Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Environmental Chemistry studies the chemical processes occurring in the environment and the impact of human activities on them. This chapter connects chemistry to real-world issues — air pollution, water contamination, soil degradation, ozone depletion, global warming, and acid rain. It also introduces green chemistry as a sustainable approach. This chapter is important for CBSE exams and often appears as short-answer and case-study questions. Understanding the chemical reactions behind environmental phenomena is essential for scoring well.
Key Definitions
| Term | Definition |
|---|---|
| Environmental pollution | Undesirable changes in the physical, chemical, or biological characteristics of air, water, or land that harmfully affect human life, other organisms, or the environment |
| Pollutant | A substance present in the environment in greater than natural abundance that causes adverse effects |
| Contaminant | A substance that does not occur in nature but is introduced by human activity |
| Source | The place or process from which a pollutant originates |
| Sink | The medium that removes or stores a pollutant from the environment |
| Receptor | The organism or system affected by the pollutant |
| BOD | Biochemical Oxygen Demand — the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water at 20°C over 5 days |
| COD | Chemical Oxygen Demand — the total oxygen required to oxidise all organic matter chemically (biodegradable + non-biodegradable) |
| Eutrophication | Nutrient enrichment of water bodies (especially N and P) leading to excessive algal growth and oxygen depletion |
| Greenhouse effect | Warming of Earth's surface due to atmospheric gases that trap infrared radiation |
| Acid rain | Rain with pH below 5.0, caused by SO₂ and NOₓ reacting with atmospheric water |
| Green chemistry | Design of chemical processes that minimise or eliminate the use and generation of hazardous substances |
| Atom economy | Measure of how efficiently reactants are incorporated into the final product; % atom economy = (MW of desired product / MW of all products) × 100 |
Atmospheric Pollution
Tropospheric Pollution
The troposphere (0–11 km) contains most of the air mass. Tropospheric pollutants include:
Gaseous Pollutants
| Pollutant | Major Sources | Effects |
|---|---|---|
| CO | Incomplete combustion of fossil fuels, automobile exhaust | Binds with haemoglobin (carboxyhaemoglobin), reducing O₂ transport; fatal at high concentrations |
| SO₂ | Fossil fuel combustion (coal), smelting, volcanic eruptions | Respiratory irritant; causes bronchitis, asthma; precursor to acid rain |
| NOₓ (NO, NO₂) | Automobile exhaust, power plants, lightning | Respiratory irritant; precursor to acid rain and photochemical smog; forms HNO₃ |
| Hydrocarbons | Incomplete combustion, petroleum refining, vehicle exhaust | Some are carcinogenic; precursors to photochemical smog |
| O₃ (tropospheric) | Formed from NOₓ + VOCs + sunlight | Respiratory irritant; damages crops and vegetation |
Particulate Matter
| Type | Size | Examples | Sources |
|---|---|---|---|
| Smoke | 0.5–5 μm | Carbon particles | Combustion of coal, wood |
| Dust | 1–100 μm | Mineral particles | Construction, mining, wind erosion |
| Fumes | 0.1–1 μm | Metallic oxide particles | Metallurgical processes |
| Mist | 0.1–10 μm | Liquid droplets | Spray, condensation |
| Smog | — | Mixture of smoke + fog | Vehicular and industrial emissions |
Types of smog:
| Type | Composition | Conditions | Location |
|---|---|---|---|
| Classical (reducing) smog | Smoke + SO₂ + fog | Cool, humid climate | London-type |
| Photochemical (oxidising) smog | NOₓ + VOCs + O₃ + PAN + sunlight | Warm, sunny climate | Los Angeles-type |
Photochemical smog formation: 1. NO₂ + hv (λ < 400 nm) → NO + O• 2. O• + O₂ → O₃ 3. O₃ + hydrocarbons → PAN (peroxyacyl nitrates), aldehydes 4. PAN is a powerful lachrymator (tear-inducing) and damages vegetation
Global Warming and Greenhouse Effect
The greenhouse effect is essential for life on Earth (without it, average temperature would be −18°C instead of +15°C). Enhanced greenhouse effect from excess gases is causing global warming.
