Soap vs. Shampoo Surfactant Lab
Surface tension results from the molecules at the surface experiencing larger attractive forces toward the liquid than towards the air.Copyright 2008 Booyabazooka, Wikimedia Commons https://commons.wikimedia.org/wiki/File:Wassermolek%C3%BCleInTr%C3%B6pfchen.svg
Students learn about the properties of solutions—such as ion interactions, surface tension and viscosity—as they make their own soap and shampoo and then compare their properties. Working as if they are chemical engineers, they explore and compare how the two surfactants behave in tap water, as well as classroom-prepared acidic water, hard water and seawater using four tests: a “shake test” (assessing the amount of bubbles produced), a surface tension test, a viscosity test, and a pH test. Then they coalesce their findings into a recommendation for how to engineer the best soap versus shampoo. The activity may be shortened by using purchased liquid soap and shampoo from which students proceed to conduct the four tests. A lab worksheet and post-quiz are provided.
Chemical engineers in the soap industry must have a clear understanding of the topics explored in this activity—including pH, viscosity, surface tension and ion interactions—in order to design surfactants for specific applications, ranging from soaps and shampoos to detergents. In this activity, students engineer soaps and shampoos and test their surfactants in different types of water. Similarly, chemical engineers systematically test their designs before making the products ready for the consumer market.
After this activity, students should be able to:
- Describe surfactant behavior in tap water, hard water, salt water and acidic water.
- Compare and contrast soap and shampoo properties.
- Describe surface tension and viscosity.