Environmental Chemistry – Quick Revision Notes 2025

1. ENVIRONMENT – BASIC TERMS

1.1 Key Definitions

• Environment: Surroundings + conditions affecting living organisms.
• Environmental Chemistry: Study of chemical processes in air, water, soil, and biota.
• Pollutant: Substance at harmful concentration (harmful quantity, time, and place).
• Primary pollutant: Emitted directly (SO₂, CO, NO, particulate matter).
• Secondary pollutant: Formed by chemical reactions (O₃, PAN, Smog).
• TLV (Threshold Limit Value): Safe exposure limit of pollutants.

Primary vs Secondary Pollutants

1.2 Important Points

• Major environmental compartments: Air, Water and Soil.
• Persistent pollutants: Don’t decompose easily (DDT, plastics, heavy metals).
• Biodegradable pollutants: Decompose naturally (sewage, paper).

➡ Exam/Interview favourite: Difference between primary & secondary pollutants in one word.

ATMOSPHERIC POLLUTION

2.1 Structure of Atmosphere

• Troposphere (up to ~12 km) – weather phenomena, pollution, smog.
• Stratosphere (~12–50 km) – ozone layer present.
• Mesosphere (~50–85 km)
• Thermosphere (~85–500 km)

2.2 Tropospheric Pollution

(A) Gaseous Pollutants

1. CO (carbon monoxide)
   • Produced from the incomplete combustion of fuel.
   • Forms carboxy-haemoglobin → reducing oxygen transport in human blood.
2. SO₂ (sulphur dioxide)
   • From fossil fuel combustion.
   • It causes acid rain and respiratory problems.
3. NO & NO₂ (oxides of nitrogen)
   • Lightning, fossil fuels combustion.
   • Contributes to photochemical smog and acid rain.
4. Hydrocarbons
   • Leakage of fuels, incomplete burning.
   • Carcinogenic; smog formation.
5. Particulate matter
   • PM₁₀ (respirable), PM₂.₅ (most dangerous).

2.3 Smog

(A) Classical Smog

• Winter smog, reducing smog, high SO₂ + smoke + fog.
• Cold, humid conditions.
• London-type smog.

(B) Photochemical Smog

• Occurs in summer, dry sunny weather.
• Formation: NOx + hydrocarbons + sunlight → O₃, PAN (peroxyacetyl nitrate), aldehydes.
• Oxidising smog, causes eye irritation and plant damage.
• PAN = most toxic component.

➡ Boards/NEET frequently ask: Differences between classical & photochemical smog.

Classical vs Photochemical Smog

2.4 Acid Rain

• Caused by H₂SO₄ + HNO₃ formation.
• pH < 5.6
• Effects: Corrosion, soil nutrient loss, aquatic life death, marble cancer (Taj Mahal).

Reactions:

SO₂ + O₂ → SO₃
SO₃ + H₂O → H₂SO₄
NO₂ + H₂O → HNO₃

2.5 Greenhouse Effect & Global Warming

• Major greenhouse gases: CO₂ > CH₄ > N₂O > CFCs > O₃.
• CO₂ contributes the maximum.
• Causes: Melting glaciers, sea level rise, heat waves.

Global Warming Potential (GWP) Order: CFCs/PFCs/SF₆ > N₂O > CH₄ > CO₂

➡ Question trend: Arrange gases by GWP or contribution.

2.6 Ozone Layer & Depletion

Important Points

• Ozone absorbs UV-B radiation.
• CFCs (freons) are major ozone-depleting substances.

Mechanism Flashcards

CF₂Cl₂ → CF₂Cl∙ + Cl∙
Cl∙ + O₃ → ClO∙ + O₂
ClO∙ + O → Cl∙ + O₂

➡ Catalytic destruction: 1 Cl radical destroys ~1 lakh O₃ molecules.

Montreal Protocol: International agreement (1987) to phase out CFCs.

3. WATER POLLUTION

3.1 Causes of Water Pollution

• Industrial discharge (Hg, Pb, Cd, Cr).
• Sewage.
• Agricultural runoff (nitrates + phosphates).
• Oil spills.
• Thermal pollution (hot water from power plants).

