Environmental Chemistry – NDA Chemistry Notes

Introduction

You breathe air. You drink water. You grow food in soil. When these three — air, water, and soil — get contaminated with harmful substances, it is called pollution. Environmental chemistry studies how chemical substances interact with the natural environment and cause pollution.

NDA 2010-I had a match list connecting ozone, nitrous oxide, carbon dioxide, and carbon monoxide to their properties. NDA 2011-I asked which method is NOT effective in removing arsenic from groundwater.

This chapter brings together pollution topics from across the syllabus. It is the closing chapter that connects chemistry to the real world.

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Understand the Topic

What is Environmental Pollution?

Pollution happens when harmful substances enter the environment and disturb its natural balance.

A pollutant is any substance that causes pollution.

Pollution is of four main types: air pollution, water pollution, soil pollution, and noise pollution.

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Air Pollution

Air pollution is the presence of harmful substances in the atmosphere.

Major Air Pollutants and Their Sources:

Carbon Monoxide (CO):

Carbon monoxide is a colourless, odourless, and tasteless gas. It is produced by the incomplete combustion of fuels (petrol, diesel, wood, coal).

Carbon monoxide is the most dangerous air pollutant from vehicles.

Carbon monoxide combines with haemoglobin in blood to form carboxyhaemoglobin.

Haemoglobin carries oxygen. When CO binds to it, oxygen cannot bind. The person suffocates even though they are breathing air. This is CO poisoning.

Carbon monoxide binds to haemoglobin about 200-250 times more strongly than oxygen. Even a small concentration of CO in air is dangerous.

Sulphur Dioxide (SO₂):

Sulphur dioxide is produced when fuels containing sulphur are burned. Power plants and industries release SO₂.

SO₂ is a major cause of acid rain.

SO₂ causes respiratory problems. It irritates the eyes, nose, and throat.

SO₂ absorbs infrared radiation. It contributes to the greenhouse effect. [NDA 2010-I] — carbon dioxide absorbs infrared radiation; this property is key for greenhouse gases]

Nitrogen Oxides (NOₓ):

Nitrogen oxides (NO, NO₂) are produced when fuels burn at high temperatures (as in diesel engines). At high temperatures, nitrogen and oxygen from air react to form NO.

Nitrogen oxides are a major cause of acid rain.

Nitrogen oxide (NO) is a gas pollutant that causes:
Leaf spotting in plants.
Bronchitis and respiratory problems in humans.
Formation of corrosive gases through photochemical reactions.

Nitric oxide does NOT directly cause silicosis. Silicosis is caused by silica dust, not NO.

Ozone (O₃) in the Stratosphere:

Ozone is NOT a pollutant in the upper atmosphere (stratosphere). There, ozone is beneficial. It absorbs ultraviolet (UV) radiation from the Sun and protects life on Earth.

Ozone gas shields the Earth from ultraviolet radiation. [NDA 2010-I Q150 Match List — ozone is associated with ultraviolet radiation]

However, at ground level (troposphere), ozone is a pollutant. It is a component of photochemical smog.

Hydrocarbons:

Unburnt hydrocarbons from vehicles are air pollutants. They contribute to smog and photochemical smog formation.

Hydrocarbons are gaseous air pollutants.

Smoke is NOT a gaseous pollutant. Smoke consists of tiny solid particles (particulate matter) suspended in air.

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Acid Rain

Normal rain is slightly acidic (pH about 5.6) because CO₂ from the air dissolves in rain to form carbonic acid. But when it becomes much more acidic (pH below 5.6), it is called acid rain.

Gases responsible for acid rain:

Oxides of sulphur (SO₂, SO₃) dissolve in rain to form sulphurous acid and sulphuric acid.
Oxides of nitrogen (NO, NO₂) dissolve in rain to form nitrous acid and nitric acid.

Only sulphur oxides and nitrogen oxides cause acid rain. Oxides of carbon (CO₂) make rain very slightly acidic (the natural pH 5.6) but are NOT the primary cause of acid rain.

Effects of acid rain:

Acid rain corrodes metal structures (bridges, railings).
Acid rain damages buildings and monuments made of marble (CaCO₃). It dissolves the calcium carbonate in the marble.
Acid rain kills fish and other aquatic life by making lakes and rivers acidic.
Acid rain damages crops and forests.

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Greenhouse Effect and Global Warming

Think of a glass greenhouse in which plants are grown. Glass lets in sunlight but traps the heat inside. The Earth’s atmosphere does something similar.

