Kitchen Chemistry Experiment 24
The Cat's Meow

This activity is used to arouse interest in a common substance, milk. Students are asked to form a hypothesis about the behavior of milk as it is acted upon by household detergent.

 

Materials	Substitutions
large glass petri dish	aluminum pie pan
wooden splint	toothpick
heavy laundry detergent
milk (1 gal)
a set of food colorings

Procedure

1. Pour milk into an aluminum pie pan to a depth of 1 cm (1/2 inch).
2. Add a couple of drops of four different colored water soluble food colorings near the edge of the container. Arrange the drops so that they are in positions equilivant to 3, 6, 9, and 12 o' clock (0deg., 90deg., 180deg., & 270deg. )
3. Dip the tip of a toothpick in detergent. Touch the surface of the milk in the center of the pie pan and hold the toothpick in place for a while. What happens?
4. Experiment with the milk and toothpick. How is it possible that the fairly quiet pan of milk is now exhibiting such activity? Suggest a hypothesis that might explain the phenomena that you observe.

 

Extensions

If the milk is diluted with water, will the phenomena occur? Would this take place if lowfat milk is used?

 

Teacher's Notes

1. The most important aspect of this activity iare the observations, hypotheses, and conclusions that they students draw. Whether or not they come up with the right answer is not important. Although the phenomena appears to be related to the detergent action on the milk, it has not be proven what causes this activity to occur.
2. Milk is more a colloid than a simple solution. It contains not only salts and sugars dissolved in water, but also small globules of fatty substances and protein which vary in diameter. The fat globules, being hydrophobic , cannot dissolve in the water. They can, however, dissolve in each other.
   Average Composition of Milk
   water 87 %
   total solids 13 % proteins (casein) 3 - 4 %
   lipids (triglycerides) 3.5-5 %
   sugars (lactose) 4.5-5 %
3. Detergent has a hydrophilic and hydrophobic end to its molecular structure. It will reduce the surface tension of water.
4. The detergent tries to surround the fat in the milk but the fat is so evenly dispersed that is simply turns over and over. This causes the swirling effect that we notice.