Pour milk into the dish.
Wait for it to settle, then add drops of different coloured food dye. Spread the coloured drops out evenly over the whole dish of milk.
Use a small spoon or medicine dropper to add a single drop of dish soap to the center of the dish.
Observe as the colours suddenly explode away from the center and roll beautifully around the edges.
Why it works...
What is milk made of? Well, mostly water. But it also contains minerals, vitamins, proteins, and droplets of fat.
Milk fat, like other oils, does not dissolve in water (it is hydrophobic). This is why we use dish soap when washing dishes. It enables us to wash away the oil, grease and fat, which normally wouldn't dissolve in water.
When we add dish soap to the milk, an interesting reaction occurs. The hydrophobic part of the molecular soap structures break up and collect the hydrophobic milk molecules. Then the hydrophilic (able to bond with water) surface of the soap structure joins up to a water molecule with the fat held inside it.
By adding dish soap to the milk, the fat molecules can now be carried by the water!
Here is where it gets really interesting for our experiment.
As the fat and soap molecules race around to connect with each other, the food dye molecules are being bumped, shoved and rolled around all over the place. Because we added food dye, we are suddenly able to witness this sudden burst of movement within the milk.
After your first reaction settles down, you can find out if there are any fat molecules left in the milk by adding another drop of dish soap. If more movement occurs, then you will know there was still fat left.
Try varying this experiment by using a few different liquids - low fat milk, water, etc. Observe the results in each type of liquid. Liquids containing the most fat will have the strongest reaction to the addition of dish soap.
This experiment is just so quick and easy to set up, and it has such an instant, beautiful reaction. Try it out with your kids, and let us know how it went in the comments below!