Sunlight passes through the atmosphere and heats the Earth’s surface. The Earth emits this heat as infrared (long-wave) radiation back toward space. Certain gases in the atmosphere absorb this outgoing infrared radiation and trap it. This is the greenhouse effect.

Greenhouse Gases:

Gas	Source	Relative Contribution
Carbon dioxide (CO₂)	Burning of fossil fuels, deforestation	Biggest contributor to greenhouse effect
Methane (CH₄)	Cattle, rice paddies, landfills, natural gas leaks	Very potent greenhouse gas
Nitrous oxide (N₂O)	Fertilisers, vehicle exhaust	Absorbs infrared radiation [NDA 2010-I]
Water vapour (H₂O)	Evaporation	Natural greenhouse gas
Ozone (O₃)	Photochemical reactions	Tropospheric ozone contributes
CFCs	Refrigerants, aerosols	Very powerful but now being phased out

Carbon dioxide is the biggest contributor to the greenhouse effect and global warming.

Excess greenhouse gases trap too much heat. This causes global warming — the gradual rise in average temperature of the Earth.

Effects of Global Warming:

Melting of ice caps and glaciers.
Rise in sea level (ocean water expands as it warms). CO₂ is responsible for the expansion of water in the ocean due to global warming]
Extreme weather events (floods, droughts, storms).
Displacement of coastal populations.

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Ozone Layer Depletion

The ozone layer is in the stratosphere (about 15-40 km above Earth’s surface). It absorbs harmful UV radiation from the Sun.

CFCs and Ozone Depletion:

CFCs (Chlorofluorocarbons) are compounds used in refrigerators and air conditioners.

In the stratosphere, UV light breaks down CFCs and releases chlorine radicals (Cl·).

Chlorine radicals destroy ozone in a chain reaction. One Cl radical can destroy thousands of ozone molecules.

CFC → Cl radical (from UV light) → Cl + O₃ → ClO + O₂ (ozone is destroyed)

Chlorine radicals (Cl·) initiate the chain reaction for ozone depletion. [NDA tested via CDS — CDS 2013-I Q111]

Gaseous hypochlorous acid (HOCl) and chlorine are photolysed by sunlight to produce chlorine radicals. This is how the chain reaction is sustained. [NDA tested via CDS — CDS 2013-I Q111]

Effect of Ozone Depletion:

Without the ozone layer, more UV radiation reaches the Earth’s surface.

UV radiation causes: skin cancer, eye cataracts, weakened immune systems in humans. It also damages crops and marine ecosystems.

CFCs are being phased out under the Montreal Protocol (an international treaty signed in 1987).

Air Pollutants and Their Effects Match List: [NDA tested via CDS — CDS 2015-I Q144]

CFC → Ozone layer depletion
SO₂ → Acid rain
Lead compounds → Harmful to human nervous system
CO₂ → Biggest contribution to greenhouse effect

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Photochemical Smog

Smog is a combination of smoke and fog. There are two types: classical smog and photochemical smog.

Classical smog is formed in cold, humid conditions from SO₂ and particulate matter. It is a reducing smog.

Photochemical smog is formed in warm, sunny conditions. It forms when sunlight reacts with nitrogen oxides and unburnt hydrocarbons from vehicles.

Primary pollutants (NO, hydrocarbons) + sunlight → Secondary pollutants (ozone, peroxyacetyl nitrate/PAN, formaldehyde, nitrogen dioxide)

Photochemical smog causes eye irritation, respiratory problems, and damage to plants.

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Toxic Substances from Common Items

[NDA tested via CDS — CDS 2010-I Q49 Match List]

Item	Toxic Substance Released
CFL lamp	Mercury (Hg)
Automobile battery	Lead (Pb)
Polymer (plastic)	Phthalates
Diesel engine	Nitrogen oxides

CFL (compact fluorescent lamp) contains a small amount of mercury vapour inside the tube. If the bulb breaks, mercury is released. Mercury is toxic to the nervous system.

Automobile batteries contain lead-acid. Lead is harmful to the nervous system, especially in children.

Diesel engines produce nitrogen oxides (NOₓ) at high combustion temperatures.

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Water Pollution

Water pollution is the contamination of water bodies (rivers, lakes, groundwater) with harmful substances.

Sources of water pollution:

Industrial effluents (waste water from factories containing heavy metals, chemicals).
Agricultural runoff (containing fertilisers, pesticides, herbicides).
Domestic sewage (containing organic waste, disease-causing bacteria).
Thermal pollution (hot water released by power plants raising water temperature).

