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.
Methodology
Materials
- Milk
- Bowl
- Washing-up Liquid
- Small amount of water in a bowl/cup
- Food Colouring in a variety of colours
- Cotton Buds
Method
- Place the milk in the bowl. No need to fill the bowl to the top, you just need enough so that the food colouring can move around in it.
- Add a few drops of each of the food colourings.
- To make the washing-up liquid solution: Mix a few drops of washing up liquid with the small amount of water and mix.
- Using a cotton bud, dip one end into the washing up liquid solution.
- Slowly dip this end of the cotton bud into the bowl of milk and food colouring and watch the food colouring move across the bowl.
This is because the washing up liquid broke the surface tension and the food colouring is now able to move freely
Precautions
- Do not let students drink the mixture or any of the ingredients.
Teacher Note
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.
Extension Activities and Worksheets
Access to worksheets and extension activities are available in the teacher resource pack to Kitchen Chemistry.
Kitchen Chemistry – Teacher Resource Pack
