Environmental Chemistry: Air, Water and Soil Pollution, Green Chemistry and Ecosystem Balance
Environmental chemistry is the study of the chemical processes that occur in the environment, and especially of the harmful changes caused by human activity. It examines how pollutants enter the air, water and soil, how they travel and react, and what harm they cause to living beings. It also studies the chemistry of the atmosphere, the greenhouse effect, the ozone layer and the natural cycles that keep the planet in balance.
For candidates preparing for APSC, ADRE, DHS, DME, SSC, Railways, NEET and other examinations in Assam, environmental chemistry is important because it links science with current affairs and government policy. Questions often cover pollution, the greenhouse effect, acid rain, water quality and green chemistry. Many of these topics are also directly relevant to Assam, with its rivers, tea gardens, forests and floods. This article provides a complete deep-study revision of the subject.
Basic Terms
- Pollutant: a substance present in the environment at a concentration high enough to cause harm.
- Primary pollutant: a pollutant released directly from a source, such as carbon monoxide from vehicles.
- Secondary pollutant: a pollutant formed in the atmosphere by reaction between primary pollutants, such as ozone in photochemical smog.
- Smog: a mixture of smoke and fog; photochemical smog is formed by the action of sunlight on nitrogen oxides and hydrocarbons.
- Acid rain: rain with a pH below about 5.6, caused by sulphur dioxide and nitrogen oxides dissolving to form acids.
- Greenhouse effect: the trapping of infrared radiation by gases such as carbon dioxide, methane and water vapour, which keeps the earth warm.
- Ozone layer: the layer of ozone in the stratosphere that absorbs harmful ultraviolet radiation from the sun.
- BOD: biochemical oxygen demand, the amount of oxygen consumed by microorganisms to decompose organic matter in water.
- COD: chemical oxygen demand, the amount of oxygen required to oxidise all the oxidisable matter in water by chemical means.
- Eutrophication: the enrichment of a water body with nutrients, causing excessive growth of algae and depletion of oxygen.
- Biomagnification: the increase in concentration of a pollutant at successive levels of a food chain.
- Green chemistry: the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.
Core Concepts and Formulas
- Clean air is roughly 78 percent nitrogen, 21 percent oxygen, and small amounts of argon, carbon dioxide and other gases, along with water vapour.
- Carbon dioxide, methane, nitrous oxide, water vapour and chlorofluorocarbons are the main greenhouse gases.
- The greenhouse gases trap infrared radiation and raise the average temperature of the earth, a process called global warming.
- The normal pH of rain is about 5.6 because atmospheric carbon dioxide dissolves to form weak carbonic acid.
- Acid rain is chiefly caused by sulphur dioxide and nitrogen oxides forming sulphuric and nitric acids.
- BOD is measured as the oxygen consumed by a water sample, usually over five days at 20 degrees Celsius, and is expressed in milligrams per litre.
- Higher BOD indicates a higher content of organic pollution, because more oxygen is needed by the decomposing microorganisms.
- Dissolved oxygen is the amount of oxygen dissolved in water and is essential for aquatic life; its normal level is about 5 to 8 milligrams per litre.
- Ozone in the stratosphere is continuously formed and destroyed by ultraviolet radiation, and this balance is disturbed by chlorine from chlorofluorocarbons.
- Concentration in parts per million = mass of solute divided by mass of solution, multiplied by ten raised to the power six.
- Global warming is linked to the enhanced greenhouse effect, and the ozone hole is linked to the catalytic destruction of ozone by chlorine radicals.
Components of the Environment
The environment consists of the atmosphere, the hydrosphere, the lithosphere and the biosphere. The atmosphere is divided into the troposphere, stratosphere, mesosphere, thermosphere and exosphere. Weather occurs in the troposphere, while the ozone layer that shields the earth from ultraviolet radiation lies in the stratosphere. The hydrosphere includes all the water on the earth's surface and underground, and the lithosphere is the solid crust. The biosphere is the zone where life exists.
Natural cycles keep the environment in balance. The carbon cycle moves carbon between the atmosphere, living things, oceans and rocks; the nitrogen cycle converts atmospheric nitrogen into usable compounds and back; and the oxygen cycle maintains the level of oxygen in the air. Human activity disturbs these cycles by adding excessive carbon dioxide, sulphur dioxide, nitrogen oxides, heavy metals and plastics. Understanding these cycles is the key to understanding pollution.
Air Pollution
Air pollution is the presence of harmful substances in the air. The main sources are the burning of fossil fuels in vehicles, industries and power plants, along with agricultural burning, construction dust and natural events such as volcanic eruptions and forest fires. The major pollutants include carbon monoxide, sulphur dioxide, nitrogen oxides, particulate matter, hydrocarbons, lead and volatile organic compounds.
