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Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Environmental Chemistry covers pollution types, their causes and effects, and green chemistry — focus on understanding pollutant sources, their environmental impact, and control measures.
Key Concepts (Quick Recap)
Environmental Pollution — Types
| Type | Definition | Major Sources |
|---|
| Air pollution | Contamination of air by harmful substances | Industries, vehicles, power plants |
| Water pollution | Contamination of water bodies | Sewage, industrial waste, agricultural runoff |
| Soil pollution | Contamination of land by chemicals | Pesticides, industrial waste, improper disposal |
Air Pollution — Key Pollutants
| Pollutant | Source | Effect |
|---|
| CO (Carbon monoxide) | Incomplete combustion of fuels | Binds with haemoglobin, reduces O₂ transport — fatal in high concentration |
| CO₂ (Carbon dioxide) | Combustion of fossil fuels, deforestation | Greenhouse gas — global warming |
| SO₂ (Sulphur dioxide) | Burning of coal, smelting | Acid rain, respiratory problems, crop damage |
| NOₓ (Nitrogen oxides) | Vehicle exhausts, power plants | Acid rain, photochemical smog, respiratory issues |
| Particulate matter (SPM) | Industries, construction, dust | Respiratory diseases, visibility reduction |
| O₃ (Ozone, tropospheric) | Photochemical reaction of NOₓ and VOCs | Respiratory irritant, crop damage |
Smog
- Classical smog (London smog): SO₂ + fog + particulates → sulphuric acid droplets. Occurs in cool, humid conditions. Reducing in nature.
- Photochemical smog (Los Angeles smog): NOₓ + hydrocarbons + sunlight → O₃, PAN, aldehydes. Occurs in warm, sunny conditions. Oxidising in nature.
Acid Rain
- Normal rain pH ≈ 5.6 (slightly acidic due to dissolved CO₂)
- Acid rain pH < 5.6 (due to H₂SO₄ and HNO₃ from SO₂ and NOₓ)
- Effects: damages buildings (marble, limestone), kills aquatic life, damages crops, leaches soil nutrients, corrodes metals
Greenhouse Effect and Global Warming
| Greenhouse Gas | Source | Global Warming Potential |
|---|
| CO₂ | Fossil fuel combustion, deforestation | 1 (reference) |
| CH₄ (Methane) | Rice paddies, cattle, landfills | 25× CO₂ |
| N₂O (Nitrous oxide) | Fertilisers, industrial processes | 298× CO₂ |
| CFCs | Refrigerants, aerosol propellants | 4,000-10,000× CO₂ |
Effects of global warming: Rising sea levels, melting glaciers, extreme weather, loss of biodiversity, changing rainfall patterns
Ozone Depletion
- Stratospheric ozone (O₃): Protective layer that absorbs harmful UV radiation
- Ozone hole: Thinning of the ozone layer, especially over Antarctica
- Cause: CFCs (chlorofluorocarbons) release Cl atoms that catalyse O₃ destruction
- Mechanism: Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂ (Cl acts as catalyst)
- Effects of UV increase: Skin cancer, cataracts, weakened immune system, crop damage
- Montreal Protocol (1987): International agreement to phase out CFCs
Water Pollution — Key Parameters
| Parameter | Definition | Significance |
|---|
| BOD | Biochemical Oxygen Demand | Oxygen needed by microbes to decompose organic matter. High BOD = polluted |
| COD | Chemical Oxygen Demand | Oxygen needed for chemical oxidation of organic matter |
| DO | Dissolved Oxygen | Oxygen dissolved in water. Clean water: 6-8 mg/L |
| pH | Acidity/alkalinity | Clean water: pH 6.5-8.5 |
Eutrophication: Excessive nutrients (N, P) from fertilisers and sewage cause algal blooms → oxygen depletion → fish kills → water body degradation
Soil Pollution
- Causes: Pesticides, insecticides, industrial waste, improper waste disposal, mining
- Effects: Reduced soil fertility, contamination of groundwater, bioaccumulation in food chain
- Control: Organic farming, proper waste disposal, bioremediation, composting
Green Chemistry — 12 Principles
1. Prevention: Prevent waste rather than clean up 2. Atom economy: Maximise incorporation of all materials into the product 3. Less hazardous chemical synthesis: Use methods that generate less toxic substances 4. Designing safer chemicals: Minimise toxicity 5. Safer solvents and auxiliaries: Avoid toxic solvents 6. Design for energy efficiency: Minimise energy requirements 7. Use renewable feedstocks: Use renewable raw materials 8. Reduce derivatives: Minimise use of blocking groups and protecting agents 9. Catalysis: Use catalysts instead of stoichiometric reagents 10. Design for degradation: Products should break down into innocuous substances 11. Real-time analysis for pollution prevention: Monitor and control in real time 12. Inherently safer chemistry: Minimise potential for accidents
I. Choose the Best Answer (MCQs)
Q1. The gas responsible for global warming is (a) O₂ (b) N₂ (c) CO₂ (d) H₂
Answer(c) CO₂ — Carbon dioxide is the most significant greenhouse gas. It absorbs infrared radiation and traps heat in the atmosphere, causing global warming.
