Environmental Chemistry
Welcome to HSLC Guru. This chapter on Environmental Chemistry for ASSEB Class 11 Chemistry explores the chemistry of our environment, the various pollutants affecting air, water and soil, the resulting hazards such as smog, acid rain, ozone depletion and global warming, and the principles of green chemistry that aim to make industrial processes environmentally benign. Understanding environmental chemistry helps us appreciate sustainable practices and the need to preserve nature.
Chapter Summary
Environmental Pollution and the Atmosphere: Environmental chemistry deals with the chemical phenomena occurring in the environment. Environmental pollution is the contamination of air, water, or soil by undesirable substances called pollutants which cause harm to living organisms. Pollutants may be biodegradable (like domestic waste) or non-biodegradable (like plastics, DDT, heavy metals). The atmosphere is divided into the troposphere, stratosphere, mesosphere, and thermosphere. Tropospheric pollution arises from gaseous pollutants such as oxides of sulphur (SO2, SO3) which produce acid rain; oxides of nitrogen (NO, NO2) released by lightning and combustion engines; oxides of carbon — CO (toxic, binds to haemoglobin) and CO2 (greenhouse gas); and hydrocarbons which are carcinogenic. Smog is of two types: classical (London) smog formed in cool humid climates from SO2 and smoke (reducing in nature); and photochemical (Los Angeles) smog formed in warm dry sunny weather from NO2 and hydrocarbons producing ozone, PAN (peroxyacetyl nitrate), and aldehydes (oxidising in nature).
Acid Rain, Stratospheric Pollution and Ozone Depletion: When the pH of rainwater falls below 5.6 due to dissolved SO2, NO2 and CO2, it is termed acid rain. Acid rain corrodes buildings (especially marble structures like the Taj Mahal — the “stone leprosy”), destroys aquatic life, and damages forests and crops. In the stratosphere, the ozone layer (15–35 km above earth) acts as a protective shield by absorbing harmful UV radiation. Chlorofluorocarbons (CFCs or freons) released from refrigerators, air conditioners and aerosol sprays migrate to the stratosphere where UV radiation breaks them down to release Cl free radicals which destroy ozone catalytically. The Antarctic ozone hole was discovered in 1985. To curb CFC emissions, the Montreal Protocol was signed in 1987, restricting the production and use of ozone depleting substances. The greenhouse effect is the trapping of infrared radiation by greenhouse gases (CO2, CH4, water vapour, N2O, CFCs and ozone) leading to a rise in global temperature called global warming. This causes melting of polar ice caps, rise in sea level, climate change and spread of infectious diseases.
Water and Soil Pollution: Water pollution is caused by pathogens (bacteria, viruses), organic wastes (sewage, agricultural runoff), chemical pollutants (heavy metals, detergents, pesticides) and industrial effluents. Biochemical Oxygen Demand (BOD) is the amount of oxygen required by bacteria to decompose organic matter in water; clean water has BOD < 5 ppm while polluted water has BOD > 17 ppm. Eutrophication is the over-enrichment of water bodies by nutrients (especially phosphates and nitrates) leading to excessive growth of algae which depletes dissolved oxygen and kills aquatic life. Potable water (fit for drinking) must be free from suspended impurities, harmful bacteria and dissolved salts; the WHO permits a maximum of 500 ppm of dissolved solids and Dissolved Oxygen (DO) above 6 ppm. Chemical Oxygen Demand (COD) measures the total amount of oxygen needed to chemically oxidise both biodegradable and non-biodegradable pollutants. Soil pollution arises mainly from excessive use of pesticides (DDT, BHC, aldrin), herbicides, and chemical fertilisers (nitrates, phosphates) which accumulate in soil, percolate to groundwater and enter the food chain through bioaccumulation.
