Magic Milk Experiment

Hypothesis

The hypothesis is that adding dish soap to milk will cause the colors to move around in interesting patterns.

Overview

Experience the wonder of the magic milk science experiment! By combining ordinary milk and soap, you'll witness a mesmerizing reaction that causes food coloring to dance and swirl in a spectacular display of color. You can capture this masterpiece onto a sheet of paper to make a magic milk painting. This easy and fun experiment is perfect for all ages. As you discover the science behind the reaction, you'll be captivated by the fascinating molecular interactions at work.

Scientific Terms

surface tension, negative charge, positive charge, uncharged, hydrophobic, fat molecules, protein molecules, water molecules, soap molecules, attraction reaction, repelling reaction.

Materials

whole milk
food coloring
dish soap, Dawn is recommended
a shallow dish that can hold the milk
cotton swabs
sheets of paper that are smaller than the size of the dish
paper towel

Procedure

Pour a little bit of milk onto the shallow dish so that a thin layer covers the bottom of the dish.
Add a few drops of red, blue, and yellow food coloring on the same spot in the center of the milk.
Dip a cotton swab in dish soap and then gently touch the center of the food coloring without stirring it.
You can then try holding the cotton swab with dish soap down on the plate of milk longer to see what happens.
Use the same cotton swab to touch the milk at the edge of the plate without stirring it.
To make your magic milk painting, carefully lay a sheet of paper over the colorful swirls, then quickly lift it off. Place it on a piece of paper towel to dry.

Results

When the dish soap touched the milk, the colors spread to the edge of the plate. When the cotton swab was held down longer, the color constantly swirled up from the bottom of the plate. When you touch the colors at the edge of the plate with the cotton swab, the color moved and mixed, creating amazing designs. Even though the colors start out mixed together, they soon burst apart in a color explosion!

Conclusion

How does the magic milk experiment work? Let's go through each ingredient in the experiment!

What does the milk do?

Milk is mostly made of water, protein, and fat molecules. For this experiment to work well, it's important to use whole milk, which has more fat molecules.

What does the food coloring do?

The food coloring dissolves into the milk, so it follows the movement of the molecules in the milk. When you see the colors moving, the milk is also moving along!

What does the dish soap do?

Dish soap is made of soap molecules, which have two different parts. One part has a negative charge that is attracted to the positive charges on protein molecules and water molecules and could be repelled by the negative charges on protein molecules. The other part of soap molecules is hydrophobic, meaning it's uncharged and repels water. That part is attracted to the hydrophobic uncharged fat molecules. These properties of soap make it a surfactant, which reduces the surface tension of liquids.

When you first put the dish soap on the milk, the high surface tension of milk is broken up by the soap. The negative end of the soap molecules line up with the positive end of the water molecules. This makes the soap molecules run out in all directions on the surface of the milk, pushing the food coloring to the edge of the plate. This will also happen when you do this experiment with a plate of water instead of milk.

However, the amazing thing about the magic milk experiment is that after the initial contact between the milk and dish soap, when you leave the cotton swab in the milk or bring it to the food coloring at the edge of the plate, the colors are constantly moving. The soap's uncharged parts interact with the uncharged fat molecules, while the soap's negative charge interacts with charges on protein molecules and water molecules. All these different attraction and repelling interactions between the soap and milk cause the liquid to twist and turn, which we see as a color explosion!

What does the paper do?

The paper will absorb both the food coloring and milk. After the paper dries, you'll have a magic milk painting!

Also consider

Swap out the whole milk for other liquids, such as plain water, chicken broth, soy milk or almond milk. The amount of fat and protein molecules are different in those liquids. What do you think will happen?
Instead of whole milk, use milk with different fat percentages, such as 1% skim milk or half & half, to see if the results are different. What do you think lowering or increasing the fat molecules in the milk would do to the movement of colors?
Store-bought milk is usually homogenized milk. What if you used non-homogenized milk, where the fat is in a thick layer on top of the milk, in the magic milk experiment? Would that cause a difference to the movement of colors?
Try different types of soap to see how they affect the magic milk experiment. Laundry detergent, hand soap, shampoo, powdered detergent or bar soaps are some other kinds of soaps to try.
The magic milk experiment uses very little dish soap, just enough to coat the cotton swab. What happens if you put more soap? Would there be more movement or less?

References

Learn the science behind the magic milk experiment from the American Chemical Society! https://www.acs.org/education/whatischemistry/adventures-in-chemistry/experiments/colors-move.html

Magic milk science experiment lesson plan with worksheets: https://www.chemed.org/wp-content/uploads/Milk-Rainbow.pdf

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