Greenhouse Gases
| Gas | Sources | Global Warming Potential (GWP) | Atmospheric Lifetime |
|---|---|---|---|
| CO₂ | Fossil fuel combustion, deforestation | 1 (reference) | 50–200 years |
| CH₄ | Rice paddies, cattle, landfills, natural gas leaks | 28 | 12 years |
| N₂O | Fertilisers, industrial processes | 265 | 114 years |
| CFCs | Refrigerants, aerosols, foam blowing agents | 4,750–14,000 | 45–100 years |
Key greenhouse effect reactions:
- CO₂ + H₂O (photosynthesis) ← balanced naturally, but excess CO₂ overwhelms the sink
- CH₄ is 28 times more potent than CO₂ per molecule but exists in lower concentrations
- CFCs are the most potent greenhouse gases per molecule but exist in very small concentrations
Consequences of global warming: 1. Rise in sea level (thermal expansion + melting ice caps) 2. Changes in rainfall patterns (droughts and floods) 3. Loss of biodiversity and habitat 4. Increased frequency of extreme weather events 5. Spread of tropical diseases to temperate regions 6. Impact on agriculture and food security
Acid Rain
Acid rain is rain with pH below 5.0 (normal rain pH ≈ 5.6 due to dissolved CO₂).
Formation reactions:
From sulphur dioxide:
- 2SO₂(g) + O₂(g) → 2SO₃(g)
- SO₃(g) + H₂O(l) → H₂SO₄(aq)
From nitrogen oxides:
- 2NO₂(g) + H₂O(l) → HNO₃(aq) + HNO₂(aq)
- 4NO₂(g) + O₂(g) + 2H₂O(l) → 4HNO₃(aq)
Effects of acid rain:
| Area Affected | Effect |
|---|---|
| Buildings and monuments | Corrosion of marble: CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O |
| Aquatic life | Lowers pH of lakes/rivers; fish eggs fail to hatch; aluminium leaching toxic to fish |
| Soil | Leaches essential nutrients (Ca²⁺, Mg²⁺, K⁺); mobilises heavy metals |
| Plants | Damages leaves (chlorosis, necrosis); reduces photosynthesis |
| Human health | Respiratory problems; skin irritation |
Control measures:
- Use low-sulphur coal or natural gas
- Install scrubbers in power plants (limestone slurry absorbs SO₂)
- Catalytic converters in vehicles (reduce NOₓ emissions)
- Liming of acidified lakes (CaCO₃ neutralises acid)
Stratospheric Pollution: Ozone Depletion
The ozone layer (stratosphere, 15–35 km) shields Earth from harmful UV radiation (200–315 nm).
Formation of ozone:
- O₂ + hv (λ < 240 nm) → 2O• (photodissociation)
- O• + O₂ → O₃ (ozone formation)
Natural destruction:
- O₃ + hv → O₂ + O•
- O₃ + O• → 2O₂
CFC-mediated destruction (catalytic cycle):
Step 1: CFCl₃ + hv → CFCl₂ + Cl• (UV breaks C—Cl bond in stratosphere)
Step 2: Cl• + O₃ → ClO• + O₂ (chlorine destroys ozone)
Step 3: ClO• + O• → Cl• + O₂ (chlorine regenerated)
Net reaction: O₃ + O• → 2O₂ (single Cl atom destroys ~100,000 O₃ molecules)
Ozone hole: Observed over Antarctica, especially during spring (September–October). Extreme cold (−85°C) leads to formation of polar stratospheric clouds (PSCs), which provide surfaces for heterogeneous reactions that convert inactive chlorine reservoirs (HCl, ClONO₂) into active Cl₂.
Montreal Protocol (1987): International agreement to phase out CFC production. Replaced by HCFCs (less ozone-depleting) and HFCs (no ozone-depleting potential, but some are greenhouse gases).