Industrial Waste, Control Strategies and Green Chemistry: Industrial wastes are categorised into biodegradable and non-biodegradable types. Hazardous industrial waste includes radioactive substances, heavy metals (Hg, Pb, Cd) and toxic chemicals which require special disposal techniques such as landfilling, incineration, recycling and bioremediation. Strategies to control environmental pollution include the 3R approach (Reduce, Reuse, Recycle), use of cleaner fuels (CNG, LPG), catalytic converters in vehicles, scrubbers and electrostatic precipitators in industries, treatment of sewage before discharge, and afforestation. Green chemistry is the design of chemical processes that minimise the use and generation of hazardous substances. Important principles include atom economy (maximum incorporation of starting materials into the final product), use of safer solvents (e.g., liquid CO2 in dry-cleaning replacing tetrachloroethene), greener bleaching agents (H2O2 in place of chlorine in paper bleaching), and the synthesis of biodegradable products. Green chemistry not only protects the environment but also enhances economic efficiency.
1-Mark Questions
Q1. What is environmental pollution?
Answer: Environmental pollution is the contamination of air, water or soil with harmful substances called pollutants that adversely affect living organisms.
Q2. Define a pollutant.
Answer: A pollutant is any substance present in the environment in concentrations greater than its natural abundance which causes harm to living organisms or the ecosystem.
Q3. Which gas causes the depletion of the ozone layer?
Answer: Chlorofluorocarbons (CFCs or freons) cause the depletion of the ozone layer.
Q4. What is the pH of normal rainwater?
Answer: The pH of normal rainwater is approximately 5.6 due to dissolved CO2.
Q5. Expand BOD.
Answer: BOD stands for Biochemical Oxygen Demand.
Q6. Name two greenhouse gases.
Answer: Carbon dioxide (CO2) and methane (CH4) are two important greenhouse gases.
Q7. What is potable water?
Answer: Potable water is water that is fit for human consumption, free from harmful microorganisms and excessive dissolved impurities.
Q8. When was the Montreal Protocol signed?
Answer: The Montreal Protocol was signed in 1987.
Q9. What is the major pollutant responsible for photochemical smog?
Answer: Oxides of nitrogen (NO2) and unburnt hydrocarbons are the major pollutants responsible for photochemical smog.
Q10. Define green chemistry.
Answer: Green chemistry is the design of chemical processes and products that minimise the generation and use of hazardous substances.
Q11. What is photochemical smog also known as?
Answer: Photochemical smog is also known as Los Angeles smog or oxidising smog.
Q12. Name a non-biodegradable pollutant.
Answer: DDT (dichlorodiphenyltrichloroethane) and plastics are examples of non-biodegradable pollutants.
2-3 Mark Questions
Q1. Distinguish between classical smog and photochemical smog.
Answer: Classical (London) smog occurs in cool humid climates and is a mixture of smoke, fog and SO2; it is reducing in nature. Photochemical (Los Angeles) smog occurs in warm dry sunny climates and contains NO2, ozone and PAN formed by sunlight acting on hydrocarbons and nitrogen oxides; it is oxidising in nature.
Q2. Explain how acid rain is formed and list two of its harmful effects.
Answer: Acid rain forms when oxides of sulphur and nitrogen released from burning of fossil fuels react with atmospheric moisture to form sulphuric acid and nitric acid. The reactions are: 2SO2 + O2 + 2H2O to 2H2SO4; 4NO2 + O2 + 2H2O to 4HNO3. Harmful effects include corrosion of marble buildings (stone leprosy of Taj Mahal) and acidification of lakes killing aquatic life.
Q3. What is eutrophication and how is it caused?
Answer: Eutrophication is the over-enrichment of a water body with plant nutrients (especially nitrates and phosphates) from agricultural runoff and detergents. It causes excessive algal growth (algal bloom). When the algae die, their decomposition consumes dissolved oxygen, which kills fish and other aquatic organisms, leading to the death of the water body.
Q4. Why is carbon monoxide a serious pollutant?
