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This lab shows how soap responds to lipids (such as the ones found in milk). It is a great way to introduce students to lipids, polar vs. nonpolar substances, and micelles which can lead to an explanation for the phospholipid bilayer of a cellular membrane.
This lab is quick and easy and the kids love it! Great for all ages, because it is easily adaptable to any grade.
Here's my lesson plan I wrote up:
Standard 1.h. Students know most macromolecules (polysaccharides, nucleic acids, proteins, lipids) in cells and organisms are synthesized from a small collection of simple precursors.
Standard 1.a. Students know cells are enclosed within semi permeable membranes that regulate their interaction with their surroundings.
Time:
Discussion about soap and lipids (10-15 minutes)
Laboratory (10-15 minutes)
Cleanup (5-10 minutes)
(Extended discussion time: 10-20 minute)
Materials:
Whole milk
Petri dishes
Toothpicks
Dawn detergent
Small beakers
Food dye (different colors)
Background knowledge:
Students should already have a general understanding of what makes up lipids, proteins, carbohydrates, and nucleic acids. A short lecture will proceed the lab. In this lecture, students will learn about how soap detergent works and the difference between nonpolar lipids and polar molecules such as water. They will also learn how soap molecules have a polar end and a nonpolar end which allow them to form micelles around oil and other fats. This will lead them to an understanding of how a cellular membrane forms.
Procedure:
1) Arrange tables so that four/five students can fit per group. Each table will have 2 petri dishes, one small beaker with Dawn detergent, and 4-5 toothpicks.
2) Fill each petri dish with milk just enough so it coats the bottom of the petri dish.
3) Have students drop one drop of different colors around the inside perimeter of the petri dish (directly on the milk).
4) When everyone has dye in their milk (be careful not to agitate the petri dish so it doesnt start mixing), allow students to dip the toothpicks in the soap and then just touch the soap into the milk, or directly into a drop of dye in the milk. They should hold it there and observe what happens.
Extended:
I ran this lab on a minimum day schedule so I did not have the students answer any questions related to it; however, it really works itself well to discussing nonpolar vs. polar substances and eventually how a phospholipid bilayer forms.
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