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This post is part of the 31 days of STEM activities for kids series. Go here to see the other posts in the series!
Surface tension fascinates me more than my children, so this experiment was mostly for me. I learned right along with my girls this time around!
Milk Surface Tension Experiment
When milk is homogenized, fat particles are equally spread throughout the milk. Because of the fat, food coloring is lighter than milk, which causes it to stay in one place. The surface tension of the milk keeps the food coloring in one place. Surface tension is caused by the force of each of the molecules in the milk pushing against each other. When the dish soap is added, it binds with the fat molecules and reduces the surface tension of the milk. This is why the food coloring seems to “dance” after it is touched with dish soap. In fact, dish soap is called a surfactant in the scientific world.
- Whole fat milk (more dramatic results, also, more delicious)
- Food coloring (a rainbow of colors if you have it. We didn’t.)
- Dish soap
- Cotton swab
Fill a shallow container with milk. Place a few drops of food coloring in different areas of the milk. Don’t shake the container or the results will be less dramatic.*****************
Dip a cotton swab into the dish soap and gently touch the food coloring. It will “dance” through the milk, leaving colorful swirls behind.
Monkey and Bo loved this experiment. Bo preferred to use her fingers to move the food coloring around. The results would have looked cooler if we had additional colors, but we only had the green so we made do. It was rather amazing how quickly the food coloring moved after the soap touched it. Monkey used the dish soap to guide the milk in specific patterns.
Both girls wanted to drink the milk afterward, but I told them they did not want to drink dish soap-flavored water. Yuck.
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Have you done this project before? Share your experiences with us in the comments!
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