Answer: Carbon monoxide is highly poisonous because it binds to haemoglobin about 300 times more strongly than oxygen forming carboxyhaemoglobin, which reduces the oxygen carrying capacity of blood and may cause headache, dizziness, suffocation and even death.
Q5. What is the greenhouse effect? Name the main gases responsible.
Answer: The greenhouse effect is the warming of the Earth’s surface caused by trapping of infrared radiation by certain atmospheric gases. Major greenhouse gases include carbon dioxide (CO2), methane (CH4), water vapour, nitrous oxide (N2O), CFCs and ozone (O3).
Q6. Define BOD and COD with their significance.
Answer: BOD (Biochemical Oxygen Demand) is the oxygen required by microorganisms to oxidise biodegradable organic matter in water. COD (Chemical Oxygen Demand) is the total oxygen needed to chemically oxidise both biodegradable and non-biodegradable matter. Both are indicators of water pollution: higher values indicate greater pollution levels.
Q7. What is meant by atom economy in green chemistry?
Answer: Atom economy is the measure of the proportion of reactant atoms that end up in the desired product. A reaction with high atom economy generates less waste and is preferred in green chemistry. It is calculated as the molecular mass of the desired product divided by the total molecular mass of all products, multiplied by 100.
5-7 Mark Questions
Q1. Discuss in detail the causes, mechanism and consequences of ozone layer depletion. What measures have been taken to prevent it?
Answer: The ozone layer in the stratosphere (15–35 km altitude) absorbs harmful UV radiation, protecting living organisms. Ozone depletion is caused mainly by chlorofluorocarbons (CFCs), used as refrigerants, propellants and foam blowing agents. CFCs are inert in the troposphere and slowly migrate to the stratosphere. Mechanism: UV radiation in the stratosphere photolyses CFCs releasing chlorine free radicals: CF2Cl2 + hv to Cl(dot) + CF2Cl(dot). The Cl free radical reacts with ozone: Cl(dot) + O3 to ClO(dot) + O2. The ClO(dot) then reacts with atomic oxygen: ClO(dot) + O to Cl(dot) + O2. The chlorine radical is regenerated and continues to destroy ozone in a chain reaction; one Cl atom can destroy thousands of ozone molecules. Consequences: increased UV radiation reaching Earth, causing skin cancer, cataracts, weakened immunity, reduced crop yields, and harm to plankton in oceans. Preventive measures: the Montreal Protocol (1987) bans the production and consumption of CFCs; alternatives like HFCs and HCFCs are being used; awareness campaigns and recycling of old refrigerants help reduce emissions.
Q2. Explain the various sources of water pollution and discuss strategies to control them.
Answer: Major sources of water pollution include: (i) Domestic sewage rich in organic matter and pathogens. (ii) Industrial effluents containing heavy metals (Hg, Pb, Cd, As), acids, alkalis and toxic organic compounds. (iii) Agricultural runoff carrying pesticides (DDT, BHC) and fertilisers (nitrates, phosphates). (iv) Thermal pollution from power plants raising water temperature and reducing dissolved oxygen. (v) Oil spills from tankers. (vi) Radioactive waste from nuclear plants. Effects: spread of waterborne diseases (cholera, typhoid, dysentery), eutrophication, fish kills, bioaccumulation of toxins. Control strategies: (a) sewage treatment plants for primary, secondary and tertiary treatment before discharge; (b) installation of effluent treatment plants in industries; (c) restricted use of pesticides and adoption of biopesticides and integrated pest management; (d) buffer zones near water bodies to filter runoff; (e) public awareness about water conservation; (f) strict enforcement of pollution control laws and water quality standards.
Q3. What is global warming? Discuss its causes, effects and remedies.