BOD (Biochemical Oxygen Demand):

BOD is the amount of oxygen consumed by bacteria to break down organic matter in water. [NDA tested via CDS]

A high BOD means there is a lot of organic pollution in the water. Bacteria consuming this organic matter use up all the dissolved oxygen. Fish and other aquatic life cannot survive without oxygen.

Clean water has a low BOD. Polluted water has a high BOD.

Eutrophication:

Excess fertilisers containing nitrogen and phosphorus drain into lakes and ponds.

This causes rapid growth of algae (algal bloom) on the water surface.

The algae blocks sunlight from reaching underwater plants.

When the algae die, bacteria decompose them and consume all the dissolved oxygen.

This oxygen depletion kills fish and other aquatic life. The lake becomes a dead zone.

This process is called eutrophication. [NDA tested via CDS]

Arsenic Contamination:

Arsenic (As) is a toxic heavy metal that contaminates groundwater in some regions. It naturally leaches from rocks into the groundwater.

Arsenic exposure causes melanosis (skin darkening), skin lesions, and cancer. [NDA tested via CDS — CDS 2013-I Q102 Match List]

Methods to remove arsenic from contaminated water:
Reverse osmosis removes arsenic effectively.
Ion exchange resins remove arsenic.
Coagulation-adsorption (using alum or ferric salts) removes arsenic.

Boiling does NOT remove arsenic from water. Arsenic is a dissolved chemical, not a biological contaminant. Boiling only kills bacteria. [NDA 2011-I]

Fluoride in Groundwater:

Excessive fluoride in drinking water causes fluorosis (damage to teeth and bones).

Ground water is most likely to be contaminated with fluoride.

Heavy Metal Pollution:

Heavy metals like mercury, lead, cadmium, and arsenic are toxic even in trace amounts.

Mercury causes Minamata disease (neurological damage, seen in Japan after industrial mercury discharge).

Lead from automobiles (from leaded petrol, now banned) and lead-acid batteries causes neurological damage. [NDA 2009-I]

Cadmium causes Itai-itai disease (painful bone disease).

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Soil Pollution and Dolomite

Soil pollution is caused by excessive pesticide use, industrial waste, and acid rain.

Dolomite (CaMg(CO₃)₂) powder is applied to acidic agricultural soil to raise the pH of the soil.

Acidic soil prevents many crops from growing properly. Dolomite is a base that neutralises the acid in soil and raises the pH to a healthier level.

This is similar to how an antacid neutralises acid in the stomach.

Excessive use of phosphate and potassium fertilisers leads to the presence of toxic substances in groundwater. [NDA tested via CDS — CDS 2016-II Q167]

Pesticides and Biomagnification:

DDT is a pesticide that is not biodegradable. It persists in the environment for many years.

DDT enters the food chain through plants and water. Each animal that eats below it in the food chain accumulates DDT. At each step up the food chain, DDT becomes more concentrated.

This is called biomagnification or bioaccumulation. The highest concentration of DDT is found in top predators (eagles, humans).

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Diseases Caused by Environmental Agents

Agent	Disease Caused
Arsenic	Melanosis (skin darkening)
Fluoride	Fluorosis (teeth and bone damage)
Silica dust (from glass/pottery/mining)	Silicosis (lung disease)
Noise (prolonged high-level noise)	Presbycusis (hearing loss)
Mercury	Minamata disease (neurological damage)
Lead	Neurological damage, especially in children
Asbestos	Mesothelioma (cancer of lung lining)

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Organic Farming

Organic farming avoids synthetic chemicals (fertilisers, pesticides). It uses natural methods to grow crops.

Benefits of organic farming:

It does NOT reduce CO₂ emissions significantly (this statement is FALSE).
It does NOT lead to toxic effects (TRUE — no synthetic pesticides or fertilisers).
It DOES improve the water-retention capacity of the soil (TRUE — organic matter improves soil structure).

So the correct facts about organic farming: it avoids toxic chemical use and it improves soil water retention. But it does not significantly reduce CO₂ emissions.

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Weedicides and Pesticides

2,4-D (2,4-dichlorophenoxyacetic acid) is a chemical commonly used by farmers to destroy weeds (it is a herbicide/weedicide).

DDT, Malathion, and Methyl bromide are pesticides (used to kill insects and pests), not weedicides.

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