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LESSON 7: Milk Rainbow
ESTIMATED TIME Setup: 5 minutes | Procedure: 5–10 minutes
- DESCRIPTION Create swirls of color in milk and explore a property of liquids known as surface tension.
- OBJECTIVE This lesson will demonstrate the property of surface tension and the effect that soap, a surfactant, has on the surface tension of milk. Students will apply soap to milk with food coloring on the surface to visualize the effect of soap on the surface tension of milk. The lesson can be extended to discuss a type of mixtures called emulsions.
- CONTENT TOPICS Scientific inquiry; states of matter; properties of matter; attractive forces (surface tension); mixtures - MATERIALS o Whole milk o Waterproof plate or shallow bowl o Food coloring o Liquid dish soap o Cotton swabs
Always remember to use the appropriate safety equipment when conducting your experiment. Refer to theSafety First section in theResource Guide on pages 421–423 for more detailed information about safety in the classroom.
Jump ahead to page 95 to view the Experimental Procedure.
OBSERVATION & RESEARCH
BACKGROUND
Matter exists primarily as a solid, liquid, or gas on the earth. Solids have a definite volume and a definite shape. Examples of solids are chairs, books, and trees. Liquids have a definite volume but no definite shape. Examples of liquids are water and orange juice. Gases have no definite shape and no definite volume. Examples of gases are the oxygen we breathe and the helium that fills balloons.
Along with differences in shape and volume, the different states of matter have other unique properties. For example,
surface tension is a property of liquids that describes the attraction of liquid particles at the surface. The strong attraction of particles at the surface of the liquid creates a surface “film” that makes moving an object through the surface of a liquid more difficult than moving the object when it is completely submerged in the liquid. Surface tension is also the reason liquids tend to keep a low surface area. For example, water droplets will tend to form into a sphere rather than spreading out flat.
Milk is a white liquid produced by female mammals and is the primary source of nutrition for young mammals
NATIONAL SCIENCE EDUCATION STANDARDS SUBJECT MATTER
This lesson applies both Dimension 1: Scientific and Engineering Practices and Dimension 2: Crosscutting Concepts from “A Framework for K–12 Science Education,” established as a guide for the updated National Science Education Standards. In addition, this lesson covers the following Disciplinary Core Ideas from that framework:
- PS1.A: Structure and Properties of Matter - PS1.B: Chemical Reactions - PS2.A: Forces and Motion - ETS2.B: Influence of Engineering, Technology, and Science on Society and the Natural World (see Analysis & Conclusion )
LESSON 7: Milk Rainbow
until they can digest other foods. Humans, especially children, often consume the milk of other mammals as a part of their diet. In the United States, cow’s milk is produced on an industrial scale and is the most commonly consumed form of milk. Milk is made mostly of water, but it also contains vitamins, minerals, fats, and proteins.
Water has a very high surface tension because of strong attractions between the water molecules (hydrogen bonding). Because milk is primarily water, it also has a high surface tension. In this experiment, the high surface tension of the milk supports the dye molecules on the surface and keeps the dye relatively centralized. (The tendency to keep a low surface area minimizes the distance that the dye will spread across the milk’s surface.)
In contrast, soap is a surfactant. A surfactant (or surface active agent) is a substance that has the ability to reduce the surface tension of a liquid. Therefore, when a drop of liquid dish soap is added to milk, the surface tension of the milk is reduced. As this occurs, the fat (butterfat) and protein particles in the milk can move more freely and easily. In addition, the soap interacts with the fat and protein particles in the milk, causing the particles to move around. This action can be seen as the dye swirls through the milk.
FORMULAS & EQUATIONS Milk is a liquid made mostly of water, but it also contains vitamins, minerals, fats, and proteins. Milk is a unique substance known as a colloid, so it does not have a specific chemical formula. A colloid is a mixture in which very small particles are spread evenly through another substance. In milk, the fat globules, proteins, vitamins, and minerals are spread throughout the water.
The molecules that make up soaps and detergents have two main parts (ends) that behave differently. One end of a soap molecule is attracted to water, while the other components are repelled by water but attracted to fats.
