Summary

In this demonstration, students will observe as the teacher creates several colorful mixtures. Using food coloring, water and milk, students will watch the colors swirl and mix. Students will have the opportunity to make a prediction about what will happen to the colors when a sample of soap is introduced to the mixture.

Grade Level

Elementary School

Objectives

By the end of this demonstration, students should be able to

• Apply a scientific inquiry process and perform an experiment
• Understand the property of surface tension
• Describe the effect of soap, a surfactant on surface tension

Chemistry Topics

This demonstration supports students’ understanding of

• Mixtures
• Solutions
• Molecules
• Polarity
• Physical Properties
• Observations

Time

Teacher Preparation: minimal (5 minutes)

Lesson: 30 minutes

Materials

• ½ cup whole milk and or low fat milk
• Clear plate or shallow bowl
• 4 different colors food coloring
• Liquid dish soap (dawn works well)
• Cotton swabs
• Little Blue and Little Yellow by Leo Lionni

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• This demonstration could be done near St. Patrick’s Day and the colors of food coloring could be put in a row and then put the dish soap just below the center and make a rainbow effect.
• The teacher will read a book like Little Blue and Little Yellow or any book about color mixing .
• First the teacher may want to have a few glasses of water available and add food coloring to the glasses to demonstrate food coloring to students. Also, you could mix some colors to see what happens when you mix certain colors.
• For the demonstration, the teacher should pour milk in a shallow bowl and then add several drops of food coloring close together, but separated, in the center of the milk.
• Dip a cotton swab in the liquid dish soap and touch the tip to the milk’s surface near the drops of food coloring .
• Observe the interaction and try touching in different places.
• Steps for Lab:
  • Pour milk into a clear shallow dish.
  • Add a few drops of four different food coloring into middle of dish.
  • Soak a cotton swab in dawn dishwashing liquid and then touch cotton swab to food coloring and milk mixture.
• Background information: Milk is composed of water, vitamins, minerals, proteins and fat. Soap allows liquids with different properties to mix with one another. Food coloring doesn’t naturally mix with materials that have fats/oils, this is why to food coloring drops don’t spread out quickly in milk, like they would in water. The soap has one end that likes to mix with fatty molecules (typically considered non-polar, and the other end likes to mix with polar molecules—like water!)
• When the soap is added it wants to mix with the different types of molecules in the milk, so it will swirl, while helping the milk and food coloring to mix as well.
• When the soap becomes spreads out through the milk you will observe it slow down and then stop. This is why milk with a higher fat content produces better mixing of color.
• As a comparison, you may want to do the demonstration with skim milk and see if the reaction is the same.