Environmental Chemistry — Pollution, Ozone, Green Chemistry

Chemistry and the Environment

Environmental chemistry studies the chemical processes occurring in the environment and the impact of human activities. Air pollution, water pollution, soil contamination, ozone depletion, and the principles of green chemistry are the core topics. This is a theory-only chapter with no numerical problems — purely conceptual.

CBSE Class 11 gives 2-3 marks. NEET rarely tests it directly but environmental questions overlap with the biology syllabus. The questions are straightforward definitions and classifications — learn the categories and examples and you score full marks.

Key Concepts

Atmospheric regions

Region	Altitude	Key feature
Troposphere	0-10 km	Weather, most pollution, temperature decreases with altitude
Stratosphere	10-50 km	Ozone layer (15-35 km), temperature increases with altitude
Mesosphere	50-80 km	Temperature decreases, meteoroids burn here
Thermosphere	80-400 km	Aurora, ionosphere (reflects radio waves)

Air Pollution

Primary pollutants — directly emitted into the atmosphere:

Pollutant	Major source	Effect
CO	Incomplete combustion (vehicles)	Binds Hb 200x stronger than O $_2$ , reduces O $_2$ transport
SO $_2$	Coal/oil combustion, volcanoes	Respiratory irritation, acid rain
NO $_x$ (NO, NO $_2$ )	Vehicle engines (high-T combustion)	Smog, acid rain, respiratory damage
Particulates (PM10, PM2.5)	Combustion, dust, construction	Lung disease, reduced visibility
Hydrocarbons (VOCs)	Fuel evaporation, solvents	Smog precursors, some are carcinogenic (benzene)
Lead	Leaded fuel, paint, batteries	Neurological damage, developmental delays

Secondary pollutants — formed by reactions in the atmosphere: O $_3$ (ground level), PAN, photochemical smog, acid rain.

Photochemical smog

Formed in warm, sunny conditions from vehicle exhaust:

\text{NO}_2 + hv \to \text{NO} + \text{O}

\text{O} + \text{O}_2 \to \text{O}_3 \text{ (ground-level ozone)}

\text{O}_3 + \text{hydrocarbons} \to \text{PAN, aldehydes, other oxidants}

Characteristics: brown haze, eye irritation, respiratory problems, plant damage. Called Los Angeles smog (warm, sunny). It is oxidising in nature.

Classical smog (London-type): forms in cold, humid conditions from SO $_2$ and soot. Reducing in nature. Causes bronchitis. The Great Smog of London (1952) killed ~4,000 people.

Feature	Photochemical smog	Classical smog
Climate	Warm, sunny	Cold, foggy
Components	O $_3$ , PAN, NO $_x$	SO $_2$ , soot
Nature	Oxidising	Reducing
Effect	Eye irritation, plant damage	Bronchitis, respiratory
City type	Los Angeles	London

Acid rain

SO $_2$ and NO $_x$ dissolve in rainwater:

\text{SO}_2 + \text{H}_2\text{O} + \frac{1}{2}\text{O}_2 \to \text{H}_2\text{SO}_4

2\text{NO}_2 + \text{H}_2\text{O} \to \text{HNO}_3 + \text{HNO}_2

Rain with pH below 5.6 is acid rain (normal rain is 5.6 due to dissolved CO $_2$ forming weak carbonic acid).

Effects:

Corrodes marble and limestone buildings: $\text{CaCO}_3 + \text{H}_2\text{SO}_4 \to \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$
The Taj Mahal’s yellowing is partly due to acid rain from Mathura refinery emissions
Kills fish in acidified lakes (pH below 4.5 is lethal for most fish)
Damages crop foliage and leaches nutrients from soil

Remember: normal rain pH = 5.6 (not 7.0, because CO $_2$ dissolves in it). Acid rain = pH below 5.6. NEET sometimes gives 7.0 as a distractor.

Water Pollution

BOD (Biochemical Oxygen Demand) — the amount of dissolved O $_2$ consumed by bacteria to decompose organic matter in a water sample over 5 days at 20°C. Higher BOD = more organic pollution. Clean water: BOD < 5 ppm. Polluted: >17 ppm.

COD (Chemical Oxygen Demand) — oxygen needed to chemically oxidise all organic and inorganic matter. COD $\geq$ BOD always, because COD measures non-biodegradable pollutants too.

