Concepts of Environmental Chemistry for JEE Main Chemistry
Environmental chemistry is the study of the origin, effect, and fate of chemical species by chemical processes that take place in the environment such as air, water, and soil. The undesirable changes in the environment that have harmful effects on plants, animals, and human beings are known as environmental pollution and the substances that cause the pollution are known as pollutants.
The generation, type, and effect of these pollutants and how these pollutants can be controlled and substituted will be studied in this article. Green chemistry is the ethical way of utilising the knowledge and principle of chemistry to minimise the pollution or deterioration of the environment. This chapter is one of the easiest chapters from the competitive exams JEE and NEET point of view.
JEE Main Chemistry Chapters 2025
Important Topics of Environmental Chemistry
Effect of environmental pollution
Air pollution
Water pollution
Air pollution control
Causes of water pollution
Green Chemistry
Important Definitions of Environmental Chemistry
Tropospheric Pollution
It occurs due to undesirable solids and gaseous particles present in the air. The major gaseous and particulate pollutants present in the troposphere are listed below:
Gaseous air pollutants contain oxides of sulphur, nitrogen and carbon, hydrocarbons, ozone, and other oxidants.
Particulate pollutants contain dust, mist, smoke, smog, fumes, etc.
1. Gaseous Air Pollutants
Oxides of Sulphur
Sulphur dioxide, oxide of sulphur, is a harmful gaseous pollutant that causes:
Respiratory diseases such as asthma, bronchitis, and emphysema in human beings.
Irritation in the eyes leads to tears and redness of the eyes.
Stiffness in flower buds which eventually leads to fall off from plants.
Oxides of Nitrogen
At high temperatures, the unburnt fossil fuel from automobiles leads to the formation of nitric oxide (NO) and nitrogen dioxide (NO2) by reaction between nitrogen and oxygen gases present in the atmosphere as shown below:
At temperatures around 1483K,
N2 (g) + O2 (g) → 2 NO (g)
NO reacts immediately with O2 present in the atmosphere, in the troposphere, and ozone in the stratosphere to form NO2.
NO(g) + O2 (g) → 2NO2(g)
Oxides of nitrogen lead to lung irritation, acute respiratory diseases, retard the rate of photosynthesis, and corrode metals.
Oxides of Carbon
Carbon Monoxide: It is a highly poisonous gas due to its strong ability to bind with haemoglobin to form carboxyhaemoglobin which blocks the transportation of oxygen in the body or tissues of humans.
Carbon Dioxide: It is a respiratory gas, released into the atmosphere by respiration, burning of fuel, decomposition of limestone, deforestation and volcanic eruptions. The increased level of carbon dioxide in the atmosphere causes global warming.
2. Particulate Pollutants
These are the minute solid particles or liquid droplets present in the atmospheric air which are present in vehicle emissions, smoke from fires, dust particles and ash from industries. Particulate pollutants are classified as viable and non-viable pollutants.
Viable particulates are minute living organisms such as bacteria, fungi moulds, algae, etc. Humans are allergic to some of the fungi present in the atmosphere. Non-viable particulates are classified according to their nature and size as follows:
Smoke: It contains solid particles or a mixture of solid and liquid particles released from cigarette smoke, combustion of organic matters, fossil fuels, garbage, etc.
Dust: It is the small solid particles of size approximately 1 μm in diameter formed from cement industries, coal works, and wood works due to crushing and grinding.
Mists: These are produced from the particles of liquid spray and condensation of vapours in air, such as insecticides, pesticides sprayed, and sulphuric acid mist remain in the atmosphere as particulate pollutants.
Fumes: These are obtained by condensation of vapours during sublimation, distillation, boiling, and several other chemical reactions of organic solvents, metals, and metallic oxides.
Smog: The word ‘smog’ is derived from smoke and fog. There are two types of smog.
Classical Smog: It is a mixture of smoke + fog + sulphur dioxide. It generally occurs in cool and humid climates and is chemically a reducing mixture and hence it is called as reducing smog.
Photochemical Smog: The main component of photochemical smog is the action of sunlight on unsaturated hydrocarbon and nitrogen dioxide released from automobiles and factories. It generally occurs in warm, dry, and sunny climates and is chemically an oxidising agent and hence it is called as oxidising smog. Photochemical smog is formed from the hydrocarbon (unburnt fuel) and nitric oxide (NO) released from the fossil fuel.
