ChemistryICSE Class 9 · Atmospheric and Environmental Chemistry

Ozone Layer, Water Pollution and Minamata Disease: Formation, Effects and Lessons (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Ozone (O₃) forms in the stratosphere when UV splits O₂ into atoms that join with O₂ (3O₂ → 2O₃) and shields the Earth from harmful UV radiation. CFCs, halons, carbon tetrachloride and nitric oxide deplete it. In water, dissolved oxygen and minerals are useful, but heavy metals, plastics, sewage and pesticides are harmful; Minamata disease is mercury poisoning through fish.

Understand Ozone Layer, Water Pollution and Minamata Disease

Ozone. Ozone ( $O_{3}$ ) is an allotrope of oxygen with three atoms per molecule; most lies in the stratosphere (about 15–35 km above the ground) as the ozone layer.

*Formation:* ultraviolet (UV) light splits oxygen molecules and the atoms join other molecules:
$O_{2} U V O + O$ ; $O + O_{2} \to O_{3}$ ; overall $3 O_{2} \to 2 O_{3}$ .

*Function:* the layer absorbs most harmful UV radiation from the Sun. Without it there would be more skin cancer, cataracts, weakened immunity, crop damage and harm to plankton at the base of ocean food chains.

*Depletion:* chlorofluorocarbons (CFCs, freons) from old refrigerators, air conditioners, aerosols and foams; halons from some fire extinguishers; carbon tetrachloride and methyl chloroform; nitric oxide from high-flying supersonic jets. They release free radicals, such as chlorine atoms, that break down ozone. CFCs are being phased out under international agreement.

Natural water. Contains dissolved gases, salts, and sometimes suspended matter and microbes.

*Useful:* dissolved oxygen for aquatic animals (more soluble in cold water); minerals (Ca, Mg, Na, K salts); a little fluoride for teeth; dissolved CO₂ for aquatic plants.
*Harmful:* heavy metals (mercury, lead, cadmium, arsenic) that accumulate; plastics that do not decompose and form microplastics; sewage, whose decay uses up dissolved oxygen and carries disease germs (cholera, typhoid, hepatitis A) and whose nutrients cause algal bloom; pesticides like DDT that concentrate up food chains.

Biomagnification. A substance that is not broken down or excreted becomes more concentrated at each step of a food chain. Example (invented data): water 0.5 ppb; small fish 25 ppb ( $50 \times$ more); large fish 1250 ppb ( $50 \times$ more again), a $2500 \times$ rise from water to large fish.

Minamata disease. Mercury poisoning first identified in the 1950s around Minamata Bay in Japan. A chemical factory discharged waste with mercury compounds into the bay; bacteria and the food chain converted and concentrated it as methylmercury in fish and shellfish eaten by local people. *Symptoms:* damage to the nervous system (numbness, muscle weakness, poor coordination, loss of vision, hearing and speech, paralysis); babies of exposed mothers born with defects. *Lesson:* heavy metals are not removed by the body or broken down in nature, so industrial waste must be treated before discharge.

Ozone Layer, Water Pollution and Minamata Disease: formulas and key facts

Ozone formation: $O_{2} U V 2 O; O + O_{2} \to O_{3}; 3 O_{2} \to 2 O_{3}$; In the stratosphere.
Biomagnification factor: $factor = \frac{concentration in organism}{concentration in water}$; Increases up the food chain.
Ozone depleters: $C F C s ∣ halons ∣ C C l_{4} ∣ methyl chloroform ∣ N O$; Named in the syllabus.

How to solve Ozone Layer, Water Pollution and Minamata Disease questions

Separate chemistry steps from environment reasoning.

1For ozone write the three equations in order.
2For depletion name the chemical, its source and the radical it releases.
3For water, split answers into useful and harmful with an example each.
4For biomagnification compute the ratio at each step.
5For Minamata give cause, pathway, symptoms and lesson.

