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Moooving Milk - A Science Experiment

An interesting and fun science experiment that allows you to see that milk is more complex than it would first appear.  This experiment will also explain how soap works.  I used this with a study on nutrition with my Girl Scout troop.

1/4 cup whole milk
1 teaspoon liquid dish washing detergent
4 wooden toothpicks
1 small container that has been thoroughly washed and dried
4 different colors of liquid food coloring*
 (optional for Step 6: 1/4 cup each of skim milk, half-and-half, and cream)


1. Pour about 1/4 cup of whole milk into the container.  This should be about inch deep.
2. Place a drop of each of 4 different food colors on the milk in the container, in opposite corners from one another.
3. Dip a toothpick into liquid dish washing detergent, then touch the toothpick into the middle of the dish. What happens?
4. Try again with more detergent, touching the milk in different areas. You can record your observations on a piece of paper, in a science journal, or in a science lab notebook.
5. Think about what you saw. Can you explain why the milk did what it did?
6. (Optional) Repeat the activity by using the same steps with different kinds of milk. Try using 1/4 cup of skim milk, then try 1/4 cup of half-and-half or cream. Write a record of what you try and how it changes.

What happened and why?   The milk you are working with is more complicated it looks. It is made up mostly of water and then equal parts of protein, fats, and sugar. The fat has been broken up and spread throughout the milk (by the process called HOMOGENIZATION) into tiny pieces of fat called globules.  When the milk was first placed into the carton, it was still and did not move. Even when the food coloring was added to the milk, things were still quiet. This is because the fat globules were steady and undisturbed. When the soap hits the milk, things begin to move.

The soap breaks up the fat globules and lets them spread across the surface of the milk. As the globules break and expand, they create movement in the milk. Normally you would never notice this, but the food coloring shows how the surface of the milk moves and changes in response to the soap breaking up the fat in the whole milk
This flowing shows how soap works. It is the same kind of thing that happens when you wash dishes or hair with soap. The soap breaks up the fat or grease and lets it to flow in the water and down the drain.  (Sounds gross right?!)

If you did step six, how were the reactions different when you used different kinds of milk?

*Be careful as some food coloring can stain.

Something to think about: Milk looks ordinary, but have you ever seen it under a microscope? If you have a change to view milk under a microscope you will see that milk is made of several different parts. Have you wondered what HOMOGENIZED means on the side of the milk container?  Or what UNHOMOGENIZED means?


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