Answer: Global warming is the gradual increase in Earth’s average surface temperature caused by the enhanced greenhouse effect. Greenhouse gases (CO2, CH4, N2O, water vapour, CFCs, O3) trap outgoing infrared radiation from Earth’s surface, raising temperature. Causes: (i) burning of fossil fuels (coal, petrol, diesel) emits CO2; (ii) deforestation reduces CO2 absorption; (iii) agriculture and livestock release CH4 and N2O; (iv) industrial activities and refrigerants emit CFCs. Effects: (a) melting of polar ice caps and glaciers; (b) rise in sea level threatening coastal cities; (c) climate change with more frequent extreme weather (heat waves, hurricanes, droughts, floods); (d) shift of vegetation belts and loss of biodiversity; (e) spread of tropical diseases (malaria, dengue) to new regions; (f) ocean acidification due to CO2 absorption. Remedies: switching to renewable energy (solar, wind, hydro); increasing energy efficiency; afforestation and reforestation; using public transport and electric vehicles; international agreements like the Kyoto Protocol and Paris Agreement; carbon capture and storage technology; and personal lifestyle changes such as reducing meat consumption and recycling.
Q4. Describe the principles of green chemistry with suitable examples.
Answer: Green chemistry is a sustainable approach to designing chemical products and processes that minimise hazards. Key principles and examples: (i) Atom economy — designing reactions where most atoms of reactants appear in the desired product, reducing waste. For example, addition reactions are more atom-economical than substitution. (ii) Use of safer solvents — replacing toxic solvents with water or supercritical CO2. In dry-cleaning, tetrachloroethene (a suspected carcinogen) is being replaced by liquid CO2 with a suitable detergent. (iii) Greener bleaching agents — paper bleaching using chlorine produces dioxins; the modern approach uses hydrogen peroxide (H2O2) in the presence of catalyst, which gives only water as by-product. (iv) Renewable feedstocks — using bio-based starting materials instead of petroleum. (v) Catalysis — catalysts reduce energy consumption and waste. (vi) Designing for biodegradability — products should break down to non-toxic compounds. (vii) Energy efficiency — carrying out reactions at ambient temperature and pressure where possible. (viii) Real-time analysis — monitoring reactions to avoid hazardous by-products. Adoption of green chemistry leads to environmental protection, economic savings and human health benefits.
Q5. Discuss tropospheric pollution caused by gaseous pollutants such as oxides of sulphur, nitrogen and carbon along with their effects.
Answer: Tropospheric pollution refers to contamination of the lowest layer of atmosphere by gaseous and particulate pollutants. (i) Oxides of sulphur — mainly SO2 from burning of coal and petroleum. SO2 is corrosive, causes respiratory disorders such as bronchitis and asthma, and contributes to acid rain damaging plants, soils and buildings. (ii) Oxides of nitrogen — NO and NO2 produced by lightning and fossil fuel combustion at high temperature in vehicle engines and power plants. NO2 is brown coloured, causes respiratory diseases, damages plant tissues, contributes to photochemical smog and acid rain. (iii) Oxides of carbon — CO is highly toxic; it forms carboxyhaemoglobin which reduces oxygen transport in blood causing headache, suffocation and death. CO2 is a major greenhouse gas; though essential for photosynthesis, increasing concentration leads to global warming. (iv) Hydrocarbons — released from incomplete combustion of fuels and vehicle exhaust; many are carcinogenic and contribute to photochemical smog. Particulate matter such as smoke, dust, mist and fumes also contribute to tropospheric pollution causing lung diseases. Control measures include using cleaner fuels (CNG, LPG), installing catalytic converters in vehicles, scrubbers in industries, and afforestation.
Multiple Choice Questions (MCQ)
Q1. Which gas is mainly responsible for ozone layer depletion?
(a) CO2 (b) CH4 (c) CFCs (d) SO2
Answer: (c) CFCs.
Q2. The pH of acid rain is generally:
(a) above 7 (b) equal to 7 (c) about 5.6 (d) below 5.6
Answer: (d) below 5.6.
Q3. Which of the following is NOT a greenhouse gas?
