Experiment Category:
Objective:
What You Need:
- Milk at room temperature (the higher the fat content the better)
- Food Coloring
- Liquid Soap
- Shallow Container (aluminum pie pans or soup plates work well)
- Toothpick
To Do and Observe:
1. Fill the pan with milk to about one inch, or until the generously coats the bottom of the pan.
2. Add a few drops of food coloring in the middle of the pan.
3. You can then add a few drops of the soap in the middle the pan, or use a toothpick dipped in the soap. If you use the toothpick with the soap, place the toothpick in to the milk solution.
4. Observe what happens to the food coloring/ milk solution.
What's Going On:
The turbulence you observe is due to a couple of factors:
The ability of soap to separate water and fat. Milk is mainly made up of water and fat. One end of the soap molecules grabs on to water and one end grabs on to fat. As the soap separates the fat from the water in the milk, it creates turbulence at the molecular level. This partly explains why the milk starts rumbling and swirling around as soon as the soap is added.
The strong surface tension of water. Water molecules are strongly attracted to each other. As the surface of a container filled with water, water molecules prefer to cling to each other instead of the air molecules. This tension creates an invisible film or skin on the surface. Since food coloring is less dense than milk, it stays near the surface of the milk. The added soap breaks the film, or surface tension, of the water and food coloring moves along with the disturbed water molecules, creating beautiful patterns.
We can se the benefits of soap when we wash dishes. The soap allows us to cut through the grease, which is fat on our pots and pans.
Parent/Teacher Tips:
Use a few different colors of food coloring to make more elaborate patterns.
For Younger Students
Students can add two separate colors and observe how they mix together and form another color when the soap is added. Students can make prediction as to what color the different combinations will yield.
For Older Students
Students can explore how soap reacts with other liquids and observe the whether these liquids are hydrophilic or hydrophobic. They can also add other liquids to the milk and compare the densities of the substances.