Bibliography

www.coolscience.org

Conclusion

My hypothesis is accepted. After I applied the drop of soap to the milk, the food coloring spread around. Why? At first, the food coloring just sat on the surface of the milk. That's because food coloring is less dense than milk, so it floats on the surface. The milk didn't mix with the food coloring in the beginning because it wasn't stirred together.The soap reduces the surface tension of the milk by dissolving the fat molecules, which is why whole milk works better. The surface of the milk outside the soap drop has a higher surface tension, so it pulls the surface away from that spot. The food coloring moves with the surface, streaming away from the soap drop. Due to the convection that results from the moving surface, the food coloring may be drawn down into the liquid, only to appear rising again somewhere else. When the soap finally gets evenly mixed into the milk, the action stops. If another drop of soap is added, the process starts all over again. This won't work in water because water doesn't contain any fat.

Independent variable: Milk and water
Dependent variable: Whether the food coloring spreads or not

Observations

When I squeezed a drop out of each food coloring bottle, the food coloring stayed in place and didn't spread. After I added the drop of soap, the food coloring spread through the surface of the milk. The food coloring moved away from the drop of soap, but it slowed down later on and settled down again. It made a swirly pattern after the experiment went on for a while. Then the food coloring stopped moving after a short time. I added another drop of soap after it stopped, and it started spreading again.

Procedure

1. Pour a layer of milk in the dish about 1/2" (1 cm) deep. Room temperature milk works better than cold milk, if you have the patience to let it sit for a while.
2. Carefully put one drop of each of the four food colors onto the surface of the milk, widely separated, and not in the center of the dish.
3. Get ready to watch what happens! Very carefully drop one drop of dish soap onto the surface of the milk in the center of the dish. (Be careful not to add the soap directly on top of the food coloring. You may want to dip a toothpick in the dish soap so that a small drop of soap remains on the end of the toothpick, then touch the drop to the surface of the milk.)
4. You can try the same thing with water and see how it won't work.

Materials

1. Dishwasher or soap
2. Milk (Whole milk works best)
3. Dish
4. Food coloring
5. You can include cotton swabs

Hypothesis

If I put a drop of dishwasher or soap into a bowl of milk and food coloring, then the soap will reduce the surface tension of the milk and cause the milk and the food coloring to stream away from the drop.

Question

Does soap affect the surface tension of milk?