The effects of air pollution range from respiratory diseases such as asthma and bronchitis to reduced visibility, damage to monuments and acid deposition. Carbon monoxide is especially dangerous because it binds to haemoglobin more strongly than oxygen and can cause suffocation. Particulate matter of fine size, especially PM2.5, penetrates deep into the lungs and is a serious health hazard. Air quality is monitored using the air quality index, and control measures include the use of cleaner fuels, catalytic converters, scrubbers, electrostatic precipitators and the planting of trees.
Smog and Acid Rain
Classical smog is a mixture of smoke and fog, common in cold and damp conditions. Photochemical smog, by contrast, forms in warm and sunny conditions when nitrogen oxides and hydrocarbons from vehicles react in sunlight to produce ozone, aldehydes and other irritants. It causes eye irritation, reduces visibility and damages crops. The brown haze seen over large cities is often photochemical smog.
Acid rain is formed when sulphur dioxide and nitrogen oxides released by the burning of coal and oil react with water vapour and oxygen in the atmosphere to form sulphuric acid and nitric acid. These acids fall with rain, snow or dust and lower the pH of soil and water. Acid rain damages forests, leaches nutrients from the soil, makes lakes acidic and kills fish, corrodes buildings and monuments, and harms human health. Control measures include the use of low-sulphur fuels, desulphurisation of flue gases and the fitting of catalytic converters.
Greenhouse Effect and Global Warming
The greenhouse effect is a natural and necessary process. Greenhouse gases in the atmosphere allow sunlight to reach the earth but trap the infrared radiation that the earth emits, keeping the average temperature suitable for life. Without this effect the earth would be far too cold. The problem arises when human activity adds excessive greenhouse gases and strengthens the effect.
Carbon dioxide from the burning of fossil fuels, methane from livestock and paddy fields, nitrous oxide from fertilisers and chlorofluorocarbons are the main contributors to the enhanced greenhouse effect. The result is global warming, which causes the melting of glaciers, a rise in sea level, changes in rainfall, more frequent floods and droughts, and threats to agriculture and biodiversity. Assam is particularly vulnerable because of its rivers, tea gardens and flood-prone plains. Mitigation requires reducing emissions, improving energy efficiency, using renewable energy and planting trees.
Ozone Depletion
The ozone layer in the stratosphere absorbs most of the harmful ultraviolet radiation from the sun. In the 1980s a severe thinning was discovered over Antarctica, called the ozone hole. The main cause was chlorofluorocarbons, used in refrigerators, air conditioners and aerosols, which release chlorine atoms in the stratosphere. A single chlorine atom can destroy thousands of ozone molecules in a catalytic cycle. Nitrogen oxides and bromine compounds also contribute.
Increased ultraviolet radiation reaching the surface causes skin cancer, cataracts, weakened immunity and damage to crops and marine life. The international response was the Montreal Protocol, which phased out the production of chlorofluorocarbons and other ozone-depleting substances. This agreement is regarded as one of the most successful examples of global environmental cooperation, and the ozone layer is slowly recovering.
Water Pollution
Water pollution is the contamination of water bodies by harmful substances, making the water unfit for drinking, agriculture or aquatic life. The main sources are domestic sewage, industrial effluents, agricultural runoff containing fertilisers and pesticides, oil spills and the dumping of solid waste. Pathogens from sewage cause diseases such as cholera, typhoid and dysentery, while industrial discharges may contain toxic heavy metals and organic chemicals.
Important indicators of water quality are the pH, the dissolved oxygen, the biochemical oxygen demand and the chemical oxygen demand. Clean water has a high dissolved oxygen and a low BOD, while polluted water has the opposite. Heavy metals such as mercury, lead, cadmium and arsenic are especially dangerous because they are not biodegradable and can accumulate in the body. Arsenic contamination of groundwater is a serious problem in parts of Assam and neighbouring regions.
BOD, COD and Eutrophication
Biochemical oxygen demand measures the oxygen used by microorganisms to break down the organic matter in water. It is an indirect measure of the amount of biodegradable organic pollution. A high BOD means that a large amount of oxygen is being consumed, leaving little for fish and other aquatic organisms, which may then die. Chemical oxygen demand measures the oxygen needed to oxidise both biodegradable and non-biodegradable matter by chemical means, and it is always higher than BOD for the same sample.
Eutrophication occurs when a water body receives excess nutrients, especially nitrates and phosphates from fertilisers and sewage. This causes an explosive growth of algae, called an algal bloom, which covers the surface and blocks sunlight. When the algae die, their decomposition consumes oxygen and creates a dead zone where aquatic life cannot survive. Control requires treating sewage, reducing fertiliser runoff and preventing the discharge of nutrient-rich effluents.
Soil Pollution, Heavy Metals and Other Forms
Soil pollution is caused by the use of chemical fertilisers and pesticides, the dumping of industrial waste, the disposal of plastics and the deposition of acid rain. Persistent pesticides such as DDT accumulate in the soil and enter the food chain, where they undergo biomagnification. Heavy metals such as mercury, lead and cadmium poison the soil and crops and can cause serious diseases; mercury poisoning caused Minamata disease and cadmium poisoning caused Itai-Itai disease in Japan.
