Environmental Pollution and Green Chemistry

ChemistryEnvironmental ChemistryFor JEE aspirants

Environmental pollution is any unwanted change in air, water or soil that harms plants, animals or people; the substance responsible is a pollutant. Green chemistry is the answer from the other side: design processes so that pollutants are never made. This page covers environmental pollution and green chemistry the way exams test them: the key reactions (acid rain, photochemical smog, ozone depletion), the numbers (pH 5.6, BOD, fluoride limits) and the reason behind each effect. Environmental chemistry is in the JEE Advanced syllabus (it was dropped from JEE Main and NEET in 2024).

On this page1Air layers2Gaseous pollutants3Greenhouse effect4Acid rain5Smog6Ozone layer7Water8Soil and waste9Green chemistry

Key Formulas - Quick Reference

★ Must learn Natural rain: $C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$ , $H_{2} C O_{3} ⇌ H^{+} + HC O_{3}^{-}$ gives pH 5.6; acid rain has $pH < 5.6$ .
★ Must learn Acid rain: $2S O_{2} + O_{2} + 2 H_{2} O \to 2 H_{2} S O_{4}$ and $4N O_{2} + O_{2} + 2 H_{2} O \to 4HN O_{3}$ .
Nitric oxide: $N_{2} + O_{2} 1483 K 2NO$ (engines, lightning), then $2NO + O_{2} \to 2N O_{2}$ .
CO poisoning: $HbCO$ is about 300 times more stable than $Hb O_{2}$ ; 3-4% carboxyhaemoglobin badly cuts oxygen supply.
★ Must learn Photochemical smog: $N O_{2} h ν NO + O$ , $O + O_{2} \to O_{3}$ , $NO + O_{3} \to N O_{2} + O_{2}$ ; products: HCHO, acrolein, PAN.
★ Must learn Ozone depletion: $C F_{2} C l_{2} UV C l^{∙} + C F_{2} C l^{∙}$ , $C l^{∙} + O_{3} \to Cl O^{∙} + O_{2}$ , $Cl O^{∙} + O \to C l^{∙} + O_{2}$ .
★ Must learn Water: fish growth stops if DO $< 6$ ppm; BOD $< 5$ ppm clean, $\geq 17$ ppm highly polluted.
★ Must learn Drinking water: $F^{-}$ about 1 ppm ( $> 2$ ppm mottling, $> 10$ ppm bone damage); $Pb$ 50 ppb; $S O_{4}^{2 -}$ 500 ppm; $N O_{3}^{-}$ 50 ppm; $Cd$ 0.005 ppm.
Marble damage: $CaC O_{3} + H_{2} S O_{4} \to CaS O_{4} + H_{2} O + C O_{2}$ .
Green ethanal: $2C H_{2} = C H_{2} + O_{2} PdC l_{2} / CuC l_{2}, water 2C H_{3} CHO$ (90% yield).

1. Pollution and the Layers of the Atmosphere

Pollutant: a solid, liquid or gas present at higher concentration than its natural abundance, released by human activity or natural events, that harms living things. Environmental chemistry studies the origin, transport, reactions, effects and fate of chemical species in the environment.

A pollutant travels a fixed path: it leaves a source, is carried by air or water, and ends in a sink (soil, water bodies, living tissue). Air pollution matters most because an adult breathes about 13 kg of air a day, several times the mass of food and water taken, so even a few ppm in air is a large dose.

Degradable pollutants

Broken down quickly by natural processes. Example: discarded vegetables.

Slowly degradable (persistent)

Stay unchanged for decades and are hard to remove: DDT, plastics, heavy metals, nuclear waste.

The atmosphere is not uniform. Its two lowest layers matter here: the troposphere (from sea level to about 10 km) and the stratosphere (about 10 to 50 km). Atmospheric pollution is therefore studied as tropospheric pollution and stratospheric pollution.

