Color Changing Milk Demo
Explore bipolar molecules - those that are simultaneously hydrophobic AND hydrophilic!
What You Need
Shallow dish
Food coloring
Milk
Cotton Swabs
What You Do
Pour enough milk in the dish to completely cover the bottom. Let the milk settle.
Add a small amount of each of the colors of food coloring. Keep the drops close together near the centre of the dish.
Next, pick up your cotton swab. Touch the tip of the swab into the centre of the milk - it’s important that you don’t stir the milk. What happens?
Now, place a drop of dish soap on the other end of the cotton swab. Dip the soapy end of the cotton swab in the middle of the dish and hold it there for a few seconds. What’s happening now?
Add more soap to the cotton swab and try again. Experiment by moving the cotton swab to different places in the dish.
What makes the food coloring in the milk move?
Explanation
Dish soap is bipolar - that is, a soap molecule has a polar end which is hydrophilic (water loving) and a non-polar end which is hydrophobic (water fearing). The hydrophilic end of the soap molecule dissolves in water while it’s hydrophobic end attaches to fat molecules in the milk.
Milk is made mostly of water, which makes it easy for the hydrophilic end of the soap molecule to dissolve, but it also contains many other things such as vitamins and minerals. Milk also contains fat. The hydrophobic end of the soap molecules want to attach to the fat molecules.
As the soap molecules and the fat molecules interact, they move around in a little soap dance. While this dance, or gymnastic routine, is happening, the food coloring is moved all around the dish, giving us an easy way to observe the activity.