NO(g) + O2 (g) → 2NO2(g)
NO2 absorbs energy from the sunlight and breaks into NO and free oxygen which is very reactive.
NO2(g) → 2NO(g) + O(g)
Free reactive oxygen atom combines to form ozone.
O(g) + O2(g) ⇌ O3(g)
The ozone reacts with NO2 formed in the above reactions and unburnt hydrocarbons to produce photochemical smog such as peroxyacetyl nitrate (PAN).
Effect of Photochemical Smog: Photochemical smog acts as an eye irritant, also irritates the nose and throat, and causes headache, chest pain, cough, and difficulty in breathing. It also causes damage to plant life, corrosion of metals, stones, building materials, etc.
Photochemical Smog can be Controlled: Catalytic converters are used in automobiles to prevent the formation of primary precursors of photochemical smog such as NO2 and hydrocarbons. The controlled or reduced formation of primary precursors such as NO2 and hydrocarbons prevents the formation of secondary precursors such as ozone and PAN which will reduce the photochemical smog.
Acid Rain
The pH of rain in normal conditions is slightly acidic, which is 5.6 due to the presence of H+ ions formed because of the reaction of rain water and carbon dioxide.
H2O (l) + CO2 (g) ⇋ H2CO3 (aq)
H2CO3 (aq) ⇋ H+ (aq) + HCO3ー(aq)
When the pH of rain water is below 5.6, it is known as acid rain. The oxides of nitrogen and sulphur are the major contributors to the acid rain as they undergo oxidation and react with water.
2 SO2(g) + O2(g) + 2 H2O (l) ⟶ 2H2SO4 (aq)
2 NO2(g) + O2(g) + 2H2O (l) ⟶ 4HNO3 (aq)
Acid rain is harmful for agriculture crops, trees, and plants as it dissolves and washes away nutrients important for their growth. It also causes respiratory ailments in humans and animals. Acid rain water also affects the aquatic ecosystem and corrodes water pipelines resulting in the leaching of heavy metals such as iron, lead, and copper into the drinking water. Acid rain also damages buildings, monuments, and other structures made of stone or metals. Taj Mahal is one of the examples of the corrosion caused by acid rain.
Global Warming and Greenhouse Effect
Most of the solar radiation is absorbed by the earth’s atmosphere and the heat that reaches the earth’s surface is trapped by some gases such as methane, carbon dioxide, ozone, chlorofluorocarbon (CFC) compounds, and water vapour in the atmosphere. These gases cause increases in the earth's temperature and this causes global warming. Carbon dioxide gas is the major contributor of global warming.
The process of heating the earth’s atmosphere when some gases present in the earth’s atmosphere traps the heat from the Sun. The gases which contribute to global warming such as methane, carbon dioxide, and ozone are known as greenhouse gases.
Water Pollution
The dissolved oxygen (DO) in water has a concentration of up to 10 ppm (parts per million). Polluted water has reduced the concentration of DO in water and the DO below 6 ppm inhibits the growth of fishes and affects aquatic life.
The amount of oxygen required by bacteria to break down the organic matter present in water samples is called biochemical oxygen demand (BOD). Clean water has a BOD value of less than 5 ppm whereas polluted water has a BOD value of 17ppm or more.
Causes of Water Pollution
Pathogens: Pathogens include bacteria and other organisms present in sewage and animal excreta that enter the drinking water which causes many diseases such as gastrointestinal diseases.
Organic Waste: The organic matter such as leaves, grass, trash etc. and excessive growth of phytoplankton causes water pollution which decreases the DO in water and affects aquatic life.
Chemical Pollutants: The water soluble inorganic chemicals such as cadmium, mercury, and nickel are dangerous to humans as they do not excrete out of the body and affect the kidney, CNS (central nervous system), liver, etc. The organic chemicals from petroleum, oil spills, pesticides, etc. discharge into the ocean and cause water pollution.
The addition of these water pollutants in water bodies increases the nutrients in the water which support a dense plant population, which kills an animal or aquatic life due to deprivation of oxygen in the water and subsequently results in loss of biodiversity, this process is known as eutrophication.
International Standard for Drinking Water
The international standard for drinking water is as follows:
Fluoride: The low concentration of fluorides in drinking water causes tooth decay. Fー ion helps in the hardening of enamel by converting hydroxyapatite [3(Ca3(PO4)2). Ca(OH)2] into harder fluorapatite [3(Ca3(PO4)2). CaF2].