Water Pollution
Sources and Types
| Pollutant Type | Examples | Sources |
|---|---|---|
| Pathogens | Bacteria, viruses, parasites | Domestic sewage, animal waste |
| Organic wastes | Sewage, food processing waste | Domestic and industrial discharge |
| Chemical pollutants | Heavy metals (Pb, Hg, Cd), pesticides | Industrial effluents, agricultural runoff |
| Nutrients | Nitrates, phosphates | Fertiliser runoff, detergents |
| Thermal pollutants | Heated water | Power plant cooling systems |
| Radioactive wastes | Uranium, caesium | Nuclear power plants, medical waste |
Eutrophication
Eutrophication is the excessive enrichment of water bodies with nutrients (especially N and P), leading to:
1. Excessive growth of algae and aquatic plants (algal bloom) 2. Depletion of dissolved oxygen as algae die and decompose 3. Death of fish and other aquatic organisms 4. Discolouration and foul smell of water 5. Loss of biodiversity
Water Quality Parameters
| Parameter | Definition | Clean Water Value | Polluted Water Value |
|---|---|---|---|
| BOD | Oxygen required by microorganisms to decompose organic matter in 5 days at 20°C | < 5 ppm | > 5 ppm (severely polluted: > 20 ppm) |
| COD | Total oxygen required to oxidise all organic matter chemically (KMnO₄ or K₂Cr₂O₇) | < 10 ppm | > 10 ppm |
| DO | Dissolved oxygen concentration | 6–8 ppm | < 4 ppm (stressful for fish) |
| pH | Acidity/alkalinity | 6.5–8.5 | Outside 6.5–8.5 |
BOD vs COD:
| Feature | BOD | COD |
|---|---|---|
| Measures | Biodegradable organic matter only | Biodegradable + non-biodegradable organic matter |
| Method | Biological (microorganisms) | Chemical (strong oxidising agent) |
| Time | 5 days | 2–3 hours |
| Value | Always ≤ COD | Always ≥ BOD |
| Use | Municipal wastewater | Industrial wastewater (often toxic to microorganisms) |
Soil Pollution
Sources
| Source | Pollutant | Effect |
|---|---|---|
| Pesticides | DDT, BHC, malathion, aldrin | Bioaccumulation in food chains; toxicity to non-target organisms |
| Fertilisers | Nitrates, phosphates | Soil acidification; eutrophication of nearby water |
| Industrial waste | Heavy metals (Pb, Hg, Cd, As) | Toxic to soil organisms and plants; enter food chain |
| Biomedical waste | Pathogens, pharmaceuticals | Disease transmission; antibiotic resistance |
| Plastics | Polythene, PVC | Non-biodegradable; persist for hundreds of years |
| Electronic waste | Lead, mercury, cadmium | Leach into groundwater |
Bioaccumulation and biomagnification:
- Bioaccumulation: Increase in concentration of a substance within a single organism over time
- Biomagnification: Increase in concentration of a substance at each trophic level in a food chain
Example: DDT concentration increases from water (0.003 ppb) → phytoplankton → zooplankton → small fish → large fish → fish-eating birds (25 ppm) — a magnification of about 10 million times.
Green Chemistry
Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It is not about cleaning up pollution after it occurs, but about preventing it at the source.
