Humans adversely affect the environment in a variety of ways, but sometimes this degradation takes time to reveal itself. The use of chlorofluorocarbons (CFCs) is one such example. CFCs are human-manufactured chemicals that, years after their creation, were proven to harm the environment.
CFCs have no natural source. These nonflammable, nontoxic compounds were developed in the mid-20th century as an inexpensive coolant for refrigerators and air conditioners and an easy-to-use propellant in aerosol cans. In 1974, Nature published research by Sherwood Rowland and Mario Molina, who explained that CFCs decrease ozone in the stratosphere. Stratospheric ozone forms a layer that helps protect Earth's surface--and inhabitants--from ultraviolet radiation that can cause sunburn, cataracts, genetic mutations, and cancer, among other ills.
Rowland and Molina's research ultimately led to the Montreal Protocol, an international agreement implemented in 1987 to decrease CFC use. In 1995, their work won them the Nobel Prize in chemistry (see "On the web"). Further research revealed CFCs are also greenhouse gases with a long lifespan in the atmosphere.
This article describes modeling activities that help students understand ozone formation in the stratosphere, the chemical effects of CFCs on the ozone layer, and the role of the Montreal Protocol.
Note that to counter a common misconception, teachers may need to explain that CFCs' current concentrations are low enough that they do not contribute significantly to global warming (see "On the web").
Using the 5E approach
Following a modified 5E approach (Bybee et al. 2006), students explore the concept of CFCs and develop their own explanation of how they deplete ozone in the stratosphere.
For the activities, you will need:
* 100-150 tennis balls (or other small items that can be thrown and caught) in boxes. Ask your school's physical education teacher for old tennis balls no longer in use;
* 2 or 3 pairs of sunglasses (or some other item to designate a student role);
* green hats or leis (or another item to designate a student role);
* large area (e.g., a hallway, outside grassy area, or part of the classroom cleared of desks); and
* a small whiteboard or chart paper.
Have students quietly consider a photograph of an elderly man exposed to ultraviolet A radiation (see "On the web").
In the open area, have students link arms in groups of two or three. Depending on class size, choose two or three students to wear the sunglasses and have these students stand behind the boxes of balls (Figure 1). Explain the following guidelines:
* The students wearing the sunglasses will toss the tennis balls to the rest of the class. For safety, emphasize the importance of gently tossing the balls underhanded.
* The receiving students must try to catch the balls while remaining linked to each other.
* When a person in a group of two catches a ball, that student should drop the ball on the ground, and the pair should split apart. These single individuals must now attach to a different person--someone in a set of two to make a group of three,...