Figure 1: Standard-atmosphere profile: the air cools to

- 56^{\circ} C

at the top of the troposphere, then warms again in the stratosphere because ozone (15-35 km) absorbs UV. Smog, acid rain and greenhouse gases belong to the troposphere; the ozone hole belongs to the stratosphere.

Layer	Height	Contains	Pollution studied
Troposphere	0 to about 10 km	air, much water vapour, clouds, dust; turbulent	gases ( $S O_{2}$ , $N O_{x}$ , CO, $C O_{2}$ ), particulates, smog, acid rain, global warming
Stratosphere	about 10 to 50 km	$N_{2}$ , $O_{2}$ , $O_{3}$ , very little water vapour	ozone layer depletion by CFCs; the ozone hole

The ozone layer stops about 99.5% of the Sun's harmful ultraviolet (UV) radiation from reaching the surface. The same ozone at ground level is a pollutant (Section 5).

2. Gaseous Pollutants in the Troposphere

Tropospheric pollutants are of two kinds: gaseous (oxides of sulphur, nitrogen and carbon, $H_{2} S$ , hydrocarbons, ozone and other oxidants) and particulate (dust, mist, fumes, smoke, smog; Section 5). Almost every gaseous pollutant comes from burning fuel.

2.1 Oxides of sulphur

Burning sulphur-containing coal and oil gives $S O_{2}$ , a gas poisonous to animals and plants. Even low levels cause asthma, bronchitis and emphysema, and irritate the eyes (tears, redness); high levels make flower buds stiff so they fall off. Oxidation of $S O_{2}$ to $S O_{3}$ is slow on its own but fast when catalysed by particulate matter, or when ozone or hydrogen peroxide is present:

2S O_{2} + O_{2} particulates 2S O_{3}

S O_{2} + O_{3} S O_{3} + O_{2}

S O_{2} + H_{2} O_{2} H_{2} S O_{4}

2.2 Oxides of nitrogen

$N_{2}$ and $O_{2}$ do not react at ordinary temperature. At the very high temperature of lightning or of an automobile engine they combine to give NO, which is oxidised to $N O_{2}$ at once; with ozone (for example in the stratosphere) the oxidation is even faster:

N_{2} + O_{2} 1483 K 2NO

2NO + O_{2} 2N O_{2}

NO + O_{3} N O_{2} + O_{2}

Lightning-made $N O_{2}$ is oxidised to nitrate, $N O_{3}^{-}$ , which rain washes into soil as a natural fertiliser. In cities, $N O_{2}$ causes the irritant red-brown haze of heavy traffic, damages leaves and slows photosynthesis, irritates the lungs (acute respiratory disease in children), and attacks textile fibres and metals.

JEE Advanced

Why do $N_{2}$ and $O_{2}$ react only in engines and lightning? For $N_{2} + O_{2} ⇌ 2NO$ , $Δ H^{\circ} = + 180.6$ kJ mol $^{- 1}$ and $Δ S^{\circ} = + 24.8$ J K $^{- 1}$ mol $^{- 1}$ . With $Δ G^{\circ} = Δ H^{\circ} - T Δ S^{\circ}$ and $K = e^{- Δ G^{\circ} / R T}$ :

$T$ (K)	$Δ G^{\circ}$ (kJ mol $^{- 1}$ )	$K$	NO in hot air at equilibrium
298	173	$4.4 \times 1 0^{- 31}$	none (about $1 0^{- 10}$ ppm)
1483	144	$8.6 \times 1 0^{- 6}$	about 1200 ppm
2000	131	$3.8 \times 1 0^{- 4}$	about 7900 ppm

The reaction is endothermic, so $K$ rises steeply with temperature (van 't Hoff). Hot exhaust cools in milliseconds, which freezes the NO made in the cylinder; it then turns into $N O_{2}$ in air. Here $x_{NO} = K x_{N_{2}} x_{O_{2}}$ with 78% $N_{2}$ and 21% $O_{2}$ .