HYPOTHESIS
uWhen drops of liquid dish soap are added to milk with drops of food coloring on the surface, the soap will reduce the surface tension of the milk and react with the fat. This interaction will cause the particles in the milk to move around and create swirls of color.
CONNECT TO THEYOU BE THE
CHEMIST CHALLENGE
For additional background information, please review CEF’s Challenge study materials online at http://www.chemed.org/ybtc/challenge/study.aspx.
- Additional information on states of matter and properties of matter, including surface tension, can be found in the Classification of Matter section of CEF’s Passport to Science Exploration: The Core of Chemistry.
Fun Fact
The fat content of milk depends on the type of milk (skim, whole, etc.) and the type of cow. Holstein-Friesian cows tend to produce milk with a lower percentage of butterfat than Jersey cows.
LESSON 7: Milk Rainbow
NOTES
EXPERIMENTATION
As the students perform the experiment, challenge them to identify the independent, dependent, and controlled variables, as well as whether there is a control setup for the experiment. (Hint: If soap is not added to the milk, do the results change?) Review the information in the Scientific Inquiry section on pages 14–16 to discuss variables.
EXPERIMENTAL PROCEDURE
- Fill the plate with whole milk, and let the milk settle for a minute.
- Add several drops of different food coloring close together, but separate, in the center of the plate of milk.
- Dip a cotton swab in the liquid dish soap, and then touch the tip of the cotton swab to the milk’s surface near the drops of food coloring. Observe the reaction.
- Then, move the swab to different areas of the plate to initiate more reactions.
DATA COLLECTION
Have students record data in their science notebooks or on the following activity sheet. What happened when the dye was first added to the milk? What happened when the soap was added? Have students answer the questions on the activity sheet (or similar ones of your own) to guide the process.
LESSON 7: Milk Rainbow
ANALYSIS & CONCLUSION
Use the questions from the activity sheet or your own questions to discuss the experimental data. Ask students to determine whether they should accept or reject their hypotheses. Review the information in the Scientific Inquiry section on pages 14–16 to discuss valid and invalid hypotheses.
ASSESSMENT/GOALS
Upon completion of this lesson, students should be able to …
- Apply a scientific inquiry process and perform an experiment. - Differentiate between the different states of matter. - Understand the property of surface tension. - Describe the effect of soap, a surfactant, on surface tension. - Describe the general composition of milk. - Compare and contrast homogeneous and heterogeneous mixtures (see Differentiation in the Classroom ). - Define and identify colloids (see Differentiation in the Classroom ).
MODIFICATIONS/EXTENSIONS
Modifications and extensions provide alternative methods for performing the lesson or similar lessons. They also introduce ways to expand on the content topics presented and think beyond those topics. Use the following examples, or have a discussion to generate other ideas as a class.
- Try the experiment using other types of milk, such as skim milk, and observe the results. You may also want to try other liquids, like water or oil. Are the results the same? Discuss why or why not? - Set up the experiment as described in the lesson, but before adding the soap, try a different test first. Dip the cotton swab in water, and touch the cotton swab to the milk. Observe what happens.
Likewise, if you put a water-soaked cotton swab in the middle of a dye droplet, what happens? Discuss these results with the class.
See Lesson 26: Swimming Specs for a simplified lesson on surface tension.
See Lesson 15: Floating Paper Clips for an introductory lesson on surface tension.
REAL-WORLD APPLICATIONS
- You may have seen commercials for liquid dish soaps that 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 or your hands, and be rinsed away by the water. - Along with being homogenized, the milk found in most grocery stores has also been pasteurized. Pasteurization is a process of heating a food, usually a liquid, to a specific temperature and for a specific length of time to kill bacteria, molds, and yeast. This process was invented by French chemist and biologist Louis Pasteur in 1863 (see the Famous Chemist section in the Resource Guide ).
COMMUNICATION Discuss the results as a class and review the activity sheet. Review the information in the Scientific Inquiry section on pages 14–16 to discuss the importance of communication to scientific progress.
