This is a fun and easy (and festive!) activity that you can do with supplies that you probably already have in the house.
- Pour milk (higher fat content is better, we used whole milk) on a dish or plate to form a shallow layer in the bottom.
- Add a cookie cutter (optional) and scatter some drops of food coloring around the milk.
- Dip the end of a cotton swab in liquid dish soap and gently tap the surface of the milk. You don’t need or want to try to stir here. You can pick up the cotton swab and move it around to tap different spots on the plate.
- Observe and enjoy!
Here is what’s going on with this one. The first thing to talk about is how soap works in general. There is a concept in chemistry that “like dissolves like” and what this means is that certain types of chemicals mix well together (dissolve) and certain types of chemicals don’t mix well together. The thing that determines whether the chemicals are “like” and will mix well is related to their polarity and whether they are hydrophobic (water fearing) or hydrophilic (water loving). One way to visualize this is to think about what happens when you pour oil (hydrophobic) and water (hydrophilic) together. They form layers instead of mixing together. Entire chemicals can be considered hydrophobic or hydrophilic, but there are also some chemicals that are both and the chemicals in soap are like that. One end of the soap molecule is hydrophilic and the other end is hydrophobic. When soap gets mixed with other solutions (like messy dishes or hands!), the hydrophobic end moves around and connects with little bits of oil and grease. This hydrophobic end also starts connecting with the hydrophobic ends of the other soap molecules. When enough soap molecules group together, they form little pockets or bubbles around the oil with the hydrophobic ends on the inside of the pocket and the hydrophilic ends on the outside of the pocket. These are called micelles and they’re pretty interesting. The outside is hydrophilic so this pocket can mix with water easily and the inside is hydrophobic so it can transport oil where it might not want to go otherwise. This is how soap collects the oils and greases from our hands and dishes and rinses it away with water.
So, back to the milk. Milk is mostly water with some fats and proteins and other vitamins and minerals mixed in. Fats are hydrophobic, so when the soap is added to the milk the hydrophobic sides of the soap molecules start moving towards the fat molecules in the milk. As all of these soap molecules swirl around collecting fats and forming micelles, the food coloring and the rest of the milk gets bumped and pushed around and we can see that bumping and pushing by how the food coloring start to spread.