Magic Milk

Author(s): Afrodita Fuentes
Discrepant Event - Teacher's Guide
SED 695B
 

Detailed Explanation of Discrepant Event

This activity shows molecular motion between molecules that have polar and nonpolar portions. Milk contains polar and nonpolar components. Soap molecules also contain polar and nonpolar regions. Therefore, the attraction and repulsion between molecules of milk and soap molecules results in rapid movements, easily seen by using food coloring.

Principles ilustrated

  • molecular interactions and motion
  • molecular polarity and nonpolarity
  • surface tension of milk

Materials for each group

  • milk (different kinds to compare is excellent)
  • 1 box of 4 food dies
  • 1 dab of liquid dishwashing soap (dawn)
  • 1 pie pan (~12 inch circumference)
  • 1 Qtips

Procedure

  1. Pour about 350 mL (12 oz)milk into the pie tin and allow to settle
  2. Drop one drop of each color of food coloring in a 3, 6, 9,12 o’clock pattern in the center of the milk
  3. Dip a clean cotton swab in the milk and do not stir for 5-10 sec.
  4. Record what happens. (this is the control)
  5. Dip a new cotton swab in liquid soap & predict what will happen with this cotton swab
  6. Place the cotton swab with soap in the middle of the food coloring and hold it for 5-10 sec.
  7. Record what was observed
  8. Try to put the cotton swab in different places in the milk and see what happens
  9. Think of and share explanations of your observations

Standards: Investigation & Experimentation

  • d. Formulate explanations by using logic and evidence.
  • i. Analyze situations and solve problems that require combining and applying concepts from more than one area of science.
  • Chemistry/Chemical bonds: 2d.Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form.
  • Chemistry/Solutions: 6b. Students know how to describe the dissolving process at the molecular level by using the concept of random molecular motion.
 

Questioning Script

Prior knowledge & experience:

  • lipids and water do not mix
  • soap cuts grease
  • "like dissolves like"
  • milk has water, sugars, proteins,and lipids
    Water - 85.5%– 89.5%
    Fat - 2.5% – 6.0%
    Proteins - 2.9% – 5.0%
    Lactose - 3.6% – 5.5%
    Minerals - 0.6% – 0.9%

Root question:

  • Simple - What would soap do to milk? or How do soap molecules interact with milk molecules?
  • More challenging - How does polarity relate to molecular motion i.e. kinetic energy?

Target response: The soap and the fats in the milk have polar and non-polar ends and with those two types of molecules come into contact with each other, ther repel each other, thus moving in different directions, making different patters (perhaps geometric patters) and carrying with them kinetic energy. The movement is easily seen with the food coloring.

Common Misconceptions: A common misconception is that liquids mix slowly unless stirred and shaken, but in this activity the repulsion of the fat molecules and the soap molecules show a very rapid movement with aid of the food coloring.

References & Links:

http://www.ncsu.edu/kenanfellows/2002/pligon/pharm/labs/kaleid.html - Kaleidoscope of milk showing molecular polarity

http://www.afns.ualberta.ca/Courses/Nufs403/PDFs/chapter2.pdf - all about milk

http://www.sandia.gov/ciim/ASK/documents/fsn-homeactivities.pdf

 


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