2.3 Hydrocarbons

Hydrocarbons (compounds of C and H only) come from incomplete combustion of fuel in vehicles. They are carcinogenic, and in plants they cause ageing, breakdown of tissue and shedding of leaves, flowers and twigs. Unburnt hydrocarbons are also a raw material of photochemical smog.

2.4 Carbon monoxide

CO is colourless and odourless, which makes it one of the most dangerous air pollutants. It forms by incomplete combustion of carbon: mainly vehicle exhaust (worse in poorly maintained vehicles), and also coal, firewood and petrol burnt with too little air.

Hb + CO ⇌ HbCO

CO binds to haemoglobin (Hb) to form carboxyhaemoglobin, about 300 times more stable than the oxygen-haemoglobin complex. Once about 3-4% of the haemoglobin is tied up as HbCO, the oxygen-carrying capacity of blood falls sharply: headache, weak eyesight, nervousness and cardiovascular disorders follow. Smokers carry extra CO; in pregnant smokers it can cause premature birth, spontaneous abortion and deformed babies.

Figure 2: Because

HbCO

is about 300 times more stable than

Hb O_{2}

, only about 22-29 ppm of CO in air (

0.002

0.003%

) is enough to reach the dangerous 3-4% carboxyhaemoglobin level.

C O_{2}

does not block haemoglobin this way, which is why CO is far more poisonous.

2.5 Carbon dioxide

$C O_{2}$ comes from respiration, burning fossil fuels, decomposition of limestone in cement making ( $CaC O_{3} \to CaO + C O_{2}$ ) and volcanoes. It stays in the troposphere. NCERT gives about 0.03% by volume; the level today is about 0.042% (420 ppm). Green plants remove $C O_{2}$ in photosynthesis; deforestation and fuel burning upset this balance, and the extra $C O_{2}$ is the main cause of global warming.

Pollutant	Main source	Key chemistry	Main harm
$S O_{2}$	coal, oil with S	$S O_{2}$ $\to$ $S O_{3}$ $\to$ $H_{2} S O_{4}$	lungs, eyes, acid rain, flower buds
NO, $N O_{2}$	engines, lightning	1483 K; NO $\to$ $N O_{2}$ $\to$ $HN O_{3}$	red-brown haze, lungs, leaves, smog, acid rain
Hydrocarbons	incomplete combustion	raw material of smog	carcinogenic; plant ageing
CO	exhaust, too little air	$HbCO$ , 300 times more stable	oxygen starvation of tissues
$C O_{2}$	fuels, cement, respiration	absorbs infrared	global warming

Key idea

Burn a fuel and you get its oxides: sulphur gives

S O_{2}

, air's own

N_{2}

gives NO (only when hot), carbon gives CO (too little air) or

C O_{2}

. Control the combustion and you control most air pollution.

3. Global Warming and the Greenhouse Effect

About 75% of the solar energy reaching Earth is absorbed by the surface, which warms up; the rest is radiated back. The warm surface gives off infrared (heat) radiation. Gases such as $C O_{2}$ , $C H_{4}$ , $O_{3}$ , CFCs, $N_{2} O$ and water vapour absorb this infrared and send part of it back, so the air near the surface stays warm. This is the greenhouse effect; its enhancement by extra gases is global warming.

Figure 3: Greenhouse gases are transparent to incoming sunlight but absorb the outgoing infrared, like the glass of a greenhouse. Without them the energy balance gives about

255 K

(

- 18^{\circ} C

); with them the average is about

15^{\circ} C

. More

C O_{2}

means more IR returned, so global warming.

A glass greenhouse works the same way: sunlight passes through the glass and heats the soil and plants, but the glass is opaque to the infrared they emit, so the heat stays inside. The natural greenhouse effect is essential: without it Earth's average temperature would be about 255 K ( $- 18$ °C) instead of about 15 °C. The trouble starts when the gases increase beyond the natural balance.