Sodium fluorides are added to drinking water to bring its concentration up to 1 ppm. A concentration of more than 2 ppm causes a mottling of teeth and harmful effect to bones and teeth.
Lead: The prescribed upper limit of concentration of lead in drinking water is about 50 ppb (parts per billion). More than the prescribed limit of the lead causes damage to the kidney, liver, reproductive system, etc.
Sulphate: The sulphate of more than 500 ppm in drinking water causes a laxative effect and a moderate level of sulphate has a mild or harmless effect.
Nitrate: The upper limit of nitrate in drinking water is 50 ppm. Excess nitrate in drinking water causes ‘blue baby’ syndrome or methemoglobinemia.
Maximum Prescribed Concentration of Some Metals in Drinking Water
Soil Pollution
Soil pollution is the presence of harmful chemicals in the soil that affect humans and plant growth.
Pesticides:
Prior to World War II, nicotine was used as a pest controlling substance for major crops.
During World War II, DDT was greatly used as a pesticide as it was very effective to control malaria and other insect-borne diseases.
Pesticide industries have shifted their attention to herbicides such as sodium chlorate (NaClO3) and sodium arsenite (NA3SO3) and many others.
Pesticides and herbicides are toxic to mammals and cause damage to multiple organs in humans, birth defects, genetic disorders, etc.
Industrial Wastes
Industrial wastes are categorised as biodegradable and non-biodegradable wastes.
Biodegradable Wastes: The wastes that can undergo aerobic or anaerobic decomposition are known as biodegradable wastes. The biodegradable wastes are generated by cotton mills, food processing units, paper mills, and textile factories.
Non-biodegradable Wastes: The wastes that do not undergo decomposition naturally and remain on earth for thousands of years. Non-biodegradable wastes are produced by thermal power plants, many industries such as iron and steel industry, fertiliser industry, industries manufacturing aluminium, zinc, and copper.
Strategies to Control Environmental Pollution
Waste Management: The government has taken many steps to manage the waste to control environmental degradation such as the ‘Clean India Mission’ or ‘Swachh Bharat Abhiyan’ which implemented two programmes listed below:
Swachh Bharat Mission - Urban (SBM-U): It aims at making urban India free from open defecation and achieving 100% scientific management of solid wastes in the country.
Swachh Bharat Mission - Rural (SBM-R): It aims at improving the quality of life in rural areas by encouraging sanitation, hygiene, cleanliness, and eliminating open defecation.
Green Chemistry
It is the innovative way of thinking and utilising the knowledge and principles of chemistry to minimise pollution or deterioration to the environment caused by the pollutants. It is also a cost-effective approach, which involves a reduction in material, energy consumption, and waste management.
The green chemistry in day-to-day life:
Dry Cleaning of Clothes: Tetrachloroethene (Cl2C=CCl2 ) was used as a solvent for dry cleaning which contaminates the groundwater and is carcinogenic. Tetrachloroethane is replaced by liquified carbon dioxide with suitable detergents which are less harmful to groundwater. Nowadays, hydrogen peroxide (H2O2) is used for bleaching clothes in the process of laundry.
Bleaching of Paper: Chlorine gas was used for bleaching paper which is replaced by hydrogen peroxide (H2O2) with a suitable catalyst.
Synthesis of Chemicals: Ethanal is now commercially prepared by one-step oxidation of ethene in the presence of catalysts such as Pd(II) or Cu (II) aqueous medium.
Green Solution to Clean Turbidity in Water: Alum was used to treat turbidity in water which was found to increase toxic ions in water. The powder of the kernel of tamarind seeds is the green solution to treat the turbidity of water.
Stratospheric Pollution: Formation and Breakdown of Ozone
Stratospheric pollution is a significant environmental concern, and understanding its impact on the Earth's atmosphere is essential for JEE Main students. One critical aspect of stratospheric pollution is the formation and breakdown of ozone in the stratosphere.
