Rainbow Milk
Watch an explosion of swirling colours in milk and discover the science of surface tension!
Adult supervision recommended for younger children.
This experiment uses food colouring which may stain surfaces and clothing. Cover your work area and wear old clothes.
You Will Need
Step-by-Step Instructions
Prepare Your Workspace
Cover your table with newspaper or a plastic sheet. Food colouring can stain! Place the shallow plate in the middle.
Pour the Milk
Gently pour milk into the plate until it just covers the bottom. You do not need a lot — about half a centimetre deep is perfect. Let it settle for a moment.
Add the Colours
Add a few drops of food colouring to the milk. Put different colours in different spots — near the middle, near the edges. Do not stir! The drops should just sit on top of the milk.
Prepare Your Magic Wand
Dip the end of a cotton bud into the dish soap so it has a small amount on the tip.
Touch and Watch!
Gently touch the soapy end of the cotton bud to the surface of the milk right in the middle of a colour drop. Watch what happens! The colours should burst and swirl away from the cotton bud in a beautiful dance.
Keep Exploring
Try touching different spots. Add more soap. Add more colour drops. Try holding the cotton bud still in one place for 10 seconds. What patterns do you see?
What Happened?
When you touched the soapy cotton bud to the milk, the colours burst away and swirled in beautiful patterns. The milk seemed to come alive!
This happened because milk is not just white water — it contains tiny droplets of fat. The surface of the milk is held together by something called surface tension, like an invisible skin on top.
When the dish soap touches the milk, it breaks the surface tension and chases after the fat molecules. The soap molecules rush outward in all directions grabbing onto fat, and as they zoom around, they push the food colouring along with them. That is what creates the swirling, dancing colours!
The Science Behind It
Surface tension is a force that makes the surface of a liquid act like a thin, stretchy skin. Water molecules pull on each other at the surface, holding it together.
Milk contains fat molecules spread throughout it. These fat molecules are held in place by the surface tension of the milk.
Dish soap is designed to break apart fat — that is how it cleans your greasy dishes! When soap touches the milk, it attacks the fat molecules, breaking the surface tension and causing rapid movement.
The food colouring is just along for the ride. It shows us the invisible movement happening in the milk by creating those beautiful swirling patterns.
Think About It
What do you think would happen if you used water instead of milk? Would the colours still dance? Why or why not?
Would skim milk (low fat) work as well as full-cream milk? What does this tell you about the role of fat?
Why does dish soap help clean greasy plates? How is that related to what happened in the milk?
Some insects can walk on water because of surface tension. What would happen if you added soap to the water they were standing on?
Knowledge Check
Test what you have learned! Select the correct answer for each question.
Question 1
What makes the colours swirl in the Rainbow Milk experiment?
Question 2
What is "surface tension"?
Question 3
Why does this experiment work better with full-cream milk than water?
Question 4
What is the job of dish soap when washing dishes?
Question 5
What happens to the colours in the milk after you touch the soap to it many times?
Key Concepts Summary
- ● Surface tension is a force that makes the surface of a liquid act like a thin skin.
- ● Milk contains fat molecules that are held in place by surface tension.
- ● Dish soap breaks surface tension and grabs onto fat, causing rapid movement in the milk.
- ● The food colouring lets us see the invisible movement happening in the milk.
- ● Soap works on dishes the same way: it breaks apart grease and fat so they wash away.