What You’ll Need
- Whole milk
- A shallow dish
- Food coloring
- Liquid dish soap
What You'll Do
- Pour milk into the dish, about one-half inch deep.
- Let the milk warm up to room temperature.
- Place drops of different food coloring in the milk. Do not stir.
- Place one to three drops of dish soap in the milk.
- Watch the colors swirl.
How It Works
Did you see the colors move as the soap spread across the surface of the milk? You probably also noticed that once soap covered the entire surface, the swirling stopped. In whole milk, fat is the secret ingredient that keeps the colors moving. As the soap molecules spread out, they stick to the tiny globules of fat. As the globules absorb the soap, they make room for more soap to spread out. This pushes and shoves the food-coloring particles all around. If you had used water instead of whole milk, the swirling of the colors would have started and stopped in an instant because water has no fat!
Extend the Fun
Younger kids: Kids can be like real scientists when they document what they see. And what better way to do that than with a picture? Offer white paper and crayons or markers so kids can draw what they observed. If you have time, have kids draw one picture of the milk and food coloring before the soap is added and one picture after. Ask them questions about what they saw: What happened when we added soap to the milk? What colors do you see in the milk? What happens when colors mix with each other? What do you think would happen if we did this again with different colors?
Older kids: Whole milk is the key to making the colors move. But what would happen if you used 2% milk or cream or almond milk instead? If you have access to other milks or creams, repeat the experiment and note the differences and similarities to whole-milk results.