Formation of Ozone:
Ozone ($O_3$) is a molecule composed of three oxygen atoms. It is vital for life on Earth because it forms the ozone layer in the stratosphere, which plays a crucial role in protecting living organisms from harmful ultraviolet (UV) radiation from the sun. Ozone is naturally formed and maintained through a delicate balance of chemical reactions in the stratosphere. The key reaction responsible for ozone formation is:
Oxygen Photolysis:
$O_2$ + UV-C light → 2O
Ultraviolet-C (UV-C) light from the sun with a wavelength less than 240 nm is responsible for breaking apart molecular oxygen ($O_2$) into individual oxygen atoms (O). These highly reactive oxygen atoms can then react with molecular oxygen to form ozone:
Ozone Formation:
$O + O_2$ → $O_3$
The formation of ozone is an ongoing process, where ozone molecules are constantly created and destroyed in a dynamic equilibrium, maintaining a relatively stable concentration in the stratosphere.
Breakdown of Ozone:
Ozone in the stratosphere is also subject to breakdown through various chemical reactions. While natural processes contribute to ozone depletion, human activities have significantly accelerated the breakdown of ozone. The most well-known contributor to ozone depletion is the release of certain chemicals known as ozone-depleting substances (ODS). The primary reactions involved in ozone breakdown are:
Chlorofluorocarbon (CFC) Reaction:
$O_3$ + UV-C light → $O_2$ + O
Ozone-depleting substances, particularly CFCs, are stable compounds that were widely used in refrigeration, air conditioning, aerosol propellants, and foam-blowing agents. When released into the atmosphere, these compounds eventually reach the stratosphere. There, they are broken down by UV-C light, releasing chlorine atoms (Cl). These chlorine atoms are highly destructive to ozone because they can catalytically destroy numerous ozone molecules. The reaction proceeds as follows:
CFC Reaction (Initiation):
Cl + $O_3$ → ClO + $O_2$
CFC Reaction (Propagation):
ClO + O → Cl + $O_2$
Cl + $O_3$ → ClO + $O_2$
CFC Reaction (Overall):
$2O_3$ → $3O_2$
The net result is the destruction of ozone, depleting the ozone layer. Ozone depletion is particularly concerning because it allows more harmful UV-B and UV-C radiation to reach the Earth's surface. Increased UV radiation poses serious health risks to living organisms and can damage ecosystems, including aquatic and terrestrial ecosystems.
Mechanism of Ozone Depletion:
Ozone depletion primarily occurs due to the presence of human-made chemicals, especially CFCs. The process involves a complex series of reactions that release chlorine atoms into the stratosphere. These chlorine atoms then catalyze the destruction of ozone.
The Mechanism can be Summarized as Follows:
CFCs, which are stable at lower altitudes, are transported to the stratosphere through various atmospheric processes.
In the stratosphere, these CFCs are broken down by UV-C radiation. For example, a CFC molecule releases a chlorine atom.
Chlorine atoms are highly reactive and can catalyze the destruction of ozone molecules. One chlorine atom can destroy multiple ozone molecules, leading to a chain reaction.
As a result, ozone molecules are converted back into molecular oxygen ($O_2$), depleting the ozone layer.
Effects of Ozone Depletion:
Ozone depletion has significant and far-reaching effects on the environment, human health, and ecosystems:
Increased UV Radiation: With a thinner ozone layer, more UV-B and UV-C radiation reach the Earth's surface. This can lead to increased cases of skin cancer, cataracts, and other health issues in humans. UV radiation can also harm animals and plants, affecting ecosystems.
Ozone Holes: Particularly over Antarctica, ozone depletion has led to the formation of ozone holes. These are regions with significantly reduced ozone concentrations, allowing for even more harmful UV radiation to penetrate the atmosphere.
Climate Change: Ozone depletion can influence the Earth's climate. Changes in the distribution of ozone in the stratosphere can affect temperature patterns and wind circulation, potentially impacting weather systems.
Aquatic Ecosystems: Increased UV radiation can penetrate aquatic ecosystems, harming phytoplankton and other aquatic organisms that form the base of the food chain. This can have cascading effects on marine life.
Terrestrial Ecosystems: Ozone depletion can damage terrestrial plants, affecting crop yields and disrupting ecosystems.
Environmental Chemistry Project
Water pollution, the contamination of water bodies with harmful substances, is a significant environmental issue with far-reaching consequences. Environmental Chemistry project aims to investigate the sources and impacts of water pollution and propose potential mitigation strategies.
Objectives
Identify and analyze the primary sources of water pollution, including industrial effluents, agricultural runoff, and domestic wastewater.
Evaluate the detrimental effects of water pollution on aquatic ecosystems, human health, and economic activities.