Greenhouse gas	Where it comes from	Note
$C O_{2}$	fossil fuels, deforestation	largest contributor
$C H_{4}$	vegetation burnt, digested or rotting without air; paddy fields, coal mines, garbage dumps, fossil fuels	forms by anaerobic decay
$N_{2} O$	natural; rising with fertilisers and fuel burning
CFCs	man-made: air conditioners, refrigerators	also destroy the ozone layer
$O_{3}$ , water vapour	natural

If the trend continues, polar ice caps melt and low-lying land floods; warmer weather also spreads infectious diseases such as dengue, malaria, yellow fever and sleeping sickness. Remedies: use fuels efficiently, use fewer vehicles (bicycle, public transport, car pools), plant trees, and avoid burning dry leaves and wood.

Exam Trick

Two identical atoms? Not a greenhouse gas. A gas absorbs infrared only if its vibration changes its dipole moment. $N_{2}$ and $O_{2}$ (99% of air) cannot, so they are never the answer to "which is a greenhouse gas". $C O_{2}$ , $H_{2} O$ , $C H_{4}$ , $N_{2} O$ , $O_{3}$ and CFCs can.

Quick Recall: tap to check

Define environmental chemistry. (NCERT 14.1)

The study of the origin, transport, reactions, effects and fates of chemical species in the environment.

List the gases responsible for the greenhouse effect. (NCERT 14.4)

C O_{2}

(the major one),

C H_{4}

N_{2} O

, CFCs,

O_{3}

and water vapour.

Why is

C O_{2}

a greenhouse gas while

O_{2}

is not?

Bending and asymmetric stretching of

C O_{2}

change its dipole moment, so it absorbs infrared; the only vibration of

O_{2}

does not.

4. Acid Rain

Even unpolluted rain is acidic, because it dissolves atmospheric $C O_{2}$ :

H_{2} O + C O_{2} ⇌ H_{2} C O_{3}

H_{2} C O_{3} ⇌ H^{+} + HC O_{3}^{-}

Normal rain water has pH about 5.6. Rain with pH below 5.6 is called acid rain. Acid rain is one form of acid deposition: acid from the air reaching the surface either wet (in rain, fog, snow) or dry (gases and particles settling directly).

Burning coal and oil in power stations and furnaces, and petrol and diesel in engines, releases $S O_{2}$ and nitrogen oxides. Oxidised in air (again catalysed by particulates) and dissolved in water, they give the two acids of acid rain:

2S O_{2} + O_{2} + 2 H_{2} O 2 H_{2} S O_{4}

4N O_{2} + O_{2} + 2 H_{2} O 4HN O_{3}

Ammonium salts formed in the air appear as a haze (an aerosol of fine particles). Aerosol particles of oxides or ammonium salts inside rain drops give wet deposition; $S O_{2}$ absorbed directly on solid and liquid surfaces is dry deposition.

Figure 4: Acid rain is rain with pH below 5.6. Natural rain is already acidic:

C O_{2}

alone brings it to about 5.6 (computed for 420 ppm

C O_{2}

). Extra acidity comes from

H_{2} S O_{4}

and

HN O_{3}

formed from

S O_{2}

and

N O_{x}

Soil and crops: dissolves and washes away plant nutrients.
Health: respiratory ailments in people and animals.
Lakes and rivers: acidic run-off harms aquatic plants and animals.
Water pipes: corroded, so iron, lead and copper leach into drinking water.
Buildings: stone and metal structures are attacked; marble turns to gypsum.

4.1 The Taj Mahal and marble damage

Air around Agra carries high levels of sulphur and nitrogen oxides from nearby industries, power plants and domestic burning of poor-quality coal, kerosene and firewood. The acid attacks marble:

CaC O_{3} + H_{2} S O_{4} CaS O_{4} + H_{2} O + C O_{2}

The marble loses its lustre and becomes discoloured and disfigured. The government's 1995 action plan for the Taj Trapezium (Agra, Firozabad, Mathura, Bharatpur) moved more than 2000 industries from coal and oil to natural gas or LPG (via a pipeline bringing over half a million cubic metres of gas a day), encouraged LPG in homes and low-sulphur diesel on nearby highways; the Mathura refinery also cut its emissions.

