Liquids and Surface Tension – Colorful Milk!

homeschool science milk surface tension lab

Kids love nothing more than getting up close and personal with science, and this homeschool science lab, with bright, fun colors, is no exception. This is the kind of lab that, potential messes aside, you can just let the kids experiment and play with. Lab coats or old clothes are recommended because I’ve yet to run a lab that involves food color without at least one kid getting more than their fair share of technicolor on hands and clothes. And, the best part is that with this homeschool science lab, you probably have everything you need right at home!

Materials:

Plastic tablecloth – cover your work surface to keep it from getting stained with food color
Aluminum pie plates or other shallow plates
Whole milk – enough to fill each tin with about 1/2-inch of milk
Liquid dish soap in small containers
Food color – liquid, not gel color
1 toothpick per child
Hand lens(es)

Procedure:

  1. ASK the kids for some examples of liquids. When somebody offers “water” as an answer, ask them how much of the planet is covered in water? (About 71% of the Earth is covered in water, and 97% of that is in the oceans). Explain that water also exists in lakes, 2% of it is in icecaps and glaciers, and in the air as water vapor.
  2. If your kids understand the basics of molecules, ask them how molecules behave when in a liquid state. (They’re packed close together but still have thermal energy so they’re slipping and sliding around each other which creates the liquid state)
  3. Pour some water into a cup. ASK the kids to closely observe the water where it touches the side of the cup with a hand lens. Ask them to make some observations. (They should see that the water is slightly higher where it meets the cup) Why is this? Explain that the surface tension of the water is actually pulling the water toward the glass away from the center of the surface.
  4. ASK who knows what tension is? (Like pulling a rope tight)
  5. HAND out pie plates with about 1 cm (1/2-inch) of whole milk in it, and some bottles of food color. Instruct the kids to put one small drop of color onto the surface of the milk, away from the center. The drops should be well spread out.
  6. Dip the end of the toothpick into the liquid soap so that it gets one drop of soap on it. Touch the soap-end of the toothpick to the milk and watch what happens.
  7. After all the oohs and aahs, ask:
  • What does the food color do when you add it to the milk?
  • What happened after you add a drop of soap?
  • After you add the soap, what did the food color do?
  • Does the reaction happen for a long time?
  • What happens if you add another drop of soap after the colors have stopped moving? (let them try)

Background:

Drops of food color sit on the surface of the milk because the food color is less dense than milk. And, if you don’t stir the colors in, they’ll just sit there in a pretty dot.

After adding a drop of soap, the magic happens. The soap lessens the milk’s surface tension by dissolving the fat molecules. This is why whole milk works better. Away from the soap, the milk’s surface tension is higher, so it pulls away from the soap (similar to what happens when you put a drop of soap into dishwater). The food color is dragged along with the milk molecules, rushing away from the soap.

Once the soap molecules are dispersed throughout the milk, the action will slow down and stop.

Extensions:

Try this experiment in a glass/clear dish and see what happens.

Add more or less soap.

Try different kinds of soap, or different kinds of milk or cream (half & half, heavy, whipping cream, almond milk).

 

homeschool science milk surface tension labhomeschool science milk surface tension lab

 

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