Propose effective water pollution control measures, emphasizing sustainable practices and technological advancements.
Methodology
Conduct a comprehensive literature review to gather information on water pollution sources, impacts, and mitigation strategies.
Collect water samples from various sources, such as rivers, lakes, and streams, to analyze their pollution levels.
Utilize laboratory techniques to measure the concentration of pollutants, such as heavy metals, organic compounds, and nutrients.
Interpret the results to identify pollution hotspots and assess the overall health of the water bodies.
Expected Outcomes
A comprehensive understanding of the sources and impacts of water pollution in the selected region.
Identification of pollution hotspots and vulnerable water bodies.
Recommendations for effective water pollution control measures, considering environmental, social, and economic factors.
A proposal for raising awareness among local communities about water pollution and its consequences.
Significance of the Project
This project aims to provide valuable insights into the multifaceted issue of water pollution, contributing to informed decision-making and the implementation of effective mitigation strategies. By understanding the sources and impacts of water pollution, we can work towards safeguarding this precious resource for future generations
JEE Main Environmental Chemistry Solved Examples
Example 1: Why does rainwater have a pH of about 5.6?
Solution: The rain has a pH of 5.6 (approximately) due to the presence of H+ ions formed because of the reaction of rain water and carbon dioxide.
H2O (l) + CO2 (g) ⇋ H2CO3 (aq)
H2CO3 (aq) ⇋ H+ (aq) + HCO3ー(aq)
Key points to remember: Rainwater gets mixed with atmospheric gas.
Example 2: Explain giving reasons. ‘The presence of CO reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells'.
Solution: CO has the strong ability to bind with haemoglobin to form carboxyhaemoglobin and reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells which blocks the transportation of oxygen in the body or tissues of humans.
Key Points to remember: CO is a strong field ligand for Fe ion.
Solved Questions from the Previous Year Question Papers
Question 1: The layer of atmosphere between 10 km and 50 km above the sea level is called
(a) Thermosphere
(b) Mesosphere
(c) Stratosphere
(d) Troposphere
Solution: The stratosphere is the second layer of atmosphere which extends between 10 km and 50 km above the sea level. The correct answer is (c).
Trick: Stratosphere starts after troposphere(0-10 km)
Question 2. Water samples with BOD values of 4 ppm and 18 ppm, respectively, are
(a) Clean and highly polluted
(b) Highly polluted and highly polluted
(c) Highly polluted and clean
(d) Clean and clean
Solution: Water which has a BOD value less than 5 ppm is considered clean water. Polluted water has a BOD value of 17 ppm or more. Therefore, the water sample is clean and highly polluted, respectively. The correct answer is (a).
Trick: High BOD means poor water quality.
Question 3: Taj Mahal is being slowly disfigured and discoloured. This is primarily due to
(a) Acid rain
(b) Soil pollution
(c) Water pollution
(d) Global warming
Solution: Acid rain is the primary reason for the discolouration of the Taj Mahal as acid rain has a pH of less than 5.6 and hence corrodes the marble of Taj Mahal and leads to its discolouration. Therefore, the correct answer is (a).
Trick: Taj mahal is made up of marble which can react with acid.
Practice Questions
Question 1: The maximum prescribed concentration of copper in drinking water is
(a) 0.05 ppm
(b) 3 ppm
(c) 5 ppm
(d) 0.5 ppm
Answer: (b) 3 ppm.
Question 2: Air pollution that occurs in sunlight is
(a) Oxidising smog
(b) Fog
(c) Reducing smog
(d) Acid rain
Answer: (a) Oxidising smog.
JEE Main Chemistry Environmental Chemistry Study Materials
Here, you'll find a comprehensive collection of study resources for Environmental Chemistry designed to help you excel in your JEE Main preparation. These materials cover various topics, providing you with a range of valuable content to support your studies. Simply click on the links below to access the study materials of Environmental Chemistry and enhance your preparation for this challenging exam.
JEE Main Chemistry Study and Practice Materials
Explore an array of resources in the JEE Main Chemistry Study and Practice Materials section. Our practice materials offer a wide variety of questions, comprehensive solutions, and a realistic test experience to elevate your preparation for the JEE Main exam. These tools are indispensable for self-assessment, boosting confidence, and refining problem-solving abilities, guaranteeing your readiness for the test. Explore the links below to enrich your Chemistry preparation.
