Get Your Charge Away from Me!
Various colored water drops falling through oil by density differences.Copyright 2010 Samuel DuPont and Ryan Cates. STARS – University of South Florida
This activity is an easy way to demonstrate the fundamental properties of polar and non-polar molecules (such as water and oil), how they interact, and the affect surfactants (such as soap) have on their interactions. Students see the behavior of oil and water when placed together, and the importance soap (a surfactant) plays in the mixing of oil and water—which is why soap is used every day to clean greasy objects, such as hands and dishes. This activity can easily be scaled to meet any desired level of difficulty.
The properties of and interactions between polar and non-polar molecules have long been studied and applied in engineering science. Chemical engineers must understand the properties of oil and water and the role that surfactants play in how they act together. The ability to mix such molecularly different substances is extremely important. For instance, engineers often use surfactants to remove oily waste from polluted environments, such as waste dumps and oil spills.
After this activity, students should be able to:
- Explain that oil and water differ in polarity and because of this difference they do not mix favorably, allowing water to form a phase-separated sphere in oil.
- Explain that when soap, a type of surfactant, is added to the oil-water system, the surface tension between them is reduced and mixing occurs.