What Do You Know About Ozone?

This exercise encourages students to share what they know about ozone and explore misconceptions. Ideally, they will come to understand that ozone is desirable in the stratosphere and a serious pollutant in the troposphere.

Background

We hear a great deal about ozone these days, but is ozone good or bad? Where do we normally find it and where does it come from? When does it benefit us and when is it dangerous? What is the ozone hole and what are we doing about it? How do scientists measure ozone and can we measure it too? These are some of the questions that come to mind as we listen to the news and try to understand what ozone is.

What is Ozone?

The oxygen we find in our atmosphere is made up of two oxygen atoms. Scientists use the symbol to indicate this oxygen compound. makes up approximately 21% of the atmosphere we breathe. Because of its chemical formulation, a single atom of oxygen is unstable. That is, it wants to combine with something else. That is why oxygen is almost always found in pairs, in its (diatomic) form, where it is more stable.

Ozone is made of three oxygen atoms, . It is less stable than , because it wants to return to the diatomic state by giving up an oxygen atom. The ability of ozone to readily give up an oxygen atom makes it a powerful oxidizing agent. Because of this, ozone is used as a bleaching agent and water purifier.

Ozone was discovered in 1839 by professor Christian Friedrich Schoenbein at the University of Basel, Switzerland. The word ozone comes from the Greek word ozein, meaning "to smell." Ozone has a pungent odor and is often detected after a thunderstorm or when close to an electric motor.

Where is Ozone Found in the Atmosphere?

Ozone is both good news and bad news! When ozone exists in the stratosphere, it protects us from the sun's harmful ultraviolet rays; however, when ozone exists in our troposphere along with the air we breathe, we consider it a major health and environmental concern.

The concentration of ozone found in the stratosphere, about 25 km (15 miles) above the earth's surface, is generally less than ten parts per million (ppm). This means that for every million molecules of air, only ten molecules are ozone. Even though ozone exists in minute quantities, it is vital for life on earth. The ozone layer filters out ultraviolet radiation from the sun, while allowing other wavelengths of light to travel on through the atmosphere.

Ozone occurs naturally in the troposphere at about 10 to 30 ppb (parts per billion). This amount can increase significantly at the ground level through a complex chemical reaction between sunlight and pollutants emitted by motor vehicles and industry. When increased to levels over 80 ppb, ozone can be very damaging to the living tissues of plants and animals and to items such as rubber and nylon. In the troposphere, ozone also acts as a potent greenhouse gas.

Learning Goals

1. Students will understand that ozone exists in both the troposphere and the stratosphere.
   
   
2. Students will be able to explain that ozone in the stratosphere is important for filtering out damaging ultraviolet radiation.
   
   
3. Students will be able to explain that ozone in the troposphere is a pollutant. It can damage respiratory tissue in living organisms.

Alignment to National Standards

National Science Education Standards

• Earth and Space Science, Structure of the Earth System, Grades 5 to 8, pg. 160, Item #8: "The atmosphere is a mixture of nitrogen, oxygen, and trace gases that include water vapor. The atmosphere has different properties at different elevations."
  

Benchmarks for Science Literacy, Project 2061, AAAS

• The Physical Setting, Processes That Shape the Earth, Grades 6 to 8, pg. 73, Item #7: "Human activities, such as reducing the amount of forest cover, increasing the amount and variety of chemicals released into the atmosphere, and intensive farming, have changed the earth's land, oceans, and atmosphere."
  
  

Grade Level/Time

• Grade level: 6 to 9
  
  
• Time:
  
  
  • First poster preparation and discussion: 30 minutes
    
    
  • Video/reading content exploration: variable
    
    
  • Poster modification: 20 minutes
    
    
  • Assessment/discussion: 20 minutes
    

Materials

• Construction paper or poster board
  
  
• Marker pens
  
  
• Tape
  
  

Procedure

1. Working in small groups of three to four, students should write on a large piece of construction paper or poster board everything they have heard about ozone. If you find that some students claim to have no idea, prompt them with some general questions. For example, do you think ozone is a solid, liquid, or gas? Where do you think ozone is? Is it good or bad? Why? You should find that students begin to come up with ideas. Remember, it is not important now that their ideas be correct, only that they generate ideas.
   
   
2. Display the lists around the room and discuss. (Are all lists consistent? What information do students agree or disagree on?)
   
   
3. Generate a list of questions students have about ozone.
   
   
4. Have students read literature on ozone or use an appropriate video. You may want to contact your regional EPA office or your state air quality department for suitable materials.
   
   
5. Have students review the generated lists and identify misconceptions. Students should then be given time to recreate a poster that incorporates their new learning.
   

Assessment Ideas

• At this stage of their learning, the assessment should be formative. You may wish to ask students (orally or on paper) to list items on the first poster that were correct and ones that were incorrect. Have students list the most surprising new fact they learned about ozone.
  

Modifications for Alternative Learners

• English Language Limited students should be paired with language skilled students to work together to interpret the reading and/or video. Students should be allowed to illustrate poster 'facts' with drawings as well as words.