The 12 Principles of Green Chemistry
| No. | Principle | Explanation | Example |
|---|---|---|---|
| 1 | Prevention | Prevent waste rather than treat or clean it up | Design synthesis to minimise by-products |
| 2 | Atom economy | Maximise incorporation of all reactant atoms into the final product | Addition reactions have higher atom economy than substitution |
| 3 | Less hazardous syntheses | Use methods that generate less toxic substances | Use H₂O₂ instead of Cl₂ for bleaching |
| 4 | Designing safer chemicals | Design chemicals that are effective but have reduced toxicity | Design pesticides that target specific pests |
| 5 | Safer solvents and auxiliaries | Avoid toxic solvents; use water, supercritical CO₂, or solvent-free methods | Use water-based paints instead of organic solvents |
| 6 | Design for energy efficiency | Minimise energy requirements; run reactions at ambient T and P | Use catalysts to lower activation energy |
| 7 | Use renewable feedstocks | Use raw materials that are renewable rather than depletable | Use biomass instead of petroleum |
| 8 | Reduce derivatives | Avoid unnecessary protecting groups and functional group interconversions | Direct synthesis rather than multi-step protection/deprotection |
| 9 | Catalysis | Use catalytic rather than stoichiometric reagents | Use enzymes or metal catalysts |
| 10 | Design for degradation | Design chemicals that break down into harmless products | Biodegradable plastics |
| 11 | Real-time analysis | Monitor and control processes in real time to prevent pollution | In-line sensors for process control |
| 12 | Inherently safer chemistry | Choose substances that minimise risk of accidents | Use less volatile, less flammable solvents |
Atom Economy
| Reaction Type | Atom Economy | Reason |
|---|---|---|
| Addition reaction | 100% | All atoms of reactants appear in the product |
| Rearrangement | 100% | Atoms are reorganised, none lost |
| Substitution | < 100% | A leaving group is expelled |
| Elimination | < 100% | A small molecule is lost |
Example: Synthesis of ibuprofen
- Old process: 6 steps, 40% atom economy, large waste
- Green process: 3 steps, 77% atom economy (BHC Company process), much less waste
Greener Alternatives in Practice
| Conventional Method | Greener Alternative | Advantage |
|---|---|---|
| Dry cleaning with perchloroethylene (C₂Cl₄) | Supercritical CO₂ cleaning | Non-toxic, no ozone depletion |
| Bleaching paper with Cl₂ | Bleaching with H₂O₂ | No chlorinated dioxins formed |
| Petrol-powered vehicles | CNG (compressed natural gas) or electric | Lower SO₂, NOₓ, and particulate emissions |
| Lead-based paints | Water-based paints | Eliminates lead exposure |
| Stoichiometric oxidising agents | Catalytic oxidation with H₂O₂ | Less waste, milder conditions |
| Organic solvents in synthesis | Solvent-free or aqueous synthesis | No VOC emissions |
Strategies for Pollution Control
Air Pollution Control
| Strategy | How It Works | Target Pollutant |
|---|---|---|
| Catalytic converters | Pt/Pd/Rh catalysts convert CO → CO₂, NOₓ → N₂, unburnt HC → CO₂ + H₂O | CO, NOₓ, hydrocarbons |
| Scrubbers | Limestone (CaCO₃) slurry absorbs SO₂ in exhaust gas | SO₂ |
| Electrostatic precipitators | Electrically charged plates remove particulate matter | Fly ash, dust |
| Unleaded petrol | Eliminates Pb emissions | Lead |
| CNG/LPG | Cleaner combustion than petrol/diesel | CO, NOₓ, particulates |
Water Pollution Control
| Strategy | Description |
|---|---|
| Primary treatment | Physical screening and sedimentation to remove suspended solids |
| Secondary treatment | Biological oxidation using aerobic bacteria to reduce BOD |
| Tertiary treatment | Chemical treatment (coagulation, filtration, disinfection) to remove dissolved pollutants |
| Effluent treatment plants (ETP) | Treat industrial wastewater before discharge |
| Sewage treatment plants (STP) | Treat domestic wastewater |
| Rainwater harvesting | Recharge groundwater; reduce urban runoff |
Soil Pollution Control
| Strategy | Description |
|---|---|
| Bioremediation | Use microorganisms to degrade organic pollutants in soil |
| Phytoremediation | Use plants to absorb, accumulate, or degrade contaminants |
| Composting | Convert organic waste into nutrient-rich soil amendment |
| Reducing pesticide use | Integrated pest management (IPM); organic farming |
| Proper waste disposal | Segregation at source; recycling; safe landfill design |
Common Mistakes to Avoid
1. Confusing BOD and COD: BOD measures only biodegradable organic matter (biological process, 5 days). COD measures total organic matter including non-biodegradable (chemical process, hours). COD ≥ BOD always. 2. Attributing ozone depletion to CO₂: CO₂ does NOT deplete ozone. CFCs and halons are the ozone-depleting substances. CO₂ causes global warming, not ozone depletion. 3. Normal rain pH: Normal rain is slightly acidic (pH ≈ 5.6) due to dissolved CO₂ forming carbonic acid. Acid rain is defined as pH < 5.0, not pH < 7. 4. CFC mechanism: Remember the catalytic cycle — one Cl atom destroys ~100,000 O₃ molecules. Write the full cycle: Cl• + O₃ → ClO• + O₂; ClO• + O• → Cl• + O₂. 5. Green chemistry is NOT environmental chemistry: Green chemistry is about designing processes to prevent pollution at the source. Environmental chemistry studies pollution that has already occurred. 6. Photochemical smog vs classical smog: Classical smog (London-type) = smoke + SO₂ + fog (reducing). Photochemical smog (LA-type) = NOₓ + VOCs + sunlight (oxidising, contains O₃ and PAN). 7. Atom economy calculation: Use the sum of molecular weights of ALL products in the denominator, not just reactants. For addition reactions, atom economy = 100%.