Radioactive pollution arises from nuclear tests, accidents and the improper disposal of nuclear waste, and it can cause cancer and genetic damage. Noise pollution, measured in decibels, causes hearing loss, high blood pressure and sleeplessness. Thermal pollution occurs when hot water from power plants is discharged into rivers and lakes, reducing dissolved oxygen. Solid waste, including non-biodegradable plastics, is a growing problem, and the management of municipal and electronic waste is now a major concern.
Green Chemistry and Waste Management
Green chemistry is the design of chemical products and processes that reduce or eliminate hazardous substances. Its principles include preventing waste rather than treating it, using renewable raw materials, designing safer chemicals, improving atom economy, using catalysts instead of stoichiometric reagents, saving energy and preferring safer solvents. Green chemistry seeks to solve environmental problems at their source rather than cleaning them up afterwards.
Waste management follows the hierarchy of reduce, reuse and recycle. Segregation of waste at the source, composting of biodegradable waste, recycling of paper, glass and metals, and safe disposal of hazardous waste are all important. In Assam, effective management of municipal solid waste, plastic waste and river pollution is essential to protect the environment and public health. Every citizen can contribute by using fewer plastics, saving water and electricity, and planting trees.
How Numerical Problems Are Solved
Environmental chemistry numericals are usually simple and unit-based. For concentration, use parts per million as the mass of solute divided by the mass of solution, multiplied by one million, and remember that for dilute aqueous solutions one milligram per litre is approximately one ppm. For BOD problems, subtract the final dissolved oxygen from the initial dissolved oxygen, and account for the dilution factor if the sample was diluted. For pH, use the relation between pH and hydrogen ion concentration. For greenhouse gas or energy problems, apply the given emission factor directly. Always check the units and the dilution, because these are the commonest sources of error.
Examination Trends
In state recruitment examinations, environmental chemistry questions are largely factual: the causes and effects of pollution, the greenhouse effect, acid rain, the ozone hole, the meaning of BOD and COD, and the principles of green chemistry. In NEET, questions often ask for the correct cause-and-effect pairing, the gas responsible for a particular problem, or the value of the criteria pollutant standard. Matching, assertion-reason and statement-based questions are common. Topics with current relevance, such as air quality, plastic pollution and climate change, are increasingly frequent.
Common Mistakes to Avoid
- Confusing BOD with COD; BOD measures biodegradable matter while COD measures all oxidisable matter.
- Reversing the relationship between BOD and water quality, since a high BOD means more pollution.
- Mixing up global warming with ozone depletion, which are separate problems.
- Assuming that the greenhouse effect is entirely harmful, when it is a natural and necessary process.
- Naming the wrong gas as the chief cause of acid rain or of the greenhouse effect.
- Forgetting that normal rain is slightly acidic because of dissolved carbon dioxide.
- Mixing up the sources of primary and secondary pollutants.
- Ignoring the role of biomagnification in making a pollutant more concentrated at higher food-chain levels.
How to Prepare This Topic
- Learn the composition of clean air and the classification of pollutants before studying their effects.
- Make a table of each pollutant with its source, its effect and its control measure.
- Study BOD, COD and dissolved oxygen carefully, because their numerical and conceptual questions are frequent.
- Link the greenhouse effect, global warming and ozone depletion to their causes and to international agreements such as the Montreal Protocol.
- Relate environmental chemistry to local issues in Assam, such as flood, river pollution and tea-garden practices, and revise previous years' papers.
Frequently Asked Questions
What is the greenhouse effect?
It is the trapping of infrared radiation emitted by the earth by gases such as carbon dioxide, methane, water vapour and nitrous oxide in the atmosphere. This natural process keeps the earth warm enough for life, but an excess of these gases due to human activity strengthens the effect and causes global warming.
What is the difference between BOD and COD?
BOD, or biochemical oxygen demand, is the oxygen consumed by microorganisms to decompose biodegradable organic matter in water. COD, or chemical oxygen demand, is the oxygen required to oxidise both biodegradable and non-biodegradable matter chemically. For the same sample COD is always higher than BOD.
How is acid rain formed and what are its effects?
Sulphur dioxide and nitrogen oxides released by burning fossil fuels react with water vapour and oxygen in the atmosphere to form sulphuric and nitric acids, which fall as acid rain. It damages forests and soil, makes lakes acidic and kills fish, corrodes buildings and harms human health.
What causes the depletion of the ozone layer?
Chlorofluorocarbons used in refrigerators, air conditioners and aerosols release chlorine atoms in the stratosphere, and each chlorine atom destroys thousands of ozone molecules in a catalytic cycle. Nitrogen oxides and bromine compounds also contribute. The Montreal Protocol was made to phase out these substances.
What is green chemistry?
Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Its principles include waste prevention, atom economy, safer solvents, energy efficiency and the use of renewable feedstocks and catalysts.