(a) CO2 (b) CH4 (c) N2 (d) H2O vapour
Answer: (c) N2.
Q4. Photochemical smog contains:
(a) SO2 and smoke (b) NO2, O3, PAN (c) CO2 and water (d) CFCs only
Answer: (b) NO2, O3, PAN.
Q5. BOD value of clean water should be:
(a) above 17 ppm (b) less than 5 ppm (c) about 50 ppm (d) zero
Answer: (b) less than 5 ppm.
Q6. The Montreal Protocol was signed in:
(a) 1972 (b) 1985 (c) 1987 (d) 1997
Answer: (c) 1987.
Q7. Eutrophication is mainly caused by:
(a) heavy metals (b) phosphates and nitrates (c) plastics (d) oil spills
Answer: (b) phosphates and nitrates.
Q8. The greenest solvent in dry-cleaning is:
(a) tetrachloroethene (b) chloroform (c) liquid CO2 (d) benzene
Answer: (c) liquid CO2.
Q9. Carbon monoxide is poisonous because it binds with:
(a) red blood cells (b) haemoglobin (c) plasma (d) platelets
Answer: (b) haemoglobin.
Q10. The bleaching agent used in paper industry as a green alternative to chlorine is:
(a) ozone (b) hydrogen peroxide (c) sulphur dioxide (d) ammonia
Answer: (b) hydrogen peroxide.
Fill in the Blanks
Q1. The ozone layer is present in the ______ region of the atmosphere.
Answer: stratosphere.
Q2. Acid rain is mainly caused by oxides of ______ and ______.
Answer: sulphur and nitrogen.
Q3. ______ smog is reducing in nature while ______ smog is oxidising.
Answer: Classical (London); photochemical (Los Angeles).
Q4. The amount of dissolved oxygen in good quality water should be more than ______ ppm.
Answer: 6.
Q5. ______ chemistry aims to minimise the generation of hazardous substances.
Answer: Green.
True or False
Q1. Carbon dioxide is the major contributor to global warming.
Answer: True.
Q2. Ozone in the troposphere is beneficial to humans.
Answer: False (tropospheric ozone is harmful; stratospheric ozone is beneficial).
Q3. A higher BOD value indicates cleaner water.
Answer: False (higher BOD indicates more pollution).
Q4. The Montreal Protocol deals with the ban of substances that deplete the ozone layer.
Answer: True.
Q5. Eutrophication increases dissolved oxygen in water.
Answer: False (it decreases dissolved oxygen).
Glossary
| Term | Meaning |
|---|---|
| Pollutant | Substance present in higher than normal concentration causing harm to environment. |
| Troposphere | Lowest layer of atmosphere, extending up to 10 km. |
| Stratosphere | Layer above troposphere containing the ozone layer. |
| Smog | Mixture of smoke and fog containing pollutants. |
| Acid rain | Rain with pH below 5.6 due to dissolved acidic oxides. |
| Greenhouse effect | Trapping of infrared radiation by certain gases warming the Earth. |
| CFC | Chlorofluorocarbon, ozone depleting substance. |
| Montreal Protocol | 1987 international treaty to phase out ozone depleting substances. |
| BOD | Biochemical Oxygen Demand, oxygen needed to decompose organic matter. |
| COD | Chemical Oxygen Demand, total oxygen required to oxidise pollutants. |
| DO | Dissolved Oxygen, oxygen dissolved in water. |
| Eutrophication | Excess nutrient enrichment causing algal bloom and oxygen depletion. |
| Potable water | Water fit for human drinking. |
| PAN | Peroxyacetyl nitrate, secondary pollutant in photochemical smog. |
| Atom economy | Fraction of reactant atoms incorporated in the desired product. |
| Green chemistry | Designing processes that minimise hazardous substances. |
| Bioaccumulation | Build-up of toxic substances in living organisms over time. |
| Biodegradable | Substance broken down by microorganisms into harmless products. |