Here is the fact that makes ozone depletion frightening rather than merely unfortunate: the chlorine atom that destroys an ozone molecule is not used up doing it. It comes back out the other side ready to go again. A single atom can work its way through thousands of ozone molecules, which is how a few million tonnes of spray-can propellant managed to thin a layer wrapped around an entire planet.
📚 What you need to know
Ozone depletion is the gradual thinning of the ozone layer in the stratosphere, caused by human-made ODSs.
CFCs are stable and do not readily degrade in the troposphere, so they gradually drift upward into the stratosphere.
In the stratosphere, CFCs absorb UV radiation and break down, releasing chlorine radicals.
Stage 1: Cl· + O3 → ClO· + O2
Stage 2: ClO· + O3 → Cl· + 2O2
Overall: 2O3 → 3O2 — ozone is converted to ordinary oxygen.
Chlorine radicals are not used up; they act as catalysts, so one radical can destroy thousands of ozone molecules.
How a chemical from a fridge reaches 30 km up
The first thing to explain in any answer is the journey, because it contains a genuine paradox. CFCs are dangerous because they are unreactive. A reactive gas released at ground level would break down within days and never get anywhere near the stratosphere. CFCs do not break down. They simply mix into the atmosphere and drift, for years, until they finally reach altitudes where the UV is intense enough to destroy them — and it is that destruction that releases the chlorine.
This time lag explains why recovery is so slow. CFCs released decades ago are still arriving in the stratosphere and still have decades of destructive life ahead of them once they get there.
The catalytic cycle
Follow the loop round once and count: two ozone molecules gone, three oxygen molecules made, and the chlorine radical exactly as it started. That is the definition of a catalyst, and it is why the quantities of CFC involved were so disproportionate to the damage.
🧩 Writing this out for full marks
State that CFCs are stable and do not degrade in the troposphere, so they drift up into the stratosphere.
Say that in the stratosphere they absorb UV radiation and break down, releasing a chlorine radical.
Give stage 1: the chlorine radical reacts with ozone to form chlorine monoxide and an oxygen molecule.
Give stage 2: the chlorine monoxide reacts with another ozone molecule, producing two oxygen molecules and regenerating the chlorine radical.
State the overall result: ozone has been converted to oxygen, so the ozone concentration falls.
Finish with the catalyst point: because the radical is regenerated, a single one can destroy thousands of ozone molecules.
The equations are given in the syllabus for this HL section, so unlike the natural ozone cycle you should be able to write them. But do not stop at the chemistry. The mark that separates a good answer from a full one is almost always the sentence explaining why regeneration matters — that the chlorine is not consumed, so the same atom keeps working. Chemistry without that sentence describes a reaction; chemistry with it explains a catastrophe.
Why the balance tips so far
Go back to the equilibrium from the previous page. Ozone was already being destroyed and reformed constantly, at matched rates. Chlorine radicals add an extra destruction pathway that the reformation reaction cannot keep up with. Two things make it so effective:
It is catalytic. The radical is regenerated at the end of each cycle, so the same atom keeps destroying ozone until something else removes it from the stratosphere — which may take years.
It also removes free oxygen atoms. Stage 2 converts ozone to ordinary oxygen molecules, so the free atoms needed to rebuild ozone become scarcer. The destruction side speeds up while the reformation side is starved.
WORKED EXAMPLE
Explain how CFCs cause ozone depletion. [4]
Point 1 — getting thereCFCs are stable and do not degrade in the troposphere, so they drift upward into the stratosphere over a period of years.Point 2 — release of the radicalIn the stratosphere they absorb UV radiation and break down, releasing chlorine radicals.Point 3 — the two stagesA chlorine radical reacts with ozone to give chlorine monoxide and oxygen; the chlorine monoxide then reacts with a second ozone molecule to give two oxygen molecules and a chlorine radical.Point 4 — the catalystBecause the chlorine radical is regenerated rather than used up, one radical can destroy thousands of ozone molecules before it is removed.4 / 4Journey, release, cycle, catalyst. If you can only remember one, remember the catalyst point.
WORKED EXAMPLE
Suggest why ozone levels continued to fall for years after CFC production was restricted. [3]
Point 1 — the time lagCFCs already released take years to drift from the surface up into the stratosphere, so molecules emitted before the ban were still arriving afterwards.Point 2 — persistenceCFCs are chemically stable, so they are not broken down on the way and remain in the atmosphere for decades.Point 3 — catalytic actionOnce a chlorine radical is released it is regenerated after each cycle, so it continues destroying ozone for a long time rather than being consumed.3 / 3This style of “suggest why” question rewards you for connecting the chemistry to the timescale. Very few students do it.
💡 Exam tip
Learn both equations and the overall reaction. At HL you are expected to be able to write them.
Explain the stability paradox: CFCs are harmful precisely because they are unreactive at ground level.
Always include the word catalyst, or the phrase “not used up”. It is the highest-value point on the page.
Use the figure thousands of ozone molecules per radical. Specific beats vague.
Note that chlorine radicals are eventually removed from the stratosphere — the cycle is not literally infinite.
Connect back to the equilibrium: ODSs increase the destruction rate so the balance can no longer hold.
⚠️ Common mix-up
Saying CFCs react with ozone directly. The CFC must be broken by UV first; it is the chlorine radical that attacks the ozone.
Thinking the chlorine is consumed. It is regenerated in stage 2, which is the whole point.
Writing that CFCs “absorb ozone”. They catalyse its conversion into oxygen.
Forgetting the upward journey. The delay between release and damage is often a whole mark.
Confusing ClO with ozone. ClO is chlorine monoxide, an intermediate radical, not an oxygen molecule.
Assuming stopping production stops the damage immediately. Molecules already released continue arriving for years.
Up next: Why the Hole Forms Over the Poles — the same chemistry, plus two peculiar features of polar weather that turn a global thinning into a seasonal hole over Antarctica.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
