Drops of food coloring are placed in milk, then detergent is added to illustrate the dramatic interactions between molecules at the surface of a liquid. This activity can be used to arouse interest in investigating a common substance such as milk. Following this activity, students in grades K-12 are usually able to define a variety of problems to investigate.
The only substance which might be considered somewhat hazardous is the detergent. Very young students may attempt to drink the colored milk. If there is enough detergent in the milk, it may cause them to have diarrhea. Students do not need access to large quantities of the detergent, only one drop per group is sufficient. Using a toothpick to transfer the detergent to the milk should prevent any dangerous amounts of detergent from getting into the milk.
The food colorings may temporarily cause staining of skin or clothing since it is full strength. Normal washings usually remove the stains.
This is a short "hands on" activity that can easily be extended within the class period or or it can be used as a home study investigation.
Set up, observations, and cleanup can be completed within 20 minutes.
To observe the unusual behavior of milk as it is acted upon by a common household detergent.
Students should pour the milk into a small aluminum pie pan or a petri dish to a depth of about one cm. The milk should have time to "sit" for a minute so that there aren't any currents from the pouring process. A couple of drops of the four different water soluble food colorings should then be added to the milk near the edge of the container at 0, 90, 180, and 270 degrees.
Observations may begin when a toothpick dipped in detergent is touched to the surface of the milk in the center of the pie pan. Students should be encouraged to "play" with the system as they wish in order to enhance their observations.
Students may be asked to suggest hypotheses which might explain the phenomena they observe. How is it possible that the fairly quiet pan of milk is now exhibiting such activity? As the hypotheses are forwarded, they can be recorded on the blackboard or overhead. This process usually results in an interesting variety of possible explanations for the unusual activity. <h4.Variations
It is most important that students get to use their current beliefs about milk, or aqueous mixtures in general, to think of plausible explanations for the observed behavior. This process leads naturally to the development of controlled experiments to test their favorite hypothesis. The phenomenon appears to be related to the detergent action on the surface tension of the milk. Exactly what causes the extent of the activity within the milk hasn't (as far as this author knows) been proven conclusively. Students usually enjoy experimenting by: (1) varying the fat content of the milk, (2) varying the age of the milk (time from homogenization), and (3) using various dilutions of the milk (skim, 2%, whole).
Milk is a more complex substance than a simple solution. It contains not only a variety of salts and sugars dissolved in water, but also small globules of fatty substances (cream) and protein which vary in diameter. The more recent the homogenizing process, the smaller the emulsified fat globules. The fat globules, being hydrophobic, cannot dissolve in the water. They can however dissolve into each other. A class discussion about the composition of milk should probably be carried out prior to the the activity.
When teaching more advanced students about the hydrophilic and hydrophobic components of milk, the general structure of a detergent molecule could also be demonstrated. In particular, the structural description could highlight the hydrophilic and hydrophobic ends. The structural formula of detergent is shown below:
A description of the average composition of bovine milk is given below:
Substance Avg. % composition water 87% total solids 13% a) proteins (mostly casein) 3.0 - 4.0% b) lipids (mostly triglycerides) 3.5 - 5.0% c) sugars (primarily lactose) 4.5 - 5.0%
When finished, simply pour the used milk down the drain and wash the pans or petri dishes with a little detergent. Rinse the milk down the drain with plenty of water to prevent a sour smell in days to come.
Reference: Submitted by John C. Hugo
White, A., P. Handler, and E. Smith, 1964. Principles of Biochemistry, Third Edition, New York: Mc Graw -Hill Book Company.