Chemistry · 9th Grade

Active learning ideas

Ozone Depletion and CFCs

Active learning helps students visualize abstract chemical processes like radical chain reactions and connect them to real-world policy decisions. When students manipulate simulations or analyze primary documents, they move from memorizing facts to constructing their own explanations of how science informs global action.

• Common Core State Standards
• HS-ESS2-6
• HS-ESS3-4

• 25–40 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Simulation Game
• 30 min · Small Groups

Simulation Game: Chlorine Radical Chain Reaction

Students use color-coded index cards representing Cl radicals, O3 molecules, and O atoms to simulate the catalytic destruction mechanism. One student plays the persistent Cl radical, showing how it is regenerated after each cycle. After the simulation, groups write out the two-step mechanism and calculate how many cycles one Cl atom could complete before removal.

Explain the natural formation and destruction of ozone in the stratosphere.

Facilitation Tip

In the HFC Replacement Tradeoffs Think-Pair-Share, assign roles within pairs (e.g., researcher, policy advisor) to ensure both students contribute meaningfully to the discussion.

What to look for

Present students with a diagram of the ozone layer and a CFC molecule. Ask them to write two sentences explaining how the CFC molecule interacts with ozone in the stratosphere, referencing the concept of a catalyst.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 02

• Case Study Analysis
• 35 min · Small Groups

Document Analysis: Montreal Protocol

Provide excerpts from the 1987 Montreal Protocol and a recent WMO ozone assessment report on stratospheric recovery. Students annotate the documents for scientific claims and supporting evidence, then discuss: what evidence convinced nations to act, and what does the current recovery data show about the protocol's effectiveness?

Analyze how chlorofluorocarbons (CFCs) catalyze the destruction of the ozone layer.

What to look for

Pose the question: 'If the Montreal Protocol was so successful, why is it important to continue monitoring the ozone layer?' Facilitate a class discussion focusing on the long-term persistence of CFCs and the possibility of new threats.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management

Activity 03

• Case Study Analysis
• 40 min · Small Groups

Data Analysis: Antarctic Ozone Hole Trends

Students graph historical ozone hole maximum area data (1980 to present) from published NASA records. They identify the peak size, describe the trend since the Montreal Protocol, and calculate a rough projected recovery timeline. Groups compare their projections and discuss sources of uncertainty in the extrapolation.

Evaluate the effectiveness of international policies like the Montreal Protocol.

What to look for

On an index card, students should write the chemical formula for ozone and one key difference between its natural formation/destruction cycle and its depletion by CFCs. They should also name one country that was a major producer of CFCs before the Montreal Protocol.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management

Activity 04

• Think-Pair-Share
• 25 min · Pairs

Think-Pair-Share: HFC Replacement Tradeoffs

Present students with the fact that HFCs were adopted as CFC replacements to protect ozone but are potent greenhouse gases. Pairs discuss whether switching to HFCs represented a net environmental gain and what the Kigali Amendment addressed. This requires students to weigh multiple chemical and policy considerations simultaneously.

Explain the natural formation and destruction of ozone in the stratosphere.

What to look for

Present students with a diagram of the ozone layer and a CFC molecule. Ask them to write two sentences explaining how the CFC molecule interacts with ozone in the stratosphere, referencing the concept of a catalyst.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

A few notes on teaching this unit

Teachers should emphasize the dynamic equilibrium of ozone formation and destruction before introducing disruptions like CFCs to build foundational understanding. Avoid rushing to solutions by having students first model the natural cycle independently, then introduce CFCs as a perturbation. Research shows that students grasp complex systems better when they first understand the unperturbed state and then observe how it changes.

By the end of these activities, students will explain the mechanism of ozone depletion using chemical equations, evaluate the effectiveness of the Montreal Protocol using data, and assess tradeoffs in chemical replacements. They will also correct common misconceptions by comparing ozone depletion to the greenhouse effect, describing the seasonal nature of the ozone hole, and explaining the delayed recovery timeline.

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Watch Out for These Misconceptions

• During the Chlorine Radical Chain Reaction Simulation, watch for students who conflate ozone depletion with the greenhouse effect.

  After the simulation, have students create a two-column chart comparing the two issues using the reaction steps they observed and the gases involved in the greenhouse effect, such as CO2 and methane.

• During the Antarctic Ozone Hole Trends data analysis, watch for students who describe the ozone hole as a permanent physical gap.

  Use the seasonal animations provided with the data set to have students sketch the ozone hole’s formation, expansion, and dispersal each year, labeling the role of polar stratospheric clouds and the polar vortex.

• During the Think-Pair-Share on HFC Replacement Tradeoffs, watch for students who believe banning CFCs immediately solved the ozone depletion problem.

  Show the timeline graph from the Montreal Protocol analysis and ask students to calculate the lag time between CFC phase-out and projected ozone recovery, emphasizing the long atmospheric lifetime of CFCs.

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Methods used in this brief

• Simulation Game
• Case Study Analysis
• Think-Pair-Share