Make Swirling Rainbows in Milk
In this science experiment, kids explore the surface tension of milk and the effects of surfactants while making amazing swirling rainbows.
Surface tension is how well the molecules in a liquid stick together on the surface of the liquid. Picture the molecules as little magnets that are trying really hard to stick together.
Soap contains "surfactants." Surfactants are used to decrease the surface tension of water; they cause the molecules to spread out.
Milk consists mostly of water and fat. When you add the food coloring, which is water with pigment but no fat, the pools of food coloring are held together in one spot by the surrounding water and fat, rather than spreading out, as they would in plain water. This has to do with the density of the liquids. (Test this by adding food coloring to water and see what happens.)
When you add soap, a surfactant, the surface tension of the milk decreases. This causes the milk molecules to spread out, and as they move, they carry the food coloring with them. Eventually the milk, the soap and the food coloring all get mixed together into a single solution that has one color.
Food coloring has a way of getting all over young scientists, so you’ll want to supervise and possibly protect the work surface and their clothing.
- Milk - 1 cup of this item per student
- Food coloring - set of different colors - 1 per class
- Plate - 1 of this item per student
- Liquid soap - 1/2 cup per class
- Q-tip or similar - 3 per student
Pour milk onto a plate or similar shallow dish. Add to a depth of at least 1/4". Let the milk settle for a moment.
Add drops of food coloring in different spots on the milk; do not stir. Use 2-3 drops per color "spot" to make a nice pool of color to spread out in the experiment.
Using a plain Q-tip, pipette or similar, touch a spot of the food coloring in the milk. What happens? Resist the temptation to stir! Now repeat but add a drop of liquid soap to your tool and drop it in the center of each pool of food coloring. Again, don't stir. Watch the amazing colors take off.
Eventually the movement of the colors will slow and stop; this happens when the milk and soap have combined to the point that soap no longer has any effect on the surface tension.
Extension or science project idea: try this with milk that has different percentages of fat (skim, 1%, 2%, whole), and/or with different types of soap.