Rainbow milk demonstrates some of the characteristics of water and soap. With a little milk, food coloring and everyday dish soap, you can produce a spectacular show of color.
Materials
Dish-washing soap (Dawn works great)
A four color box of food coloring
Aluminum pan or plastic dinner plate
Whole milk
Q-tips
What to do
1. Place enough of the milk to cover the bottom of the pan or plate.
2. In the center of the milk place a drop of each of the food colors. Try not to mix the drops of color.
3. Take a clean Q-tip and push it into the center of the drops of color for about 10 seconds and then remove it. Observe the interaction of the colors in the milk.
4. Then, dip another Q-tip in some dish-washing soap such a s Dawn.
5. Now place the soap covered Q-tip into the center of the milk for about 10 seconds and then remove it. Observe what happens in the milk. Repeat the experiment using just water, fat free or 1% milk.
Why is that?
Cow’s milk consists of about 87% water and 13% dry substance. The dry substance is suspended or dissolved in the water. These substances consist of fat, protein, lactose (milk sugar) and minerals (salts). Milk also contains trace amounts of other substances such as pigments, enzymes, vitamins, and phospholipids, (substances with fat like properties).
The food coloring is water based and will travel and diffuse better through an aqueus (water) medium than through fat. Whole milk has a larger proportion of fat, so the food coloring does not move much.
Soap detergent is bipolar, meaning that one end of the molecule is attracted to the water and the other end is attracted to the fat in the milk. This change in the water/fat interaction allows for the rapid diffusion of the food coloring.
Water also has surface tension which is the tendancy for water molecules to stick together. Initially this property keeps the drops of food coloring together. The soap breaks this surface tension by breaking the bonds between water molecules and the colors begin to fly everywhere.