An exploration into the cohesive forces of molecules
Grade Level:
K-12
Subjects:
Physical Science:
properties of matter
surface tension of liquids
Tools & Materials
Shallow bowl, tupperware or pie tin
Milk (any variety or fat content will work)
Food Coloring (a variety of colors is best)
Dish soap
Cotton swabs or toothpicks
To Do & Notice
1. Question: How does soap affect the surface tension of milk?
2. Make a Claim:
What do you think will happen if a drop of soap touches the surface of milk with drops of food coloring?
3. Test Ideas:
Fill the bowl with about half an inch of milk. (Note: this experiment works best when the milk is at room temperature)
Carefully add one drop of each color of the food coloring to the surface of the milk by holding the food coloring close to the surface before gently squeezing. Be sure to evenly space the drops of food coloring around the edges of the bowl.
Dip your cotton swab (or toothpick) into a small bowl of dish soap.
Take the soapy cotton swab (or toothpick) and touch the center of the milk with it.
Watch what happens to the milk and food coloring.
Wait for 30 seconds, then try adding a second drop of dish soap.
Option: Try the experiment again with a different variety or fat content of milk. How does it differ? How does the fat content impact the results?
4. Analyze & Interpret Data:
What happened to the food coloring when you first put it on top of the milk? Did it sink or float on top? Did it stay in one place or mix in with the milk?
What happened when you added the dish soap? What direction did the food coloring move? Did the food coloring stop moving or did it continue to move even after 30 seconds? What caused this reaction to happen? What happened when you added the second drop of dish soap?
5. Communicate Findings:
Share your results with SWEP by sending us photos or videos of your experiment. Post your photos on social media and tag us @sweptahoe on Instagram and/or to @swep4 on Facebook. Be sure to hashtag and follow #SWEPsnippets. If you cannot post directly yourself, send your photos or video to SWEP (Jenna@4swep.org).
What’s Going On
Density Differences
You probably first noticed that the drops of food coloring just sat on the surface of the milk where you placed them. That's because food coloring is less dense than milk, so it floats on the surface, and the colors do not mix because you didn't stir the milk.
Surface Tension
Milk is mostly made up of water, but it also contains vitamins, proteins, minerals, and fat. This liquid is stretched along the surface and the molecules cling onto each other, creating a strong bond along the surface of the milk. This surface layer creates a barrier between the air and the liquid known as surface tension. The dish soap wants to bond to the fat in the milk. The dish soap molecules race around trying to connect to the fat molecules in the milk. This continues until all of the soap and fat molecules become bonded and the mixture is evenly mixed. The more fat molecules that are in the milk (whole milk and half & half) the more action you will see because the dish soap will be actively moving and bonding.
What makes the Tie Dye?
The small drop of dish soap breaks the surface tension of the milk by dissolving the fat molecules, and causes the milk to move, bend, twist, and roll in the bowl. The surface of the milk outside the soap drop has a higher surface tension, so it pulls the surface away from that spot. The food coloring moves with the surface, streaming away from the soap drop. Due to the convection that results from the moving surface, the food coloring may be drawn down into the liquid, only to appear rising again somewhere else. That's why it's best to use a clear bowl so you can see what's happening. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops. Addition of another drop of soap may start the process again.
Resources
Here is a video of this Tie Dye Milk experiment from a student's perspective explanation of how & why it works: https://www.youtube.com/watch?v=5BTi1w-n70c
Going Further
More Surface Tension Experiments:
How many paper clips can you float in a cup of water? https://www.youtube.com/watch?v=_bsHMVuPSGs
How many paper clips can you add to the bottom of a cup of water before the water overflows? https://www.youtube.com/watch?v=7DZP92X_n2I
Thanks to our partners at Excellence in Education for supporting STEAM based experiments.
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