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LESSON 7 ACTIVITY SHEET: Milk Rainbow
ANALYZE & CONCLUDE
- What happens when you first place the drops of food coloring on the milk’s surface? __________________________
- What happens to the food coloring when you touch the milk with the cotton swab soaked in soap? ______________
- What are the components of milk? (What makes up milk?) ______________________________________________
- What effect does the soap have on the surface tension of the milk? ________________________________________
- Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
PERFORM YOUR EXPERIMENT
- Fill the plate with whole milk, and let the milk settle for a minute.
- Add several drops of different food coloring close together, but separate, in the center of the plate of milk.
- Dip a cotton swab in the liquid dish soap. Then, touch the tip of the cotton swab to the milk’s surface near the drops of food coloring. Observe.
- Try touching the cotton swab to different areas of the plate of milk to initiate more reactions.
You Be The ChemistYou Be The Chemist ®Activity Guides | page^ Activity Guide | page 9999
LESSON 7 ACTIVITY SHEET: Milk Rainbow
- What other substances are considered surfactants? ______________________________________________________
- Why do the components of grocery store milk not separate? ______________________________________________
EXPAND YOUR KNOWLEDGE—ADVANCED
- Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an image of the example.
Term Definition Example (write or add image)
Mixture
Homogeneous mixture
Heterogeneous mixture
Colloid
Emulsion
LESSON 7 ACTIVITY SHEET: Milk Rainbow ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
ANALYZE & CONCLUDE
- What happens when you first place the drops of food coloring on the milk’s surface? __________________________
- What happens to the food coloring when you touch the milk with the cotton swab soaked in soap? ______________
- What are the components of milk? (What makes up milk?) ______________________________________________
- What effect does the soap have on the surface tension of the milk? ________________________________________
- Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
PERFORM YOUR EXPERIMENT
- Fill the plate with whole milk, and let the milk settle for a minute.
- Add several drops of different food coloring close together, but separate, in the center of the plate of milk.
- Dip a cotton swab in the liquid dish soap. Then, touch the tip of the cotton swab to the milk’s surface near the drops of food coloring. Observe.
- Try touching the cotton swab to different areas of the plate of milk to initiate more reactions.
When droplets of food coloring are placed onto the milk’s surface, the food coloring stays suspended on the surface in a small area.
When you touch a cotton swab soaked in soap to the milk, the colors spread throughout the milk creating colorful swirls.
Milk is made mostly of water, but it also contains vitamins,
minerals, fats, and proteins.
Soap is a surfactant, which reduces the surface tension of the milk and allows the food coloring to move around the milk.
Answer 1: Valid because the data support my hypothesis. Answer 2: Invalid because the data do not support my hypothesis. I would reject my hypothesis and could form a new one, such as …
LESSON 7 ACTIVITY SHEET: Milk Rainbow ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
- What other substances are considered surfactants? ______________________________________________________
- Why do the components of grocery store milk not separate? ______________________________________________
EXPAND YOUR KNOWLEDGE—ADVANCED
Have students complete this section if you used the advanced differentiation information, or challenge them to find the answers to these questions at home and discuss how these terms relate to the experiment in class the next day.
- Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an image of the example.
Term Definition Example (write or add image)
Mixture A physical combination of two or more substancesthat can be physically separated.
Homogeneous mixture A type of mixture that is considered to be the samethroughout; the substances are evenly mixed.
Heterogeneous mixture
A type of mixture in which the makeup is not the same throughout; the substances are not evenly mixed.
Colloid
A mixture, between homogeneous and heterogeneous, in which very small particles are spread evenly throughout another substance.
Emulsion
A colloid that consists of liquids spread throughout other liquids; the liquids in an emulsion do not completely mix and are often unstable.
Other soaps and laundry detergents are examples of surfactants.
Raw milk will actually separate because it is an emulsion. However, most milk purchased at grocery stores goes through a process called homogenization. This process breaks down the fat in the milk
so it has a more uniform consistency, which, in turn, keeps the components of milk from separating.
