think would happen if water had been contaminated with soap.  Would the surface tension decrease, or would it increase?  How would this change affect the animals that live in the water?

### Develop a hypothesis that answers the experimental question. Write this statement below.

Procedure:    Surface tension can be measured and observed by dropping water (drop by drop) onto a penny.

Experiment:   Carefully use a dropper to place drops of water on a penny. Count how many drops of tap water can fit on your penny.  Do four trials and take an average.

 Trial 1 Trial 2 Trial 3 Trial 4 Average Tap Water

Repeat the process using water that has soap in it.  Soap types can vary, your instructor will provide the soapy water (it may be dish soap, shampoo, or laundry soap.)

 Trial 1 Trial 2 Trial 3 Trial 4 Average Soapy Water

Organize Data - make a bar graph that compares the trials.  Be sure to label your X and Y axis.  Color is optional.

## Data Analysis

1.  Write a sentence that explains how soap affects the surface tension of water based on the results of your experiment.

2.  Identify the independent (manipulated) variable and the dependent (responding) variable in your penny experiment.

3.  What was the control in the penny experiment?  What is the purpose of this control?

4.  What other questions can you develop (and even test) about surface tension. Write at least two follow-up questions that could be tested using the penny technique.

5. Consider the experimental question and how it relates to a water strider.  What happens to water striders on lakes that have been contaminated with soapy?      Make a CLAIM to answer this question and support your claim with EVIDENCE from the experiment you just conducted.

## Related Resources

Properties of Water – Investigation exploring cohesion, adhesion, heat capacity, presented as lab station activities

Graphing Practice – given data sets, such as video games scores and shirt colors, students create line and bar graphs, activity paired with growing sponge animals while students wait on results

Is Microwaved Water Harmful to Plants – use germinating seeds that have been soaked in distilled water and microwaved water