Ozone Layer, Water Pollution and Minamata Disease in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Sewage and fish kills

Suppose a lake has 8 mg of dissolved oxygen per litre (invented value) and untreated sewage causes bacteria to use up 1.5 mg per litre each day with no replenishment. Fish cannot live below 4 mg per litre.

After how many days are the fish in danger?

Try it first, then show the solutionSolution

Oxygen that may be lost $= 8 - 4 = 4$ mg per litre.
Time $= \frac{4}{1.5} \approx 2.7$ days.

Answer: In under 3 days.

Memory hook: Decay eats oxygen.

Tricks to remember Ozone Layer, Water Pollution and Minamata Disease

Short memory aids. Say each one aloud once, then use it on a question the same day.

Three oxygens make ozone
O₃: from O₂ + O by UV.
Use it when: Formation.
Heavy metals climb the chain
Small amounts in water become large in top predators.
Use it when: Biomagnification.
Minamata = mercury
Minamata Bay, 1950s: methylmercury in fish.
Use it when: Disease.

Ozone Layer, Water Pollution and Minamata Disease: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Where is the ozone layer? · 1 mark · MCQBoard level

The ozone layer lies mainly in the:
(a) troposphere
(b) stratosphere
(c) the ground level
(d) the oceans

Try it first, then show the solutionSolution

About 15–35 km above the Earth in the stratosphere.

Answer: (b) stratosphere

Example 2: The role of ultraviolet light · 2 marks · Short answerBoard level

Which step in the formation of ozone needs ultraviolet radiation, and why? Give the equation of that step and the overall equation.

Try it first, then show the solutionSolution

The splitting of an oxygen molecule into atoms, $O_{2} U V O + O$ , needs UV energy to break the strong O=O bond. (1)
The atoms then join other oxygen molecules, $O + O_{2} \to O_{3}$ ; overall $3 O_{2} \to 2 O_{3}$ . (1)

Answer: O₂ → 2O needs UV; overall 3O₂ → 2O₃

Example 3: Oxygen to ozone · 3 marks · Short answerHarder board level

Twelve oxygen molecules are converted into ozone. How many ozone molecules form, and how many oxygen atoms are present before and after? Does the mass change?

Try it first, then show the solutionSolution

$3 O_{2} \to 2 O_{3}$ : $12 \times \frac{2}{3} = 8$ ozone molecules. (1)
Atoms before: $12 \times 2 = 24$ ; after: $8 \times 3 = 24$ . (1)
The mass is unchanged: atoms are only rearranged. (1)

Answer: 8 molecules; 24 atoms both; mass unchanged

Example 4: Sewage and oxygen · 3 marks · Short answerHarder board level

A lake holds 8 mg per litre of dissolved oxygen (invented). Bacteria feeding on sewage use up 1.5 mg per litre per day. Fish die below 4 mg per litre. Find the time for fish to be in danger and explain why sewage makes this worse than summer warmth alone.

Try it first, then show the solutionSolution

Loss allowed $= 8 - 4 = 4$ mg per litre. (1)
Time $= \frac{4}{1.5} \approx 2.7$ days. (1)
Warmth only lowers oxygen solubility; sewage adds a continuing oxygen demand and algal bloom as well. (1)

Answer: About 2.7 days

Example 5: Mercury in the food chain · 4 marks · Case-basedHOTS / competency

In a bay the mercury level (invented data) is 0.5 ppb in water, 25 ppb in plankton-eating small fish and 1250 ppb in large fish eaten by people.
(a) Find the factor of increase at each step and overall.
(b) Name this effect.
(c) Name the disease caused by eating such fish and one symptom.
(d) State one precaution.

Try it first, then show the solutionSolution

(a) $\frac{25}{0.5} = 50$ ; $\frac{1250}{25} = 50$ ; overall $\frac{1250}{0.5} = 2500$ . (1½)
(b) Biomagnification. (½)
(c) Minamata disease (mercury poisoning); numbness or damage to the nervous system. (1)
(d) Treat industrial waste to remove heavy metals; avoid fish from polluted waters. (1)

Answer: (a) 50, 50, 2500 (b) biomagnification (c) Minamata disease (d) treat waste

Where students lose marks in Ozone Layer, Water Pollution and Minamata Disease

The slips examiners see most often, and what to write instead.

