Milk and soap painting

Make a swirl of colour patterns, formed as the mixture of molecules in milk are reorganized by soap.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Atoms, Molecules (3-7)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
  • large plate, any colour
  • full fat milk, half cup or a little more, to reach the edges of the plate
  • food dye colours in dropper bottles
  • Q-tip
  • dish soap

Pour the half cup of milk onto the plate, or until it fills the plate.
Add drips of food dye into the centre of the milk.
Dip the Q-tip in the dish soap, to soak it up.

Dip the Q-tip into the food dye colours. The colours will scoot away from the soap.
If you hold it in place, the dish soap will keep coming out of the Q-tip and the colours will spread further and further.
Try touching the milk in several places.

What's happening?
The fat in the milk is in tiny droplets, separate from the watery part of the milk. (Milk is a type of colloid called an emulsion. The fat droplets can be seen under a microscope.)
The food dye stays in the watery part of the milk, separate from the fat.
The dish soap has molecules with two parts - one part likes to touch fat molecules, and the other part likes to touch water. As the soap spreads out and attaches its fat-loving part to the milk fat, the food dye is pushed aside. Each time dish soap is touched to the milk, the watery part of the milk and the food dye is pushed around.


Try with low and non-fat milk to see the difference.