Cat's Meow - Experiment 21

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 household detergents act upon it.

Materials Substitutions
large glass petri dish aluminum pie pan
wooden splint toothpick
liquid dish detergent
milk
food color (4 different colors)

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, differently colored, water-soluble food coloring near the edge of the container. Arrange the drops so that they are in positions equivalent to 3, 6, 9, and 12 on a clock (0º, 90º, 180º, and 270º).
  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 observed.

Extensions
If the milk is diluted with water, will the phenomenon occur? Would this take place if low-fat milk was used?

Teacher’s Notes

  1. The most important aspects of this activity are the observations, hypotheses, and conclusions that the students draw. Whether or not they come up with the right answer is not important. Although the phenomenon appears to be related to the detergent action on the milk, it has not been proven that this is what causes the activity to occur.
  2. Milk is a colloid. 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 into 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. Detergents have a hydrophilic and hydrophobic end in their molecular structures. This structure reduces 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.

Disposal
All solutions may be poured down the sink.