Written By Kelly Lundstrom & Kennda Lynch January 31, 2012 Milk Dye ACTIVITY PLAN

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1 Milk Dye ACTIVITY PLAN Objective: Students will use the scientific method to test the difference between using whole milk and skim milk in this milk and food dye experiment. Students will explore ideas of density, surface area, convention, and the role of the structure of different types of milk in the experiment. Science Standards Addressed: SC Matter exists in different states such as solids, liquids, and gases. SC Mixtures of matter can be separated regardless of how they were created; all weight and mass of the mixture are the same as the sum of the weight and mass of the parts. SC Distinguish between physical and chemical changes, noting that mass is conserved during any change. Additional Background Information: The drops of food coloring stay on the top surface of the milk when dropped onto the milk. This is due to the fact that the food coloring is less dense than the milk, so it floats on the surface. The colors do not mix because they were not stirred. When the soap is added, the soap reduces the surface tension of the milk by dissolving the fat molecules. Students should notice that this experiment works better for the whole milk versus the skim milk because the whole milk has a higher percentage of milk fat. 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. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops. Addition of another drop of soap will start the process again. (modified from: ) Materials: skim milk, whole milk, (11) clear pie tins, (4-5) sets of food-dye, (11) droppers, soap, (11) plastic beakers, newspapers Questions to Ponder 1. Why does the whole milk react differently than the skim milk in this experiment? 2. What is the role of the food dye in this experiment? References

2 Name Science Club 1/31/2012 Milk and Food Dye Experiment Procedure 1. Set up your workstation by getting one long sheet of paper for your table, 2 pie tins, and one soap dropper. 2.Fill 2 red cups to the marked line inside the cup with milk. One cup should have whole milk, one cup should have skim milk. 3. Pour the skim milk into one pie tin and the whole milk into another. 4.Carefully put one drop of each of the four food colors onto the surface of the milk in each, widely separated, and not in the center of the dish. Note: you will have to share the food dye with other groups. Your set-up should look something like this: 5.Get ready to watch what happens! Very carefully drop one drop of dish soap onto the surface of the milk in the center of each dish. (Be careful not to add the soap directly on top of the food coloring.)

3 Observations ñ What happens to the food coloring when you first put it on the milk? Why do you think that is? ñ What happens when you add the drop of soap? ñ What direction does the food coloring move when you first add the drop of soap? ñ What direction does the food coloring move after the experiment has been running for a while? ñ Does the movement go on forever? What happens? ñ What happens if you add another drop of soap after the colors have stopped moving? ñ What did you observe that was different between using whole milk versus skim milk? ñ Chemically, what is main difference between whole milk and skim milk? Why is that important for this experiment?

4 Milk-Dye Experiment WHAT S HAPPENING??!! ñ What happens to the food coloring when you first put it on the milk? Why do you think that is? You should see the food dye sitting on top of the milk in small dots. This is because the density of the food coloring is much less than the density of the milk and so it sits on top of the milk instead of mixing. The milk also has what we call surface tension, so the lighter food coloring will sit on top of the milk. Definition: Density density is how much something of a set volume weighs. This is generally measured by taking a cubic centimeter of a substance (cm^-3). If we take a cubic centimeter of some commonly known substances, we can see that they weigh different amounts: ñ What happens when you add the drop of soap? The soap is what we call a surfactant. A surfactant is a compound that lowers the surface tension of a fluid and allows unmixable liquids (like oil and water) to mix. So, for example, this is why you use dish soap to clean up your plate after eating a greasy cheeseburger. The soap lowers the surface tension of the grease and pulls it away from your plate. ñ What direction does the food coloring move when you first add the drop of soap? The food coloring should move away from the drop of soap and look like it is disappearing long the edge of the plate. ñ What direction does the food coloring move after the experiment has been running for a while? The food coloring will eventually look like it is coming back up from the center of the initial drop. This movement is because the soap breaks down the fat in the milk, reduces the surface tension. The milk outside of the soap drop has a higher surface tension and pulls the fluid away from the soap which carries the food dye with it. This causes what we call convection.

5 Definition: Convection The transfer of heat through a fluid (liquid or gas) caused by molecular motion ñ Does the movement go on forever? What happens? Nope! Once the soap is evenly mixed with the milk the reaction should stop. ñ What happens if you add another drop of soap after the colors have stopped moving? The reaction should start again. ñ What did you observe that was different between using whole milk versus skim milk? You should see the food dye convecting (moving) faster in the whole milk vs. the skim milk. ñ Chemically, what is main difference between whole milk and skim milk? Why is that important for this experiment? The whole milk has more lipids (another word for fat). Whole milk has about 8 grams of fat while skim or 1% milk have 1 gram or less of fat. Definition: Lipids Lipids are another type of organic molecule (contains carbon). When you think of fats, you should know that they are lipids. Another type of lipid is cholesterol (see below).