Wet Pennies
Students conduct a simple test to determine how many drops of each of three liquids—water, rubbing alcohol, vegetable oil—can be placed on a penny before spilling over. Because of their different surface tensions, more water can be piled on top of a penny than either of the other two liquids. However, the main point of the activity is for students to come up with an explanation for their observations about the different amounts of liquids a penny can hold. To do this, they create hypotheses that explain their observations, and because middle school students are not likely to have prior knowledge of the property of surface tension, their hypotheses are not likely to include this idea. Then they are asked to come up with ways to test their hypotheses, although they do not need to actually conduct these tests as part of this activity.
The same phenomenon that lets a water strider walk on water also allows students to place an astonishing number of drops of water on a penny.Copyright Robert Suter, Vassar College http://faculty.vassar.edu/suter/1websites/bejohns/mateselection/files/female.htm
Chemical engineers apply their understanding of natural scientific properties, such as surface tension, as they design experiments to create new materials and products that behave as they desire.
After this activity, students should be able to:
- Give an example of a hypothesis that is based on an observation of a natural phenomenon.
- Give an example of an experiment designed to address a specific hypothesis.