Eutrophication: Excess nutrients (phosphates from detergents, nitrates from fertilisers) → algal bloom → algae die → bacteria decompose them using DO → O $_2$ depleted → fish die. This is a major problem in Indian water bodies.

Biomagnification: Persistent pollutants (DDT, mercury) concentrate at each trophic level. Top predators accumulate dangerous levels. DDT biomagnification caused eggshell thinning in raptors.

Soil Pollution

Sources: pesticides (DDT, BHC, aldrin), industrial waste, electronic waste, mining residues.

Pesticide classification:

Insecticides (DDT, malathion) — target insects
Herbicides (2,4-D, atrazine) — target weeds
Fungicides (organomercury compounds) — target fungi

Persistent organic pollutants (POPs) do not break down easily and accumulate in food chains. The Stockholm Convention (2001) aims to eliminate the worst POPs.

Ozone Layer

Ozone ( $\text{O}_3$ ) in the stratosphere absorbs UV-B and UV-C radiation. CFCs (chlorofluorocarbons) release Cl radicals that catalytically destroy ozone:

\text{Cl} + \text{O}_3 \to \text{ClO} + \text{O}_2

\text{ClO} + \text{O} \to \text{Cl} + \text{O}_2

One Cl atom can destroy ~100,000 O $_3$ molecules. The Antarctic ozone hole was detected in 1985. The Montreal Protocol (1987) phased out CFCs globally. The ozone layer is now slowly recovering.

Distinguish ozone depletion (stratosphere, CFCs, UV radiation) from global warming (troposphere, CO $_2$ /CH $_4$ , infrared trapping). Different causes, different layers, different treaties (Montreal Protocol vs Kyoto/Paris). NEET tests this distinction regularly.

Greenhouse Effect and Global Warming

Greenhouse gases (CO $_2$ , CH $_4$ , N $_2$ O, CFCs, water vapour) absorb outgoing infrared radiation and re-emit it, warming the surface. The natural greenhouse effect keeps Earth habitable (~+33°C). The enhanced effect from excess emissions is causing global warming.

Gas	GWP (vs CO $_2$ )	Major source
CO $_2$	1	Fossil fuels, deforestation
CH $_4$	~25	Rice paddies, cattle, wetlands, landfills
N $_2$ O	~298	Fertilisers, combustion
CFCs	1000-10000	Refrigerants (now banned)

Global temperature has risen ~1.1°C since pre-industrial times. Consequences: sea level rise, ice melt, extreme weather, coral bleaching.

Green Chemistry

The design of chemical products and processes that reduce or eliminate hazardous substances. Twelve principles — the most important for exams:

Prevention — prevent waste rather than clean it up
Atom economy — maximise incorporation of starting materials into the product
Less hazardous synthesis — use and generate non-toxic substances
Safer solvents — avoid toxic solvents; use water or supercritical CO $_2$
Energy efficiency — minimise energy use; prefer reactions at ambient conditions
Catalysis — use catalytic reactions over stoichiometric reagents (catalysts are reusable, reagents are consumed)
Renewable feedstocks — use bio-based starting materials where possible

Example: The synthesis of ibuprofen was redesigned from a 6-step process (atom economy ~40%) to a 3-step catalytic process (atom economy ~77%), producing less waste.

Solved Examples

Stratospheric ozone absorbs UV-B and UV-C, protecting life from DNA damage. Ground-level ozone is a secondary pollutant formed from NO $_x$ + sunlight — it is a powerful oxidant that damages lung tissue, causes breathing problems, and harms plants. Same molecule, different altitude, opposite effects.

A factory discharges effluent with BOD = 50 ppm into a river. Clean river water has DO = 8 ppm. If the effluent consumes 50 mg O $_2$ per litre but only 8 mg is available, the river becomes anaerobic. Fish and other aerobic organisms die. This is why BOD limits are enforced on industrial discharges.

The Taj Mahal is made of marble ( $\text{CaCO}_3$ ). Acid rain from the Mathura refinery: $\text{CaCO}_3 + \text{H}_2\text{SO}_4 \to \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$

The calcium sulphate product is powdery and flaky — it crumbles off, exposing fresh marble to further attack. This process is called “marble cancer” or “stone leprosy.” The Supreme Court of India ordered industries around Agra to switch to cleaner fuels to protect the monument.

Common Mistakes to Avoid

Confusing primary and secondary pollutants. Ozone at ground level is secondary (formed from NO $_x$ + sunlight). Ozone in the stratosphere is naturally occurring and protective. CO, SO $_2$ and particulates are primary (emitted directly).