Q2. Acid rain is caused by (a) CO₂ and CO (b) SO₂ and NOₓ (c) O₃ and O₂ (d) CH₄ and CFCs
Answer(b) SO₂ and NOₓ — Sulphur dioxide (SO₂) reacts with water to form H₂SO₄, and nitrogen oxides (NOₓ) react with water to form HNO₃. These acids lower the pH of rain below 5.6, causing acid rain.
Q3. BOD stands for (a) Biological Oxygen Deficit (b) Biochemical Oxygen Demand (c) Biological Oxygen Demand (d) Biochemical Oxygen Deficit
Answer(b) Biochemical Oxygen Demand — BOD is the amount of dissolved oxygen required by microorganisms to break down organic matter in water. High BOD indicates polluted water.
Q4. Ozone depletion is mainly caused by (a) CO₂ (b) SO₂ (c) CFCs (d) NOₓ
Answer(c) CFCs — Chlorofluorocarbons (CFCs) release chlorine atoms in the stratosphere that catalyse the destruction of ozone. One Cl atom can destroy up to 100,000 O₃ molecules.
Q5. Which of the following is a secondary air pollutant? (a) CO (b) SO₂ (c) O₃ (tropospheric) (d) NO
Answer(c) O₃ (tropospheric) — Primary pollutants are emitted directly (CO, SO₂, NO). Secondary pollutants are formed by reactions of primary pollutants in the atmosphere. Tropospheric ozone is formed by photochemical reactions of NOₓ and hydrocarbons.
Q6. The pH of acid rain is (a) Above 7 (b) 5.6 (c) Below 5.6 (d) Exactly 7
Answer(c) Below 5.6 — Normal rain has pH ≈ 5.6 (slightly acidic due to dissolved CO₂). Acid rain has pH below 5.6 due to the presence of H₂SO₄ and HNO₃ from SO₂ and NOₓ.
II. Fill in the Blanks
1. The full form of BOD is Biochemical Oxygen Demand. 2. The pH of normal rainwater is approximately 5.6 (slightly acidic due to dissolved CO₂). 3. Acid rain has a pH below 5.6. 4. The main greenhouse gases are CO₂, CH₄, N₂O, CFCs, and water vapour. 5. Ozone depletion is mainly caused by CFCs (chlorofluorocarbons). 6. The Montreal Protocol was signed in 1987 to phase out CFCs. 7. Eutrophication is caused by excessive nitrogen and phosphorus (nutrients) in water bodies. 8. Photochemical smog is formed by the reaction of NOₓ and hydrocarbons in the presence of sunlight. 9. Classical smog is also called London smog or reducing smog. 10. Photochemical smog is also called Los Angeles smog or oxidising smog. 11. The ozone layer is found in the stratosphere (15-35 km above Earth). 12. Green chemistry aims to reduce or eliminate the use and generation of hazardous substances.
III. True or False (with corrections for false statements)
Q1. CO is a greenhouse gas.
AnswerFalse — Correction: CO (carbon monoxide) is NOT a greenhouse gas. It is an air pollutant that binds with haemoglobin and reduces oxygen transport. The greenhouse gases are CO₂, CH₄, N₂O, CFCs, and water vapour.
Q2. Acid rain damages marble buildings.
AnswerTrue — Acid rain (containing H₂SO₄ and HNO₃) reacts with marble (CaCO₃) to form CaSO₄ and Ca(NO₃)₂, which are soluble and wash away. This is called stone leprosy. CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O.
Q3. BOD is higher in clean water than in polluted water.
AnswerFalse — Correction: BOD is LOWER in clean water (typically < 5 mg/L) and HIGHER in polluted water. High BOD means more organic matter is present, which requires more oxygen to decompose.
Q4. Ozone in the troposphere is harmful.
AnswerTrue — Tropospheric (ground-level) ozone is a harmful pollutant that causes respiratory problems, crop damage, and is a component of photochemical smog. Stratospheric ozone (the ozone layer) is beneficial as it protects us from UV radiation.
