Lesson

Engage

The teacher should prepare for this demonstration by pouring two teaspoons of sodium polyacrylate into the nonclear cup without any students seeing it. Have students gather around (but not too close!) and pour water into the cup. Swirl for about 15 seconds, then flip the cup upside down. Nothing should come out. Have students make observations and inferences.

Explain that the lesson will be focused on polymers, a specific type of molecule made up of different units. These can have many different properties. There are different types of polymers, and the one they will be looking at today are superabsorbent polymers; polymers that can absorb a high amount of water comparable to their size. They will split into smaller groups and each test a different variable related to superabsorbent polymers.

Explore

1. Ask students: What types of variables could you test? [potential answers: how much water can be absorbed, mixing water with other substances, different types of superabsorbent polymers]
2. Choose one variable to use as an example. Review what an independent variable is (the variable that you have control of changing) and what a dependent variable is (represents what the output is). Ask students: Is [insert chosen test variable here] an independent or dependent variable? [independent]
3. Ask students: What is the dependent variable? [how much water is absorbed]
4. As students: If the variable you are testing is how much water different amounts of superabsorbent polymer can absorb, what are some controls variables to keep in mind? [test each sample with the same amount of water, use the same cup, same temperature of water, same type of water (i.e. tap or distilled), etc.]
5. Break students up into groups (maximum of four students per group), and have students create their own experiment (if you feel comfortable setting this up for each group, feel free to do so). As students work, the teacher should circulate between groups to ensure that correct data is being taken. Potential variables are:
   a. Absorbency of distilled vs. tap water
   b. Water vs. salt water vs. ammonia and water
   c. Salt water vs. sugar water
   d. Different types of superabsorbent polymers (sodium polyacrylate found in diapers, silica gel packs, chamois cloth)
   e. Varying amounts of superabsorbent polymers with the same amount of water
   f. Same amount of superabsorbent polymers with different amounts of water
6. Students should keep track of how much liquid they add to the polymer and how much is actually absorbed by the polymer.
7. As students work, they should complete the data table. Groups should also be prepared to share their results with the rest of the class.
8. Once all groups are done, each should report their findings to the class, which should include graphs of their results. It is up to students to decide what information should be shown graphically, and what kind of graph would best convey the information.

Explain

Student groups should share their results with the class, as well as whether the results they found were predicted or not. Tell students that polymers are made of repeating subunits, called monomers (it may be helpful to connect a series of pipe cleaners together to demonstrate). Monomers are linked together to create polymers (show the molecular model of plastic from Middle School Chemistry as a visual). Polymers are naturally found or manmade. When polymers are manmade or have been altered, they are referred to as synthetic polymers.

Though the monomers that make a polymer are repetitive, monomers can be bonded to different end groups to gain different properties. For example, the ends of the sodium polyacrylate are sodium and carbonyl, a molecule made of carbon, hydrogen, and oxygen atoms. When water is introduced, these two substances break into ions, which attract the water because water us polar. Because these polymers attract water so well, they are called ‘hydrophilic’ molecules. In fact, because these molecules are so good at absorbing water (they ‘unravel’ allowing more areas for water molecules to bond), they are referred to as superabsorbent polymers.

Other polymers, like plastics, do not attract water well and instead repel it. These are called ‘hydrophobic’ substances. Water is not attracted to these substances because of a lack of charge attraction.

Ask students: What are some other examples of hydrophobic and hydrophilic substances? [hydrophobic: oil, wax, etc. / hydrophilic: paper towels, sponges, etc.]

Elaborate

To continue their investigation of polymers, students may look at Happy/Unhappy Balls, available from Arbor Scientific. With these, further discussion on end groups and structure can be explored.

Evaluate

Discussion Questions

• What other types of variables could have been tested with the superabsorbent polymers?
• How could the idea of superabsorbent polymers be helpful in real life, aside from diapers?

Multiple Choice Items

1. The Greek term mono- means alone, whereas the term poly- means many. With this in mind, why do you think monomers and polymers are named as such?
   a. Many polymers are in the world, but only one monomer
   b. Monomers link together to make polymers*
   c. Polymers link together to make monomers
   d. Monomers are made of many different parts
2. What property of water allows it to be absorbed by superabsorbent polymers?
   a. It is wet
   b. It is a liquid, and solids absorb liquids
   c. The charges of water are attracted to all the different parts of the polymer*
   d. The charges of water are attracted to the end groups of the polymer
3. Which of the following is not a good use of synthetic superabsorbent polymers?
   a. Cleanup of liquid waste spills
   b. Coating materials to become super water-resistant*
   c. Taking in water better for plants in areas that suffer from drought conditions
   d. Absorbency in diapers

Open-Ended Questions

Do you think polymers can absorb other liquids aside from water? Why or why not? [Yes, as long as there is an attraction between the liquid and the superabsorbent polymer, absorption could take place. This means that the liquid must be polar]

Other [Project-based; Performance task]

The graph and presentation of results could be used as an evaluative tool.

Cross-Disciplinary Extensions

Connect to Math

Extrapolating conclusions and information from a graph, creating a graph, advantages/disadvantages of different types of graphs

Connect to Social Studies

Further discussion of the use of superabsorbent polymers in the clean up of environmental waste could be done as a class, or with students doing an individual project, like a brochure.

Next Generation Science Standards

This lesson supports the following:

Practices of Science and Engineering

• Asking questions and defining problems
• Planning and carrying out investigations
• Analyzing and interpreting data
• Obtaining, evaluating, and communicating information

Cross-Cutting Concepts

• Cause and Effect: Mechanism and Explanation
• Structure and Function

Disciplinary Core Ideas, Grades 6-8

Physical science

• Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. (MS-PS1-1)
• Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals). (MS-PS1-1)
• Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it. (MS-PS1-2),(MS-PS1-3)

Engineering Design

• The more precisely a design task’s criteria and constraints can be defined, the more likely it is that the designed solution will be successful. Specification of constraints includes consideration of scientific principles and other relevant knowledge that are likely to limit possible solutions. (MS-ETS1-1)