Mixing up photochemical and classical smog. Photochemical = warm, sunny, oxidising (O $_3$ , PAN). Classical = cold, foggy, reducing (SO $_2$ , soot). The climate and chemical nature are opposite.

Confusing BOD and COD. BOD = biological oxygen demand (measured over 5 days by bacteria). COD = chemical oxygen demand (measured by strong oxidant like K $_2$ Cr $_2$ O $_7$ ). COD is always $\geq$ BOD because it oxidises non-biodegradable matter too.

Saying normal rain is pH 7. Normal rain is pH 5.6 because atmospheric CO $_2$ dissolves in it to form weak carbonic acid ( $\text{H}_2\text{CO}_3$ ). Acid rain has pH below 5.6.

Thinking green chemistry means using natural products. Green chemistry is about designing processes to minimise waste and toxicity — this can involve synthetic chemicals too. The key is prevention of pollution at source, not elimination of chemistry.

Exam Weightage and Strategy

Environmental Chemistry carries 2-3 marks in CBSE Class 11 boards. NEET rarely tests it as a standalone question but environmental concepts overlap with biology (Environmental Issues chapter). The questions are definitional — know the terms, the types and one example for each.

Three classification tables cover the chapter: (1) primary vs secondary pollutants with examples, (2) photochemical vs classical smog, (3) BOD vs COD. Add the ozone depletion mechanism and 3 green chemistry principles. That is your complete exam toolkit.

Practice Questions

Q1. What is the cause of ozone depletion? Name the chemicals responsible.

CFCs (chlorofluorocarbons) like Freon. UV radiation in the stratosphere breaks C-Cl bonds, releasing Cl radicals that catalytically destroy ozone. One Cl atom destroys ~100,000 O $_3$ molecules before being deactivated. The Montreal Protocol (1987) banned CFC production globally.

Q2. What is eutrophication? How does it kill aquatic life?

Excess nutrients (phosphates, nitrates from fertilisers and detergents) cause algal bloom → algae block sunlight → submerged plants die → algae also die → bacteria decompose dead matter using dissolved oxygen → DO drops below survival level for fish → mass die-off. Prevention: reduce fertiliser runoff, use phosphate-free detergents, treat wastewater.

Q3. Distinguish between biodegradable and non-biodegradable pollutants with examples.

Biodegradable: broken down by microorganisms — sewage, food waste, paper, animal dung. Non-biodegradable: persist in the environment — DDT, plastic, heavy metals (Hg, Pb), PCBs. Non-biodegradable pollutants are more dangerous because they accumulate in ecosystems through biomagnification.

Q4. What is green chemistry? State any two principles.

Chemistry that designs products and processes to reduce or eliminate hazardous substances. (1) Prevention: prevent waste at source rather than treating it after formation. (2) Catalysis: use catalytic reactions over stoichiometric ones — catalysts are reusable and produce less waste. Other key principles include atom economy and use of safer solvents.

Q5. How does acid rain damage buildings?

Acid ( $\text{H}_2\text{SO}_4$ , $\text{HNO}_3$ ) reacts with calcium carbonate in marble/limestone: $\text{CaCO}_3 + \text{H}_2\text{SO}_4 \to \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$ The calcium sulphate product is powdery and flakes off, exposing fresh stone. Over time, sculptures lose detail and walls corrode. The Taj Mahal, Parthenon and many European cathedrals have suffered acid rain damage.

FAQs

What is the greenhouse effect?

CO $_2$ , CH $_4$ and other gases trap infrared radiation reflected from Earth’s surface, warming the atmosphere. Without this natural greenhouse effect, Earth would be about -18°C — too cold for life. The problem is the enhanced greenhouse effect from excess CO $_2$ emissions since the Industrial Revolution.

What is the difference between global warming and ozone depletion?

Global warming is caused by greenhouse gases trapping heat in the troposphere. Ozone depletion is caused by CFCs destroying stratospheric ozone. Different causes, different atmospheric layers, different problems — but both are environmental crises. CFCs contribute to both (ozone depletion + greenhouse warming), which adds confusion.

What is smog?

A combination of “smoke” and “fog.” Photochemical smog forms in sunny, warm conditions (Los Angeles type) and contains ozone and PAN. Classical smog forms in cold, foggy conditions (London type) and contains SO $_2$ and soot. Modern Indian cities experience a mix of both types, especially during winter inversions.