Surfactants

http://investigative.weebly.com/blog/surfactants

Essential Question
How does surface tension work? How can surface tension be broken?

Objectives

 * Infer why the color first remains in the same area, and then spreads throughout the milk.
 * Explain the difference between polar and nonpolar materials.
 * Distinguish the difference between the milk and the solutes and suspended solids in the milk.

Introduction & Student Background


Minimal introduction is needed. After the activity, you can explain this phenomenon to the students.

The secret of the bursting colors is the chemistry of that tiny drop of soap. Dish soap, because of its bipolar characteristics (nonpolar on one end and polar on the other), weakens the chemical bonds that hold the proteins and fats in solution. The soap's polar, or  hydrophilic  (water-loving), end dissolves in water, and its  hydrophobic  (water-fearing) end attaches to a fat globule in the milk.

The molecules of fat bend, roll, twist, and contort in all directions as the soap molecules race around to join up with the fat molecules. During all of this fat molecule gymnastics, the food coloring molecules are bumped and shoved everywhere, providing an easy way to observe all the invisible activity. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops.

See here and here for more info

Vocabulary
Polar bond: a chemical bond that shares electrons unevenly. Because water is polar, this makes polar molecules hydrophilic.

Nonpolar: a chemical bond that shares electrons evenly.

Materials

 * Different varieties of milk (2%, skim, whole)
 * Soak proof paper plates or durable plates
 * Food coloring
 * Dish soap

Lesson Plan

 * 1) Pour milk into plate (make sure that there is plenty of “flow” room).
 * 2) Place one drop of each color of food coloring on top of the milk (it should be pretty stationary).
 * 3) Introduce a drop of soap into the plate (via a cotton swap, fingertip, or just drop it in there) and watch the colors begin to integrate into the milk.

How it Fits to Standard

 * Carry out investigations by using instruments, observing, recording, and drawing evidence-based conclusions.
 * Substances have unique properties based on their atomic structure.

Follow Up
What would happen if you used water instead of milk? What would be different, and why? What kind of milk created the greatest swirl?