Q5. Green chemistry promotes the use of non-renewable fossil fuels.
AnswerFalse — Correction: Green chemistry promotes the use of renewable feedstocks and sustainable processes. One of its 12 principles is to use renewable raw materials rather than depleting non-renewable resources like fossil fuels.
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Acid rain | (a) CFCs | (b) SO₂ and NOₓ |
| 2. Ozone depletion | (b) SO₂ and NOₓ | (a) CFCs |
| 3. Global warming | (c) Organic matter in water | (d) CO₂ and CH₄ |
| 4. BOD | (d) CO₂ and CH₄ | (c) Organic matter in water |
| 5. Photochemical smog | (e) Heavy metals in soil | (f) NOₓ + hydrocarbons + sunlight |
| 6. Eutrophication | (f) NOₓ + hydrocarbons + sunlight | (g) Excess N and P in water |
V. Answer Briefly (2-mark questions)
Q1. Distinguish between classical smog and photochemical smog.
Answer
| Feature | Classical Smog | Photochemical Smog | |---|---|---| | Also called | London smog | Los Angeles smog | | Formed from | SO₂ + fog + particulates | NOₓ + hydrocarbons + sunlight | | Nature | Reducing | Oxidising | | Occurs in | Cool, humid conditions | Warm, sunny conditions | | Colour | Grey | Brown | | Main components | H₂SO₄ droplets, soot | O₃, PAN, aldehydes |
Q2. What is eutrophication? How does it affect water bodies?
Answer
Eutrophication is the excessive enrichment of water bodies with nutrients (nitrogen and phosphorus) from fertilisers, sewage, and detergents. Effects: 1. Excessive growth of algae (algal blooms) 2. Algae block sunlight, preventing photosynthesis by submerged plants 3. When algae die, decomposers consume large amounts of oxygen 4. Oxygen depletion (low DO) kills fish and other aquatic organisms 5. Water becomes foul-smelling and unusable
Q3. What is the ozone hole? How is it formed?
Answer
The ozone hole is the thinning of the ozone layer in the stratosphere, particularly over Antarctica. Formation mechanism: 1. CFCs (chlorofluorocarbons) released from refrigerants and aerosols reach the stratosphere 2. UV radiation breaks CFCs, releasing Cl atoms 3. Cl atoms catalyse ozone destruction: Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂ 4. One Cl atom can destroy up to 100,000 ozone molecules 5. This causes the ozone layer to thin, allowing more UV radiation to reach Earth
Q4. What is green chemistry? State any two principles.
Answer
Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Two principles: 1. Prevention: It is better to prevent waste than to treat or clean up waste after it is formed 2. Atom economy: Synthetic methods should be designed to maximise the incorporation of all materials used in the process into the final product
VI. Answer in Detail (5-mark questions)
Q1. Discuss the causes, effects, and control measures of air pollution.
Answer
Causes of Air Pollution: 1. Industries: Thermal power plants (SO₂, fly ash), cement factories (dust), steel plants (CO, SO₂) 2. Vehicles: Petrol/diesel combustion produces CO, NOₓ, hydrocarbons, and particulate matter 3. Burning of fossil fuels: Coal in power plants, petroleum products in vehicles 4. Agricultural activities: Crop residue burning, use of chemical fertilisers (NH₃) 5. Natural sources: Volcanic eruptions, forest fires, dust storms Effects of Air Pollution: 1. On human health: Respiratory diseases (asthma, bronchitis), lung cancer, cardiovascular problems 2. On plants: Chlorosis, necrosis, reduced growth, crop damage 3. On materials: Corrosion of metals, damage to buildings (acid rain on marble) 4. On atmosphere: Global warming (CO₂), ozone depletion (CFCs), acid rain (SO₂, NOₓ), smog Control Measures: 1. Use of clean fuels: CNG, LPG, biofuels instead of coal and diesel 2. Catalytic converters: Convert CO and NOₓ to CO₂ and N₂ in vehicle exhausts 3. Electrostatic precipitators: Remove particulate matter from industrial emissions 4. Scrubbers: Remove SO₂ from industrial exhaust gases 5. Afforestation: Plant trees to absorb CO₂ and reduce air pollution 6. Legislation: Air (Prevention and Control of Pollution) Act, emission standards
Q2. Explain water pollution with its sources, effects, and control measures.
