Grade Level: 9–12 Based on ideas by: Carlos Olivo, High School Teacher
A simple but dramatic demonstration of surface tension, molecular bonds, and the interaction between soap and fat molecules. Excellent for chemistry or introductory physical science—and students absolutely love watching the colors burst and swirl.
Objective
Students will:
Observe how soap interacts with fat molecules
Understand how surface tension and molecular bonding influence the movement of liquids
Describe the chemical reaction between soap and fat
Analyze real-time visual evidence of bond disruption
Materials
Buttermilk (whole milk can also be used, but higher fat = better results)
Shallow dish or saucer
Four colors of food coloring
Liquid dishwashing soap
Sink or washable workspace (it can get messy!)
Plan
Prepare the area: Perform this experiment in or near a sink. Clean-up is easy, but spills happen quickly.
Pour the milk: Add a small amount of buttermilk into the dish—just enough to cover the bottom.
Add the colors: Place one drop of each food coloring onto the surface of the milk.
Avoid the center of the dish
Space the colors apart so they don’t touch
Add the soap: Carefully place one drop of liquid dish soap directly into the center of the milk.
Observe: Immediately, the colors will shoot, swirl, mix, and dance across the dish.
Why It Works
Milk contains fats and water.
Food coloring remains mostly on the surface due to surface tension.
Dish soap breaks apart fat molecules by disrupting the molecular bonds.
As the soap spreads through the milk, it pulls and drags the surface layer with it, causing the dramatic swirling effect.
More fat = more dramatic movement, which is why buttermilk or whole milk works best.
Extension Ideas
Compare results using skim, 2%, and whole milk
Explore effects of different soaps
Record observations and relate them to polarity, hydrophobic/hydrophilic interactions, and surfactants