Benefits of Using Vedantu for JEE Main 2025 - Chemistry Environmental Chemistry
Embark on a journey of mastering Environmental Chemistry for JEE Main with Vedantu's comprehensive approach. Simplified explanations, personalised guidance, and interactive sessions ensure a deep understanding, while practice questions and tests, flexible learning, and an exam-oriented focus cater to diverse learning needs, making resources readily accessible online. More such points to consider are:
Comprehensive Learning on Environmental Chemistry: Vedantu ensures thorough coverage of Environmental Chemistry, aligning with JEE Main requirements, and providing students with a deep understanding of relevant concepts.
Simplified Explanation for Easy Understanding: Complex topics in Environmental Chemistry are presented in a simplified manner by Vedantu, enhancing accessibility and easing comprehension for JEE Main aspirants.
Personalised Guidance for Environmental Chemistry: Vedantu offers personalised guidance specifically for Environmental Chemistry, with experienced tutors addressing individual doubts and providing additional support to enhance clarity.
Interactive Sessions for Real-time Discussions: Live interactive sessions in Environmental Chemistry facilitate real-time discussions, enabling active participation, encouraging students to ask questions, and enhancing engagement with the material.
Practice Questions and Tests for Environmental Chemistry: Vedantu provides a variety of practice questions and tests related to Environmental Chemistry, aiding students in reinforcing their learning and preparing effectively for JEE Main exams.
Flexible Learning in Environmental Chemistry: The platform allows flexible learning schedules for Environmental Chemistry, enabling students to study at their own pace and revisit lessons as needed, accommodating diverse learning styles.
Exam-oriented Approach to Environmental Chemistry: Vedantu's approach in teaching Environmental Chemistry is tailored to JEE Main exams, ensuring that students are well-prepared and confident in tackling questions specific to this chapter.
Accessible Resources for Environmental Chemistry: All resources, including study materials and mock tests for Environmental Chemistry, are readily accessible online, offering convenience for students to study anytime and anywhere.
Conclusion
In this article, we dove into the world of Environmental Chemistry for the JEE Main exam. We discovered the key concepts and problem-solving strategies related to this crucial topic. We'll explore environmental processes, pollution control, and the impact of human activities on our planet. Our comprehensive guide covers everything you need in one place. You can easily access downloadable PDFs with clear explanations, definitions, and practice questions. This resource is a valuable tool to help you excel in your exams, providing a solid foundation in Environmental Chemistry.
FAQs on Environmental Chemistry Chapter - Chemistry JEE Main
1. Is Environmental Chemistry an important chapter for the JEE Main 2026 exam?
Yes, Environmental Chemistry is a consistently important chapter for JEE Main 2026. While it may not have the highest weightage, it is a high-scoring, theory-based chapter from which 1-2 direct questions are frequently asked. Mastering this chapter can provide a quick and easy score boost as the questions are often based on NCERT concepts.
2. What are the main types of smog, and what is the key chemical difference between them for JEE Main?
The two main types of smog are Classical Smog and Photochemical Smog. The key chemical difference lies in their composition and chemical nature:
- Classical Smog: Occurs in cool, humid climates. It is a mixture of smoke, fog, and sulfur dioxide (SO₂). Chemically, it is a reducing mixture, so it is also called reducing smog.
- Photochemical Smog: Occurs in warm, dry, and sunny climates. It is formed by the action of sunlight on unsaturated hydrocarbons and oxides of nitrogen (NOx). It contains high concentrations of oxidising agents like ozone (O₃) and peroxyacetyl nitrate (PAN), making it an oxidising mixture.
3. What are BOD and DO, and how are their values used to determine water quality in JEE Main problems?
BOD (Biochemical Oxygen Demand) is the amount of oxygen required by bacteria to break down organic waste in a water sample. DO (Dissolved Oxygen) is the actual amount of oxygen present in the water. For JEE Main problems, their relationship is key:
- Clean Water: Has a low BOD (less than 5 ppm) and a high DO level (around 10 ppm).
- Polluted Water: Has a high BOD (17 ppm or more) because of excess organic matter, which leads to a low DO level (below 6 ppm), harming aquatic life.
