To research air bags on the Internet. To use that information to design and test a version of an air bag.
This lesson begins with an Internet exploration of how air bags in cars were developed to solve a very practical problem: saving lives in automobile accidents. It also examines some of the safety issues surrounding air bags. Students do a chemistry lab in which they mix baking soda and vinegar in a Ziploc® bag, with the goal of determining the correct ratio of acid and base that leaves no appreciable amount of either reactant behind, yet fully inflates the bag. The inflated bag is analogous to the air bags in cars. Then students perform a crash test with a raw egg.
According to Benchmarks for Science Literacy, students "…must use different tools to do different things in science and to solve practical problems. Through design and technology projects, students can engage in problem-solving related to a wide range of real-world contexts." (Benchmarks for Science Literacy, p. 43.) Research suggests that middle-school students are better able to understand experiments where the goal is a desired outcome, rather than ones that try to analyze a cause and effect relationship. (Benchmarks for Science Literacy, p. 334.)
It may be helpful to review the chemistry behind airbags by going to How Airbags Work, which provides a detailed description of the functioning of an air bag.
Distribute the Air Bag Study Guide student sheet and have students go to How Airbags Work. Students should read through page 4 of the resource. At this site, they will learn how an airbag inflates and is supposed to save the life of a passenger. As students go through this resource, they should answer the questions on their study guide.
Once students have finished their Air Bag Study Guide, you should go over the questions with them:
- What is the intended purpose of an air bag?
- According to the article, what is a major concern with air bags?
- What is being done to make air bags safer?
- According to the article, what are three rules for drivers of cars with passenger-side air bags?
- What gas fills up the air bag?
- Explain how an air bag works.
- How long does it take for the air bag to inflate?
This will lead into a discussion of the chemistry behind airbags. The engineers who design air bags are able to determine the exact quantity of chemicals needed to inflate the air bag to the proper volume, and in the correct amount of time. If the air bag is under inflated or inflates too quickly, the passenger will still be injured by the steering wheel. If the air bag over inflates or inflates too slowly, the passenger will hit the inflating airbag and be injured. It is critical to get just the right amount of gas in the air bag.
Distribute the It's a Crash Test, Dummy student sheet. Students are now ready to begin the lab. During the exploration, students will attempt to find the amount of sodium bicarbonate (baking soda) required to react with 25 ml of acetic acid (vinegar) so that the bag is inflated, with no appreciable quantity of either reactant left. This will allow students to develop some idea of a limiting reactant. (Depending on the atmospheric pressure, the amount of baking soda required to fully inflate the bag should be between 1.5 and 3.0 grams.)
In this reaction, acetic acid combines with sodium bicarbonate to form water, carbon dioxide, and sodium acetate. The chemical equation for this reaction is:
HC2H3O2 + NaHCO3 ---> H2O + CO2 + NaC2H3O2
The carbon dioxide is the gas that fills the bag, and the sodium acetate will be dissolved in the water. It is important for students to realize that this is not exactly how an air bag works, but is a good analogy that allows them to simulate the inflation of a protective device in the classroom.
Be prepared for some messes with the egg drop test. It is recommended that a plastic sheet be placed under the "drop zone" to assist in cleaning up. It is important to find a container for the egg drop that is the right size; three-pound butter tubs or small shoe boxes work well. Also be sure that students wash hands well after handling raw eggs.
In order to summarize, ask these questions:
- Why were air bags developed?
- Why are air bags dangerous for small adults and children?
- What are some options for reducing the risk to small adults and children?
Then assess student understanding by asking students to form and defend an opinion based on information learned:
- Should air bags be mandatory in all cars? Should consumers have the right to turn them off?
The student worksheets (Air Bag Study Guide and It's a Crash Test, Dummy) could be collected and used to assess understanding. In addition to grading the questions, points could be awarded for a successful crash test.
Newton's Apple has a nice lesson called Air Bags and Collisions that would work as an extension to this activity.
Some suggestions from Newton's Apple:
- See if you can find other examples of impact protection devices. Where are they most common? Why do you use pads for gymnastics? What about helmets and knee pads? Do they transfer momentum?
- Toss a balloon filled with water into the air. Stop and analyze what happens when you successfully catch the balloon. What ways have you helped transfer the momentum?
- Design a few posters that promote seat belt use. Hang them up around the school to promote automobile safety. Why would parents want to put small children in safety seats in the back seat of a car?
- People have different opinions about mandatory seat belt and air bag use. In fact, this discussion often includes bicycle and motorcycle helmets for safety, too. Divide your class into two debating teams and discuss the following question: Can a society that believes in individual decisions regarding safety issues require individuals to use devices that may save their lives in an accident?