All Resources

Swirling Milk

In general, fat and water do not mix; liquids like these that do not form a homogeneous (uniform) mixture are called immiscible.

When food colouring is placed on the surface of milk, the drop remains intact with little spreading. The water-based food colouring does not mix with milk easily, because milk is a suspension of fat molecules in water and food colouring is a water-like dye.

Once the soap is added to the milk, it spreads over the surface and causes the food colouring to move quickly thorough the milk and out to the edges of the plate. Since milk is mostly water, it has a surface tension like water. Liquid soap wrecks the surface tension by breaking the bonds between water molecules.

After the initial zing of the food colouring, swirling of colours ensue due to the chemical structure of soap. Soap molecules have both properties of non-polar and polar at opposite ends of their molecules.

The fat in milk is non-polar. The non-polar hydrocarbon tail of the soap dissolves into the fat. The fat-like end of the soap molecules join with the fat in the milk and as the soap spreads across the surface of the milk, more and more fat molecules are pulled by the spreading soap. When the fat molecules congregate, the polar water in the milk is pushed away, taking the food colouring with it. The movement of the milk decreases as the soap becomes “used up", or attached to all the molecules for which it has room.

Objectives

  • Appreciate how soap allows fats or oils to mix with water based solutions.

  • Appreciate the liquids have surface tension and that soap affects it.

Materials

  • Per Student or Group:
    Homogenized milk (at room temperature)
    Aluminum pie plate
    Food colouring
    Dish detergent/liquid soap
    Cotton swabs or small droppers

Key Questions

  • What happens right away?
  • What happens after 1 minute?

What To Do

  1. Pour room-temperature milk in to the pie plate, completely covering the bottom. Allow it to settle.
  2. Add up to 5 drops of food colouring to the milk (any combination of colours is okay). Keep the drops close together in the centre of the plate.
  3. Dip the cotton end of the swab in to the soap. Place the soapy end of the swab in the middle of the milk and hold it there for 2 seconds (or use the dropper to introduce a drop of soap into the middle of the milk).
  4. Watch the reaction closely. Notice what happens immediately and what happens over time.
  5. Continue to experiment by adding another drop of soap.

Extensions

  • What happens if you use a different kind of milk? Try using skim, 1%, 2% and cream. Predict if each will react the same as the whole milk and then test to see if you’re right!

Other Resources

Science World Resources | Tourbillon de lait | French version of this resource