4. Which pollutants are responsible for acid rain, and what are the key chemical reactions involved?
The primary pollutants responsible for acid rain are oxides of sulfur (SO₂) and oxides of nitrogen (NO₂). These gases react with oxygen and water in the atmosphere to form strong acids. The key reactions are:
- 2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq) (Sulfuric acid)
- 4NO₂(g) + O₂(g) + 2H₂O(l) → 4HNO₃(aq) (Nitric acid)
Normal rainwater is slightly acidic (pH ≈ 5.6) due to dissolved CO₂, but acid rain has a pH below 5.6.
5. What are the international standards for key ions like Lead (Pb) and Fluoride (F⁻) in drinking water that are relevant for JEE Main?
For JEE Main, you should be aware of the prescribed concentration limits for certain ions in drinking water:
- Fluoride (F⁻): The ideal concentration for preventing tooth decay is up to 1 ppm. Concentrations above 2 ppm can cause brown mottling of teeth (fluorosis).
- Lead (Pb): The upper limit for lead in drinking water is 50 ppb (parts per billion). Exceeding this limit can cause damage to the kidneys, liver, and central nervous system.
- Nitrate (NO₃⁻): The maximum limit is 50 ppm. Excess nitrate can cause methemoglobinemia, or 'blue baby syndrome'.
6. Why is photochemical smog called an 'oxidising smog' while classical smog is a 'reducing smog'?
This classification is based on the dominant chemical nature of their components. Photochemical smog is termed 'oxidising' because it contains high concentrations of powerful oxidising agents like ozone (O₃), peroxyacetyl nitrate (PAN), and nitrogen dioxide (NO₂). In contrast, classical smog is a mixture of smoke, fog, and sulfur dioxide (SO₂). Since SO₂ is a well-known reducing agent, the overall chemical environment of classical smog is reducing.
7. How do chlorofluorocarbons (CFCs) catalytically destroy the ozone layer? Explain the reaction mechanism.
CFCs cause ozone depletion through a free-radical catalytic cycle. The mechanism proceeds in two main steps after CFCs reach the stratosphere:
- Initiation: High-energy UV radiation breaks the C-Cl bond in a CFC molecule, generating a highly reactive chlorine free radical (Cl•).
CF₂Cl₂(g) + UV → Cl•(g) + •CF₂Cl(g) - Propagation (Catalytic Cycle): The chlorine radical attacks an ozone molecule, and is then regenerated to attack another one.
- Cl•(g) + O₃(g) → ClO•(g) + O₂(g)
- ClO•(g) + O(g) → Cl•(g) + O₂(g)
The net reaction is O₃(g) + O(g) → 2O₂(g). Because the chlorine radical (Cl•) is regenerated in the second step, a single CFC molecule can destroy thousands of ozone molecules.
8. Compare the environmental impact of Carbon Monoxide (CO) and Carbon Dioxide (CO₂). Why is CO considered more immediately poisonous to humans?
While both are major pollutants, their mechanism of harm is very different. Carbon Monoxide (CO) is immediately poisonous because it binds to the iron in haemoglobin about 200-300 times more strongly than oxygen. This forms carboxyhaemoglobin, which prevents red blood cells from transporting oxygen, leading to asphyxiation. In contrast, Carbon Dioxide (CO₂) is not directly toxic in the same way. Its primary environmental harm is as a greenhouse gas, trapping heat in the atmosphere and contributing to global warming.
9. What is the role of 'Green Chemistry' in environmental protection, and how does it differ from traditional pollution control?
Green Chemistry is a proactive approach focused on preventing pollution at its source. Its goal is to design chemical products and processes that reduce or eliminate the use and generation of hazardous substances. This is fundamentally different from traditional pollution control, which is a reactive approach known as remediation. Remediation deals with treating or managing waste and pollutants after they have already been created.
10. Explain the process of eutrophication. How does nutrient enrichment in a water body lead to a decrease in dissolved oxygen?
Eutrophication is the process where a water body becomes overly enriched with nutrients, primarily nitrates and phosphates. This triggers a chain of events that depletes oxygen:
- Nutrient Loading: Runoff from fertilisers and sewage introduces excess nutrients.
- Algal Bloom: These nutrients cause a rapid, dense growth of algae and other aquatic plants.
- Decomposition: When these algae die, they are decomposed by aerobic bacteria.
- Oxygen Depletion: The decomposition process is oxygen-intensive, consuming large amounts of dissolved oxygen (DO) from the water, which drastically increases the Biochemical Oxygen Demand (BOD).
This resulting state of low oxygen, called hypoxia, kills fish and other aquatic life, destroying the ecosystem.