Exam Tips
- Most-asked questions: Acid rain formation (2–3 marks), ozone depletion mechanism (3 marks), greenhouse gases and GWP (2 marks), BOD vs COD (2 marks), green chemistry principles (3 marks), eutrophication (2 marks)
- Acid rain equations: Practise writing the SO₂ → H₂SO₄ and NOₓ → HNO₃ reaction sequences
- Ozone depletion cycle: Write the catalytic cycle with Cl• as the catalyst. Net: O₃ + O → 2O₂
- BOD and COD: Know the definitions, differences, and typical values. Clean water: BOD < 5 ppm; polluted water: BOD > 5 ppm
- Green chemistry: At least 4–5 of the 12 principles should be memorised with examples. Atom economy formula is frequently asked.
- Montreal Protocol: Know the year (1987) and that it phased out CFCs
- Biomagnification: Explain with the DDT example — concentration increases at each trophic level
Quick Revision Notes
Atmospheric Pollution
- Tropospheric: CO, SO₂, NOₓ, hydrocarbons, particulates, smog
- Classical smog: reducing, smoke + SO₂ + fog (London-type)
- Photochemical smog: oxidising, NOₓ + VOCs + sunlight + O₃ + PAN (LA-type)
Global Warming
- Greenhouse gases: CO₂ (GWP = 1), CH₄ (GWP = 28), N₂O (GWP = 265), CFCs (GWP = 4750–14000)
- Greenhouse effect: gases absorb and re-emit infrared radiation from Earth
- Consequences: sea level rise, climate change, biodiversity loss
Acid Rain
- Caused by SO₂ → H₂SO₄ and NOₓ → HNO₃
- Normal rain pH ≈ 5.6; acid rain pH < 5.0
- Effects: marble corrosion, lake acidification, soil leaching, plant damage
- Control: scrubbers, catalytic converters, low-sulphur fuels
Ozone Depletion
- Ozone layer: 15–35 km altitude, absorbs UV-B (200–315 nm)
- CFCs release Cl• which catalytically destroys O₃
- One Cl atom destroys ~100,000 O₃ molecules
- Montreal Protocol (1987): phased out CFCs
- Ozone hole: observed over Antarctica during spring
Water Pollution
- BOD: biological, 5 days, biodegradable organics only (< 5 ppm = clean)
- COD: chemical, hours, all organics (≥ BOD always)
- Eutrophication: nutrient enrichment → algal bloom → O₂ depletion → fish kill
- Biomagnification: pollutant concentration increases at each trophic level
Soil Pollution
- Sources: pesticides, heavy metals, biomedical waste, plastics
- DDT biomagnification: concentration increases up the food chain
Green Chemistry
- 12 principles: prevention, atom economy, safer synthesis, safer chemicals, safer solvents, energy efficiency, renewable feedstocks, reduce derivatives, catalysis, design for degradation, real-time analysis, safer chemistry
- Atom economy = (MW desired product / MW all products) × 100
- Addition reactions = 100% atom economy