Observing a polyurea-coated watermelon dropping from a 148-foot drop as a phenomenon, students will develop an understanding of how the coating is formed via a chemical reaction and what properties make the substance so durable. Through models and observational data, students will explain how the molecular structure of polyurea keeps the watermelon inside from smashing when dropped from a tower. This lesson addresses both MS-PS1-1 and MS-PS1-2 of the Oklahoma Academic Standards for Science. This lesson includes optional modifications for distance learning. Resources for use in Google Classroom are included.
How does the atomic structure of a substance influence its properties?
Students watch a video demonstration of watermelons with and without a polyurea coating dropped from a tower and attempt to explain what they observed.
Students watch additional video clips showing the chemical reaction that produces polyurea (brand name LINE-X®). Then, students draw models to explain what they observe and build additional models to explore the chemical structure of polyurea and its original components.
Students share their models and discuss how they know a chemical reaction occurred, how polyurea is formed, and describe the relationship between atomic structure and a substance's properties.
In small groups, students experiment with "Cat's Cradle" string games as a practical demonstration of the characteristics of polyurea that make it virtually indestructible.
Students create a final model that explains how polyurea is formed, why its atomic structure contributes to its near-indestructible quality, and the mechanism for how polyurea protected the watermelon observed in the Engage.
Lesson Slides (attached)
Student Graphic Organizers handout (attached, one per student)
Video Timing Guide (attached, for teacher reference)
Model building supplies, such as cotton balls, marshmallows, pom-poms, toothpicks, pipe cleaners, straws, popsicle sticks, etc.
Yarn or string, cut into a variety of lengths
Use the attached Lesson Slides to guide the lesson. You may wish to review the lesson objectives and Essential Question on slides 3-4. Begin the lesson on slide 5. Give students the first page of the Student Graphic Organizers handout. Have students complete the I Notice, I Wonder strategy while watching the "Indestructible Coating?!" video. The image on slide five is linked to the video, and the full URL can be found in the Resources below if needed. Be sure to stop the video at the 2:10 mark. Then, go to slide 6 and ask students the following questions:
What do you think the polyurea substance around the watermelon is made of?
What properties does the material have that make it so strong?
How do you think it was able to protect the melon?
Go to slide 7 to present the phenomenon goal to the students: "How does the polymer coating protect the watermelon?" During the lesson, they will be trying to explain why the polymer coating is able to protect the watermelon.
Continue with the "Indestructible Coating?! video, letting students watch from the 2:40–3:26 marks. This section shows the chemical reaction that forms the polyurea coating in real time. Play the clip at least twice to give students an opportunity to pick up on the specific details during their observations. The observable indicators that a chemical reaction has occurred in the video include the release of a gas, change in temperature, and change in consistency (i.e., new properties) of the material in the cup.
Go to slide 8. Invite the class to share some of their observations about the video, then ask students to draw a model on Explore section of the Student Graphic Organizers handout that shows how the LINE-X® is formed. Students could share these models with partners, a few volunteers could share their models with the class, or you can set the models aside without sharing out for now.
Go to slide 9. Ask students to recall what "Substance A" (clear liquid) and "Substance B" (dark liquid) looked like. Then, ask them to explain why the substances are different from each other. If they struggle to develop answers you might ask guiding questions like, "Why is Substance A a clear liquid?" or "What is Substance B made of that causes it to be black?" Have students add the distinction to their model.
Discuss the models and the ideas they represent as a whole group. Accept a variety of answers from students, but emphasize those ideas that suggest that Substance A and Substance B are made of different particles.
Go to slide 10. Pose the following prompt to students: "Using the materials provided, build a model that shows how Component A and B combine to form the LINE-X® coating. Be sure your model represents what makes each component different from each other." Allow time for students to build their models.
Go to slide 11. After the models are complete, students should share their models with the class. You may choose to have this sharing be formal or informal. Ask students to identify similarities among their models. Record their ideas somewhere all students can see to help them draw conclusions and connect ideas (such as in a lesson slide, in a Google Doc, on poster paper, or similar). If students' models do not represent all the major concepts they need to discover, it may help to make those observations yourself for the class (e.g., "I notice in most of your models that all three substances are shaped differently," and "When I look at your models I see that many of you built LINE-X® out of the same parts you used for Substance A and B."). If this is necessary, be sure to ask students to explain why they built their models in these ways.
Following the discussion of the students' models, show the "Indestructible? Inconceivable!" video from the 2:10–4:05 marks. Go to slide 12. From the combination of model sharing, the video explanation, and a class discussion of content, students should have built a conceptual understanding of the content.
Go to slide 13. Conclude this portion of the lesson by asking students to explain how polyurea/LINE-X® forms. Re-voice student answers to formally summarize how polyurea forms.
Go to slide 14 to introduce students to relevant vocabulary. (Note that the video uses the word "exothermic," but students do not need to know or remember this term since the video defines the word every time it is used.)
Introduce the following vocabulary terms to students (displayed on slide 14):
Atoms: the particles that make up matter
Elements: different types of atoms; the elements and the way they react with each other is what gives substances their unique properties
Molecules: two or more atoms bonded together; they can be the same type of atom, or combinations of different types
Polymers: substances made of many copies of the same type of molecule bonded together
Reactive: This is a video-specific word (~2:18) and is not necessary for students to remember, but for the purpose of this lesson, they need to know that Substance A makes very strong bonds with other molecules.
Go to slide 15. Ask students if they have enough information yet to explain why the LINE-X® coating was able to protect the watermelon. At this point, the answer is probably no. Let them know that the activity they are going to do next will help them fill in the gaps in their knowledge by investigating further.
Go to slide 16. Assign students specific "Cat's Cradle" string game designs or let them choose one for themselves. Student pairs might choose to try two different designs of varying complexity, or a single design with the complexity distributed across the classroom. Pairs should attempt to create the same design with their strings and make observations about the experience (e.g., which length was easiest to use, how the length of the string related to the complexity of their design, what happened to the string during the activity).
Go to slide 17. In students' pairs or in small groups, have students summarize what they learned while playing string games (i.e., longer threads get tangled more easily and can make more complicated designs; shorter threads are difficult to make designs with and aren't as easy to tangle). Continue with the questions on slide 18. Then, play the rest of the video (4:05–end) for students.
The Claim, Evidence, Reasoning (CER) strategy would fit well with the discussion. Students' claim would be about the strength of LINE-X®; their evidence would be what they observed during the string-games, and their reasoning would be the scientific explanation for how the watermelon is protected. Give students the second page of the Student Graphic Organizers handout if you wish to use this activity.
Go to slide 19. Students should create a final model that illustrates how the LINE-X® forms and how its structure protects the watermelon when it is dropped. This model might be drawn or constructed as was previously done in the lesson. It could also be created digitally, or students could demonstrate it with a kinesthetic model. In addition, each student should provide an explanation, either written or oral, of their model and how it represents the concepts they learned during the lesson. It would be reasonable for students to work in pairs or small groups, especially for a kinesthetic model, since the explanations are independent.
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