There are two experiments in this video. The first is Tie-dye Milk and the second is Runaway Pepper. Although they are two separate experiments they demonstrate the same scientific principle of surface tension.
This is because the washing up liquid broke the surface tension and the food colouring is now able to move freely
The movement that you observe is due to a couple of factors: The ability of soap to separate out water and fat by a chemical reaction. Milk is mainly made up of water and fat. One part of the soap molecule grabs on to a water molecule and one end grabs on to a fat molecule. This separates the water and fat in the milk and creates a turbulent effect. This partly explains why the milk starts rumbling and swirling around as soon as the soap is added. The second factor is the strong surface tension of water. Water molecules are strongly attracted to each other. This tension creates an invisible film or skin on the surface. The added soap breaks the film, or surface tension, of the water and food colouring moves along with the disturbed water, creating beautiful patterns. We see the benefits of soap when we wash dishes. The soap allows us to cut through the grease, which is fat on our pots and pans. The same principle is at work here.
Access to worksheets and extension activities are available in the teacher resource pack to Kitchen Chemistry.
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