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.
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
This demonstration supports students’ understanding of
- Physical Properties
Teacher Preparation: minimal (5 minutes)
Lesson: 30 minutes
- 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
- 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.
- 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.
For the Student
- Draw a picture of what happens when food coloring is added to water:
- Draw a picture of what happens when food coloring is added to milk:
- What is the biggest difference that you see when food coloring is added to water compared to when food coloring is added to milk?
- Draw a picture of what happens when soap is added to the food coloring and milk:
- What is the purpose of soap? Why do you use it? How is this similar to adding soap to the milk and food coloring mixture?