Introduction: This is a fun and simple investigation into solubility and polarity that can be done at home or in a classroom. We’ve also included two explanations for how this works (for younger and older students) and modifications for more advanced students to use the scientific method.
- Rectangular kitchen container (like Tupperware)
- 2% Milk
- Food Coloring (water-based, not gel-based)
- Eye droppers or Q-tips
- Pour a thin layer of milk in a rectangular container.
- Add a few drops of food coloring about 2 inches from the container’s edge.
- Pick up soap with the eye-dropper.
- Add a drop of soap in the milk about 2 cm behind the food coloring, and watch the food coloring move away from the soap.
Why does this work?
Milk contains fat and water. The food coloring is mostly made of water. Because fat and water are very different from each other, they don’t mix well; therefore, the food coloring will not move through the milk by itself. Soap is a special type of chemical (called amphipathic) that can mix with both fat and water. When the soap is added to the milk, the soap surrounds the fat and hides it from the water and the food coloring. Then, the food coloring easily mixes with the water.
For more advanced students, the concepts of hydrophobic and hydrophilic molecules can be introduced. The fat in milk is hydrophobic (water fearing) and the water is hydrophilic (water loving). Generally, hydrophobic and hydrophilic molecules do not mix well, but the milk contains other molecules (sugars and proteins) that allow the milk fat to mix with the water. This is an emulsion. Soap is amphipathic; it has both a hydrophobic part and a hydrophilic part. The hydrophobic part of the soap will mix with the milk fat, and the hydrophilic part of the soap will mix with water. In effect, the soap surrounds and hides the milk fat from the watery parts of the milk. With the fat surrounded by soap, the food coloring, which is also hydrophilic, is able to mix with the milk.
This activity can be modified to test different questions. Have students build hypotheses based on the questions you ask them For example:
- Will the type of milk affects the results?
- Whole milk has more fat than 2% milk and would require more soap to mix the food coloring with the milk.
- Fat-free or skim milk has little fat and would easily mix with the food coloring.
- Will different soaps, detergents, or cleaners do a better job at mixing the milk and food coloring?
- Dawn dish soap works well, but other soaps may work better or worse depending on what ingredients they include.
- Can other liquids besides soaps help the food coloring mix with the milk?
- Oil is hydrophobic and would probably make it more difficult to mix the food coloring and the milk.
- Lemon juice is acidic and might cause the milk fat to curdle.
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