The exploding colors of milk (Meenakshi Gangopadhyay)

Title: Exploding colors of milk

Principle(s) Investigated: Surface Tension

Standards : Next Generation Science Standards : HS-PS1-3 Structure and Properties of Matter

Materials:

Milk whole and 2%;

Plate or some other type of shallow dish

Red, yellow, green and blue food coloring

Cotton swab

Liquid dish soap or liquid laundry detergent

Paper towels

All ingredients are commonly available in the kitchen or at the grocery store.

Procedure:

Slowly pour enough milk into the plate so that the bottom is completely covered. Allow the milk to settle for a moment. Add one drop of each color of food coloring to the milk, forming a horizontal line of drops near the bottom of the plate. To make a simple, four-color rainbow, add the colors in the following order, going from left to right: red, yellow, green then blue.Take one end of the cotton swab and rub some liquid dish soap on it. Touch the soap-covered end to area right below the middle of the line of drops, on the plain milk.

Try the same thing with whole milk and 2% milk and report your findings here

Student prior knowledge:

In a liquid, the molecules (small individual particles that can have positive and negative charges on their surfaces) can, just like magnets, attract and repel one another. In the case of a water molecule surrounded on all sides by other water molecules there is roughly the same amount of pulling and pushing force. The overall effect is no change. But at the surface, where the water molecules are exposed to air on one side, the water molecules experience more pulls downward, toward the other water molecules below them, than upward toward the air. This causes the water at the surface to contract, minimizing its surface area. This phenomenon is called surface tension.

Certain compounds affect water's surface tension: Some are are hydrophilic (or polar), which are attracted to water. Others, however, are hydrophobic (or nonpolar) and repel water. Some compounds have both properties. When such a compound is added to water, the hydrophilic end will try to get close to the water molecules whereas the hydrophobic end will push away from them. This pulling and pushing separates the water molecules from one another and consequently decreases surface tension. Compounds that lower water’s surface tension are known as surfactants.

Explanation:

Detergent, such as liquid dish soap, are mostly surfactants. These can lower the surface tension of water as well as milk, which contains water and molecules of fat. Surfactants have a hydrophilic part that wants to interact with the water and a hydrophobic part that wants to interact with the fat molecules. Because of this, when the cotton swab with soap touched the milk, the soap separated the fat from the water in the milk, dissolving the fat (which is how soap cleans greasy, dirty dishes). This also decreased the milk's surface tension. As the soap spread out from the cotton swab, it decreased the milk's surface tension around it, and the higher surface tension surrounding this area pulled the milk (along with its food coloring) toward it.

Questions & Answers:

1. Why does the whole milk react differently than the skim milk in this experiment?

The food coloring in the whole milk bowl will move the fastest because the dish soap bonds, or holds tight, with the fat in the milk. This bond is so strong that the water and the food coloring are pushed out. Everything else has to dance out of the way to make room for the dish soap and fat bond. Since there is less fat in 2 percent and 1 percent milk, you won't see as much movement. In the bowl of skim milk, the dish soap has no fat to bond with, so the food coloring isn't pushed away.

2. Will there be more movement of color if more soap is added? Why

If enough soap is added, the soap and milk become evenly mixed and the milk (and food coloring) no longer move when more soap is added.

3. What is the role of the food dye in this experiment?

The food coloring molecules interact and ride along with all the other molecules, and that’s what makes the colors move!

Applications to Everyday Life:

1. In commercials for liquid dish soaps claim the soap has the ability to “cut the grease.” Grease is mostly fat. While one part of the soap is attracted to water, other parts bind to the fat. Moving the soapy water around allows the soap to pull the grease away from the dishes, and be rinsed away by the water.

2. The separation of oil and water is also caused by the difference in the surface tension of the two liquids. When water molecules come together, they form a network of hydrogen bonds which results in very high surface tension. Both oil and water have high surface tensions, due to which the adhesion between them is weak and they do not mix.

3. The mercury used in the thermometer does not stick to the wall of the tube because of the surface tension, if it did the measurement of the temperature wouldn't be correct.

Photographs:

Videos: You Tube Video