4.2 Controlling acid rain

Cut the $S O_{2}$ and $N O_{x}$ at the source: fewer fossil-fuel vehicles, low-sulphur fuels, natural gas instead of coal. Fit catalytic converters in cars: a ceramic honeycomb coated with Pd, Pt and Rh converts unburnt fuel, CO and $N O_{x}$ (at about 573 K) into $C O_{2}$ and $N_{2}$ :

2CO + O_{2} Pt, Pd, Rh 2C O_{2}

2NO + 2CO Pt, Rh N_{2} + 2C O_{2}

Soil already acidified is neutralised with powdered limestone ( $CaC O_{3}$ ).

5. Particulates and Smog

5.1 Particulate pollutants

Particulates are minute solid particles or liquid droplets in air, from vehicle exhaust, smoke, dust and industrial ash. Viable particulates are living (bacteria, fungi, moulds, algae): some fungi cause allergies in people and diseases in plants. Non-viable particulates are classified by nature and size:

Type	What it is	Examples
Smoke	solid, or solid + liquid, particles from burning organic matter	cigarette smoke; smoke from fossil fuel, garbage, dry leaves; oil smoke
Dust	fine solid particles (over $1 μ m$ ) from crushing, grinding, attrition	sand blasting, saw dust, pulverised coal, cement, fly ash, dust storms
Mists	spray droplets and condensed vapour	sulphuric acid mist; stray herbicide and insecticide sprays
Fumes	condensed vapour from sublimation, distillation, boiling, reactions	organic solvents, metals, metal oxides

The effect depends on particle size. Particles larger than about $5 μ m$ are trapped in the nasal passage; particles of about $1 μ m$ and smaller reach deep into the lungs (air-quality indices now track PM10 and PM2.5, particles below 10 and $2.5 μ m$ ). Lead from leaded petrol was once the main airborne lead in Indian cities; unleaded petrol has replaced it across India. Lead interferes with the development and maturation of red blood cells.

5.2 Two kinds of smog

Smog = smoke + fog, the most common air pollution of cities.

Feature	Classical (London) smog	Photochemical (Los Angeles) smog
Climate	cool, humid	warm, dry, sunny
Made of	smoke, fog and $S O_{2}$	products of sunlight acting on $N O_{x}$ and unsaturated hydrocarbons
Chemical nature	reducing ( $S O_{2}$ )	oxidising ( $O_{3}$ , $N O_{2}$ , PAN)
Key species	$S O_{2}$ , soot, $H_{2} S O_{4}$ droplets	$O_{3}$ , NO, $N O_{2}$ , HCHO, acrolein, PAN
Time	night and morning, winter	afternoon peak, summer

Exam Trick

London reduces, LA oxidises. Classical smog is coal + fog + $S O_{2}$ , a reducing mixture; photochemical smog is cars + sunshine, full of oxidants ( $O_{3}$ , PAN). Swap the city and you swap every property in the table.

5.3 How photochemical smog forms

Burning fuel releases two primary pollutants: unburnt hydrocarbons and NO. When they build up, sunlight starts a chain in which NO is converted into $N O_{2}$ . $N O_{2}$ absorbs sunlight and splits; the oxygen atom makes ozone; ozone turns NO back into $N O_{2}$ (a brown gas that adds to the haze):

N O_{2} h ν NO + O (i)

O + O_{2} ⇌ O_{3} (ii)

NO + O_{3} N O_{2} + O_{2} (iii)

Reactions (i)-(iii) alone make no net ozone. Hydrocarbon radicals, however, convert NO into $N O_{2}$ without using ozone, so ozone accumulates. Ozone and $N O_{2}$ , both strong oxidising agents, then oxidise the unburnt hydrocarbons to formaldehyde, acrolein and peroxyacetyl nitrate (PAN):

3C H_{4} + 2 O_{3} 3C H_{2} = O + 3 H_{2} O

Figure 5: Reactions (i)-(iii) only shuttle oxygen atoms around; hydrocarbons break the cycle, so

O_{3}

accumulates and oxidises them to formaldehyde, acrolein and PAN, the eye-irritating secondary pollutants.