Mistake: Ozone layer placed in the lower atmosphere
It is in the stratosphere.
Wrong working
The ozone layer is near the ground.
Right working
It lies at about 15–35 km (stratosphere).
Do this: Name the layer.
Usually costs: 1 mark.
Mistake: Ozone formation equation unbalanced
The overall equation is 3O₂ → 2O₃.
Wrong working
O₂ → O₃
Right working
3O₂ → 2O₃
Do this: Count atoms.
Usually costs: 1 mark.
Mistake: Minamata disease attributed to lead
It is mercury poisoning.
Wrong working
Lead poisoning.
Right working
Mercury (methylmercury) poisoning.
Do this: Minamata = mercury.
Usually costs: 1 mark.

Exam tips

Write the three ozone equations in order and the overall one.
Name at least three ozone-depleting chemicals.
For Minamata give cause, pathway, symptoms and lesson in four lines.
Use the words "accumulates" and "food chain" for heavy metals.

Practice questions on Ozone Layer, Water Pollution and Minamata Disease

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The chlorine free radicals that destroy ozone in the stratosphere come mainly from:
(a) carbon dioxide
(b) chlorofluorocarbons
(c) sulphur dioxide
(d) nitrogen
Hint
Old refrigerants.
Answer
(b) chlorofluorocarbons
Q2 · 1 mark · Very short answerBoard level
Mercury poisoning from contaminated fish is called what?
Hint
Japan, 1950s.
Answer
Minamata disease.
Q3 · 2 marks · Short answerHarder board level
Why do bacteria that decompose sewage make the water unsuitable for fish?
Hint
Oxygen demand.
Answer
The bacteria use up the dissolved oxygen as they decompose the organic waste, so fish suffocate; the sewage also carries disease germs.
Q4 · 3 marks · Short answerHarder board level
The mercury level in water is 0.4 ppb, in small fish 20 ppb and in large fish 1000 ppb (invented data). Find the factor of increase at each step and from water to large fish.
Hint
Divide successive values.
Answer
Water to small fish: $\frac{20}{0.4} = 50$ ; small to large fish: $\frac{1000}{20} = 50$ ; overall $\frac{1000}{0.4} = 2500$ .
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): CFCs are being phased out.
Reason (R): They release chlorine radicals in the stratosphere that destroy ozone.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Ozone depletion.
Answer
(a)
Q6 · 3 marks · Long answerHOTS / competency
List four harmful effects of ozone depletion on living things and explain why harm to plankton affects food chains.
Hint
UV reaches the surface.
Answer
More UV causes skin cancer, cataracts, weaker immunity and crop damage. Plankton are at the base of ocean food chains, so damage to them reduces food for small fish and then for larger animals, harming the whole chain.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Function of the ozone layer?Show answer
It absorbs most harmful UV radiation.
2.Overall equation of ozone formation?Show answer
3O₂ → 2O₃.
3.Two ozone-depleting chemicals?Show answer
CFCs and halons.
4.Cause of Minamata disease?Show answer
Methylmercury from industrial mercury waste, concentrated in fish.
5.What is biomagnification?Show answer
Increase in concentration of a non-degradable substance up a food chain.

Ozone Layer, Water Pollution and Minamata Disease: frequently asked questions

How is ozone formed in the atmosphere?

UV light splits oxygen molecules into atoms, which combine with oxygen molecules: O₂ → 2O and O + O₂ → O₃; overall 3O₂ → 2O₃.

What causes Minamata disease?

Eating fish and shellfish contaminated with methylmercury from mercury waste discharged by a factory into Minamata Bay, Japan.

Why is sewage harmful to water bodies?

It uses up dissolved oxygen, carries disease germs and adds nutrients that cause algal bloom.