Answer
Sources of Water Pollution: 1. Industrial waste: Chemicals, heavy metals (Pb, Hg, Cd), thermal pollution 2. Domestic sewage: Organic waste, pathogens, detergents (phosphates) 3. Agricultural runoff: Fertilisers (nitrates, phosphates), pesticides, herbicides 4. Oil spills: Petroleum products in oceans, killing marine life 5. Thermal pollution: Hot water from power plants reduces DO in water Effects of Water Pollution: 1. Eutrophication: Excess nutrients cause algal blooms → oxygen depletion → fish kills 2. Biomagnification: Heavy metals and pesticides concentrate up the food chain 3. Waterborne diseases: Cholera, typhoid, dysentery from contaminated water 4. Loss of biodiversity: Death of aquatic organisms 5. Groundwater contamination: Nitrates, heavy metals leach into aquifers Control Measures: 1. Sewage treatment: Primary (screening, sedimentation), secondary (biological), tertiary (chemical treatment) 2. Effluent treatment: Industrial waste must be treated before discharge 3. Reduce pesticide/fertiliser use: Organic farming, integrated pest management 4. Rainwater harvesting: Reduces dependence on contaminated water sources 5. Legislation: Water (Prevention and Control of Pollution) Act 6. Public awareness: Proper disposal of waste, avoid pouring chemicals down drains
Q3. What is green chemistry? Explain its significance with examples.
Answer
Green Chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It aims to make chemistry sustainable and environmentally friendly. Key Principles (12 Principles of Green Chemistry): 1. Prevention of waste 2. Atom economy 3. Less hazardous chemical synthesis 4. Designing safer chemicals 5. Safer solvents and auxiliaries 6. Design for energy efficiency 7. Use of renewable feedstocks 8. Reduce derivatives 9. Catalysis 10. Design for degradation 11. Real-time analysis for pollution prevention 12. Inherently safer chemistry for accident prevention Examples of Green Chemistry in Practice: 1. Dry cleaning with liquid CO₂: Replaces toxic perchloroethylene (C₂Cl₄) with liquid CO₂ as a solvent 2. Synthesis of ibuprofen: Reduced from 6 steps with large waste to 3 steps with high atom economy (95% atom economy) 3. Catalytic hydrogenation: Using H₂ and catalysts instead of hazardous reducing agents 4. Biodegradable plastics: Polylactic acid (PLA) from corn starch instead of petroleum-based plastics 5. Water-based paints: Replacing organic solvent-based paints with water-based alternatives Significance: Green chemistry reduces pollution at the source (rather than cleaning up after), saves costs by reducing waste, protects human health and the environment, and promotes sustainable development.
Common Mistakes to Avoid
- Confusing BOD and DO: BOD (Biochemical Oxygen Demand) is HIGH in polluted water (more oxygen needed to decompose waste). DO (Dissolved Oxygen) is LOW in polluted water (less oxygen available for aquatic life). High BOD = polluted; High DO = clean.
- Confusing stratospheric and tropospheric ozone: Stratospheric ozone = GOOD (protects from UV). Tropospheric ozone = BAD (pollutant, causes respiratory problems). These are completely different contexts.
- Confusing classical smog and photochemical smog: Classical smog = SO₂ + fog, reducing, winter, London. Photochemical smog = NOₓ + hydrocarbons + sunlight, oxidising, summer, Los Angeles.
- Calling CO a greenhouse gas: CO (carbon monoxide) is an air pollutant, NOT a greenhouse gas. The greenhouse gases are CO₂, CH₄, N₂O, CFCs, and H₂O.
- Forgetting that acid rain pH is below 5.6: Normal rain is already slightly acidic (pH 5.6) due to dissolved CO₂ forming carbonic acid. Acid rain has pH below 5.6 due to H₂SO₄ and HNO₃.
- Confusing biomagnification and bioaccumulation: Bioaccumulation = accumulation of a substance in one organism over time. Biomagnification = increasing concentration up the food chain (e.g., DDT in birds of prey).
Exam Checklist
- [ ] Can you list the major air pollutants and their sources and effects?
- [ ] Can you distinguish between classical smog and photochemical smog?
- [ ] Can you explain acid rain — causes, mechanism, and effects?
- [ ] Can you explain the greenhouse effect and list greenhouse gases?
- [ ] Can you explain ozone depletion — causes (CFCs), mechanism, and effects?
- [ ] Can you define BOD, COD, and DO and explain their significance?
- [ ] Can you explain eutrophication — causes, mechanism, and effects?
- [ ] Can you list the 12 principles of green chemistry?
- [ ] Can you give examples of green chemistry in practice?
- [ ] Can you discuss control measures for air and water pollution?
