Stratospheric Ozone: A Balancing Act

This activity helps students understand the concept of equilibrium as applied to a model system and to stratospheric ozone. Although the materials are easy to come by, this model set-up does require a fair amount of preparation. You may want to do this as a classroom demonstration.

Background

Over the past several years there has been much discussion about the bad effects of ozone. But keep in mind that ozone occurs in two different layers of the atmosphere — the troposphere and the stratosphere. The stratospheric ozone, the so-called good ozone, protects the planet from the harmful effects of the sun's ultraviolet (UV) rays.

About 90% of all ozone is found in the stratosphere, where it plays an important role by absorbing harmful UV radiation. Some of the chemical pollutants that we are releasing into the atmosphere are destroying the ozone in this layer. It is the destruction of this stratospheric ozone, the subsequent formation of the ozone hole, and the general global reduction in stratospheric ozone thickness that should cause grave concern among all people.

The amount of ozone in the stratosphere is the net result of production and loss processes. Ozone is produced by photolysis (breaking apart of molecules by light) of oxygen high in the stratosphere where ultraviolet light is most intense. Ozone is lost by conversion back to molecular oxygen () through reactions whose net effect is:

O + -----> 2

So in the stratosphere, ozone is always being created and destroyed. Normally, this natural cycle is in perfect balance. It keeps just the right amount of ozone in the ozone layer.

With the discovery of the "ozone hole" over Antarctica in 1985, scientists determined that something was destroying ozone faster than nature could replace it. This something, a group of human-produced chemicals called chlorofluorocarbons (CFCs), has shifted the balance in the natural process of ozone production and ozone destruction. CFCs catalyze, or speed the break up of, ozone molecules when UV rays are also present. One CFC molecule can help destroy up to 100,000 ozone molecules over its lifetime of 100 years, and ozone production cannot keep up. In the graphic below, the destructive cycle of a chlorine atom is shown.

1. UV radiation breaks off a chlorine atom from a CFC molecule.
   
   
2. The chlorine atom attacks an ozone molecule (), breaking it apart and destroying the ozone.
   
   
3. The result is an ordinary oxygen molecule () and a chlorine monoxide molecule (ClO).
   
   
4. The chlorine monoxide molecule (ClO) is attacked by a free oxygen atom releasing the chlorine atom and forming an ordinary oxygen molecule ().
   
   
5. The chlorine atom is now free to attack and destroy another ozone molecule (). One chlorine atom can repeat this destructive cycle thousands of times.
   

The following animation shows the destruction of an ozone molecule by a chlorine atom.

In this activity, students will build a model that represents the natural balance of stratospheric ozone production and destruction. They will then alter their model to represent the changes humans have caused in this ozone balance.

Learning Goals

Students will be able to explain the concept of equilibrium as applied to the model system and to stratospheric ozone.

Alignment to National Standards

National Science Education Standards

• Unifying Concepts and Processes, Evidence, Models, and Explanation, Grades K to 12, pg. 117, paragraph 2: "Models are tentative schemes or structures that correspond to real objects, events, or classes of events, and that have explanatory power. Models help scientists and engineers understand how things work. Models take many forms, including physical objects, plans, mental constructs, mathematical equations, and computer simulations."
  
  
• Unifying Concepts and Processes, Evolution and Equilibrium, Grades K to 12, pg. 119, paragraph 2: "Equilibrium is a physical state in which forces and changes occur in opposite and off-setting directions: for example, opposite forces are of the same magnitude, or off-setting changes occur at equal rates. Steady state, balance, and homeostasis also describe equilibrium states. Interacting units of matter tend toward equilibrium states in which the energy is distributed as randomly and uniformly as possible."
  
  

Benchmarks for Science Literacy, Project 2061, AAAS

• Common Themes, Constancy and Change, Grades 9 to 12, pg. 275, Item #1: "A system in equilibrium may return to the same state of equilibrium if the disturbances it experiences are small. But large disturbances may cause it to escape that equilibrium and eventually settle into some other state of equilibrium."
  
  
• Common Themes, Constancy and Change, Grades 9 to 12, pg. 275, Item #3: "Things can change in detail but remain the same in general (the players change, but the team remains; the cells are replaced, but the organism remains). Sometimes counterbalancing changes are necessary for a thing to retain its essential constancy in the presence of changing conditions."
  
  
• Common Themes, Models, Grades 6 to 8, pg. 269, Item #1: "Models are often used to think about processes that happen too slowly, too quickly, or on too small a scale to observe directly, or that are too vast to be changed deliberately, or that are potentially dangerous."
  

