There are many cereal brands out there that pride themselves on the fun that kids can have dying their milk different colours. While typically we are encouraged not to “play with your food”, the rainbow milk activity demonstrates how food can actually be a gateway to learning.
Science connection:
The following activity will teach you about surface tension. Surface tension is caused by the attraction of the particles on the surface layer of a liquid that then results in a minimized surface area.
What you will need:
- A clear cereal bowl
- Whole milk
- Food colouring
- Liquid dish soap
- A toothpick or cotton swab
What you will do:
- Pour your whole milk into the bowl (make sure it isn’t filled completely to the brim).
- Let your milk sit for a few minutes to get it to reach room temperature.
- Put about 5 or 6 drops of food colouring onto the surface of the milk (don’t stir the milk at all) and make sure the drops are spaced out.
- What do you notice about the food colouring? Does the food colouring stay relatively in once spot? Does it mix in with the milk?
Next, take your cotton swab and dip it in your dish soap. Next, gently place the cotton swab (the side with the dish soap on it) onto the surface of the milk, avoiding direct contact with any drops of food colouring.
What did you notice about the food colouring after you adding your dish soap?
Initially the drops of food colouring remained intact and on the surface of the milk, because the food colouring is less dense than the milk with the help of surface tension. However, dish soap attacks the fat molecules found in milk and weakens the surface tension and attraction of the particles. The food colouring drops move along with the milk’s surface and therefore away from the spot where you made contact with dish soap. The food colouring will also begin to spread out and then will eventually settle. In order to make the milk and colours move again, repeat the process by putting dish soap on your swab and pick a new spot in your bowl to place it on.
You can try this activity with different types of milk (skim milk, cream, 1%, etc.) and see how the amount of fat in the milk affects the experiment.
This experiment is also a cool way of seeing how different colours can be made by mixing colours together!
