Amazing detergent

Follow these instructions to do an activity with colourful milk and learn about surface tension.

You will need

  • saucer
  • milk
  • food colouring in four colours
  • detergent.

What to do

  1. Fill the saucer with milk.
  2. Add one drop of each food colour to the milk around the edge of the saucer.
  3. Add one drop of detergent into the centre of the saucer.

What's happening

The colours swirl and zoom around the saucer.

Milk stays together as one liquid because of surface tension. This acts like a skin and keeps the milk in a puddle. When you add the detergent, it breaks the surface tension of the milk in one spot.

The pull of the surface tension from the milk at the edge of the saucer causes the milk in the centre to move to the outside, taking the colours along with it.

The colours keep moving until the detergent stops affecting the milk.


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