3.2 Indicators of Water Pollution

BOD (Biochemical Oxygen Demand)

• Oxygen is consumed by microbes in decomposing organic matter.
• Higher BOD = more polluted water.
• Clean water: BOD < 5 ppm.

COD (Chemical Oxygen Demand)

• Oxygen is required to oxidise organic matter chemically.

➡ Frequently asked: Difference between BOD & COD.

3.3 Eutrophication

• Excess nutrients (N, P) → algal bloom → oxygen depletion → fish death.
• Sources: Fertilisers, sewage.

3.4 Water-borne Diseases

• Cholera, Dysentery, Typhoid.

3.5 Heavy Metal Pollution

• Lead (Pb): Neurological impairment (especially in children), kidney damage, hypertension, cardiovascular issues.
• Minamata disease: Mercury poisoning.
• Itai-Itai disease: Cadmium poisoning.

➡ Hot Question: Match diseases with pollutants.

3.6 Methods of Water Treatment

• Sedimentation
• Flocculation / Coagulation
• Filtration
• Chlorination
• Ozonation
• Reverse Osmosis (RO)

4. SOIL POLLUTION

4.1 Sources

• Solid waste (plastics).
• Pesticides (DDT, BHC, aldrin).
• Industrial chemicals.
• Acid rain effects.

4.2 Pesticides

(A) Classified as:

• First-generation: Natural (nicotine, pyrethrin).
• Second-generation: Synthetic (DDT, organophosphates, carbamates).

(B) DDT

• Highly stable, non-biodegradable, and biomagnification.
• Persistent Organic Pollutant (POP).

(C) Biomagnification

• Increase in pollutant concentration at each trophic level.
• Highest in top predators.

➡ Most commonly asked: Why is DDT concentration highest in fish-eating birds?

5. SOLID WASTE MANAGEMENT

5.1 Types

• Municipal solid waste.
• Hazardous waste.
• Biomedical waste.
• e-waste.

5.2 Disposal Methods

• Landfills
• Incineration
• Composting
• Recycling

➡ Exam: Compare landfill vs incineration.

6. GREEN CHEMISTRY

6.1 Meaning

• Designing chemical processes/products to reduce or eliminate hazardous substances.

6.2 12 Principles of Green Chemistry

1. Prevent waste.
2. Atom economy.
3. Less hazardous synthesis.
4. Safer chemicals.
5. Safer solvents.
6. Energy efficiency.
7. Use renewable feedstocks.
8. Reduce derivatives.
9. Catalysis.
10. Design for degradation.
11. Real-time monitoring.
12. Inherently safe processes.

➡ Most important: Atom economy, safer solvents, catalysis.

6.3 Applications

• CO₂ used as blowing agent in polystyrene.
• Bio-diesel from vegetable oils.
• Green solvent: Supercritical CO₂, water.
• Photochemical reactions using visible light.
• Aqueous-phase reactions replacing harmful solvents.

➡ Direct NCERT lines often come from here.

7. VERY IMPORTANT QUESTIONS

7.1 Short Questions

• Define BOD and explain its significance.
• What is photochemical smog? Write its reactions.
• What are PAN and O₃ in smog?
• Write two effects of acid rain.
• Explain biomagnification with DDT example.
• What is the role of CFCs in ozone depletion?
• Define eutrophication.
• Differentiate between primary and secondary pollutants.
• What is green chemistry?

8. PREVIOUS YEAR QUESTIONS

Q1. What are the major causes of acid rain? Mention its effects.
Q2. Explain the formation of photochemical smog. Name its major components.
Q3. What is BOD? How does it indicate water pollution?
Q4. Why is DDT banned in many countries? Explain its effect via biomagnification.
Q5. Write two applications of green chemistry.
Q6. How do CFCs lead to ozone depletion? Give equations.
Q7. Distinguish between biodegradable and non-biodegradable pollutants.
Q8. What is global warming? Name two greenhouse gases.