Grade Level/Time

• Grade level: 6 to 9
  
  
• Time:
  
  
  • Instructions and data collection: 45 minutes
    
    
  • Questions and discussion: 30 minutes
    

Materials

• Large, transparent, plastic storage container
  
  
• 4 plastic gallon jugs filled with water
  
  
• Several 2-liter plastic pop bottles
  
  
• 12-inch length of plastic tubing (1/4 inch diameter works well)
  
  
• Silicone sealant
  
  
• Device to regulate water flow (valve, clamp, holes with stoppers)
  
  
• Small soldering iron
  
  
• Waterproof tape or modeling clay
  
  
• Large needles
  
  
• Ring and ring stand (or improvised platform several inches high)
  
  
• Marking pens
  
  
• Small bucket
  

Procedure

You will have to first set up an apparatus that will allow water to be poured and collected. If you have access to a sink, you can modify the materials to use the sink as your regulated water source.

1. Cut the bottom off of a 2-liter plastic pop bottle.
   
   
2. Bore a hole in the cap of the bottle.
   
   
3. Place the plastic tubing through the hole in the cap and seal with silicone sealant.
   
   
4. Place the clamp on the tube several inches below the cap.
   
   
5. Screw the cap back on the bottle and place in the ring stand over the large storage container. You now have the ability to regulate water flow.

Part 1: A Delicate Balance

1. Using a marking pen, draw a line around the outside of a second bottle—about halfway between the top and bottom.
   
   
2. Poke a small hole near the bottom of the second 2-liter plastic pop bottle (the small soldering iron works well for this).
   
   
3. Place the 2-liter pop bottle in the middle of the storage container on the overturned small bucket. Insert the plastic tube from the water source bottle.
   
   
   
   
4. Begin pouring the water from the gallon jugs into the open bottle.
   
   
5. Using the clamp on the tube, adjust the flow of water in such a way that the water level in the bottle reaches and stays at the marked line.
   
   
6. Now slowly reduce the flow of the water. Make note of changes in the water level.
   
   

Part 2: Changing the Balance

1. Using a new 2-liter pop bottle, draw a line around the outside of the bottle — about halfway between the top and bottom.
   
   
2. Poke several holes below the marked line that can then be plugged up with modeling clay or covered with waterproof tape. (Another option is to begin the activity with an intact bottle and 'stab' holes with a large needle as the activity progresses.)
   
   
3. Place the bottle in the middle of the storage container on the overturned small bucket. Insert the plastic tube from the water source bottle.
   
   
4. Start off with all but one hole covered and adjust the water flow so the water level in the bottle reaches and stays at the marked line.
   
   
   
   
5. Uncover another hole (or poke with a large needle) and note what happens to the water level. Time the reduction of the water level every 15 seconds for 2 minutes.
   
   
6. Uncover another hole and continue to measure the drop in water level every 15 seconds.
   
   
7. Cover one hole with tape or modeling clay, but do not readjust the water flow. Continue to measure the water level.
   
   
8. Finally, cover all but one hole, and time how long it takes for the water level to reach the marked level.
   

Questions and Observations

1. Describe what occurs to the water level in Part 1, when the water flow is increased. (The water in the bottle rises.)
   
   
2. What occurs when the water flow is decreased? (The water level gradually drops.)
   
   
3. Graph the water level in Part 2. The vertical axis represents water level and the horizontal axis represents time. Indicate in your graph when one plug is removed, when two plugs are removed, and then when each of the plugs is replaced.
   
   
4. What effect did adding holes have upon the water level? (Water level gradually fell.)
   
   
5. This exercise is a model of a dynamic balance. What is meant by a dynamic balance? (Inputs and outputs are equal, so everything remains in equilibrium.)
   
   
6. Scientists say that ozone in the stratosphere would naturally be in dynamic balance. What does this mean? (The ozone is created and destroyed at a constant rate; as much is produced as is destroyed.) How is ozone made? (Oxygen molecules are split apart by UV radiation, and one oxygen atom combines with an oxygen molecule to produce ozone.) How is ozone destroyed? (A photochemical reaction with UV light, chlorine, and ozone breaks apart the bond, producing an oxygen molecule and an oxygen atom.)
   
   
7. What are we modeling by adding the extra holes in the container in Part 2? (Ozone depletion caused by CFCs.)
   
   
8. Relate this activity to ozone in the stratosphere.
   

Assessment Ideas

• Use question #8 above as your assessment question, though it might be more effective to ask each student to draw two diagrams: 1) a diagram of the water-level model. They should be given the freedom to illustrate it however they want to show the features of the model they think are the most important, and 2) a diagram of stratospheric ozone showing specifically how the water model is related to the atmospheric process.
  

Modifications for Alternative Learners

• No specific modifications necessary, although relating the model to the atmosphere should be done as visually as possible to assist the English Language Limited students.