Figure 6: Box-model result for a sunny, polluted city: NO from traffic peaks first (0.25 ppm), hydrocarbon radicals turn it into

N O_{2}

(0.27 ppm), and only when NO is used up does

O_{3}

pile up, peaking after midday (0.31 ppm). Ozone is a secondary pollutant: no exhaust pipe emits it.

5.4 Effects and control

Ozone and PAN are powerful eye irritants; ozone and NO irritate the nose and throat, and at high levels cause headache, chest pain, dry throat, cough and difficulty in breathing. Photochemical smog cracks rubber, damages plants and corrodes metals, stone, building materials and painted surfaces.

Control the primary precursors ( $N O_{2}$ and hydrocarbons) and the secondary ones ( $O_{3}$ , PAN) fall automatically. Catalytic converters stop NO and hydrocarbons at the tail pipe, and plants such as Pinus, Juniperus, Quercus, Pyrus and Vitis metabolise nitrogen oxide, so planting them helps.

Key idea

Ozone in smog is a secondary pollutant: no exhaust pipe emits it. It builds up only when sunlight and hydrocarbons turn NO into

N O_{2}

, so cutting

N O_{x}

and hydrocarbons cuts ozone and PAN.

6. Stratospheric Pollution: the Ozone Layer

6.1 How ozone is formed and broken

The upper stratosphere holds a layer of ozone that absorbs harmful UV radiation (NCERT quotes $λ$ about 255 nm), which would otherwise cause skin cancer (melanoma). UV splits $O_{2}$ into atoms, and an atom adds to $O_{2}$ to give ozone:

O_{2} UV O + O

O + O_{2} + M O_{3} + M

Why the M? NCERT writes $O + O_{2} ⇌ O_{3}$ with UV over the arrow. Strictly, UV drives only the backward step (ozone absorbs UV and splits into $O_{2} + O$ ); the forward step needs a third molecule M ( $N_{2}$ or $O_{2}$ ) to carry away the energy released, or the new $O_{3}$ would fly apart again.

Ozone is thermodynamically unstable and decomposes to $O_{2}$ , so a dynamic equilibrium exists between its production and decomposition. Pollution tips this balance.

6.2 CFCs and the chlorine chain

Chlorofluorocarbons (CFCs, freons) are non-reactive, non-flammable and non-toxic, so they were used in refrigerators, air conditioners, plastic foam and for cleaning computer parts. That same inertness lets them survive, mix with air and drift up to the stratosphere, where strong UV breaks them into chlorine radicals:

C F_{2} C l_{2} UV C l^{∙} + C F_{2} C l^{∙} (i)

C l^{∙} + O_{3} Cl O^{∙} + O_{2} (ii)

Cl O^{∙} + O C l^{∙} + O_{2} (iii)

Figure 7: In clean air ozone is made and broken at equal rates. CFCs add a chlorine atom that is regenerated every turn (it is a catalyst), so a single

C l^{∙}

can destroy about

1 0^{5}

ozone molecules.

Chlorine radicals are regenerated in step (iii), so CFCs act as carriers that keep delivering chlorine to the stratosphere. The Montreal Protocol (1987) phased out CFCs; their replacements (HCFCs, then HFCs) release little or no chlorine.

Count the chlorine. Add (ii) and (iii): $C l^{∙}$ and $Cl O^{∙}$ cancel and the net change is $O_{3} + O \to 2 O_{2}$ . A species that appears on both sides is a catalyst, so one $C l^{∙}$ can destroy about $1 0^{5}$ ozone molecules before it is trapped.

6.3 The ozone hole

In the 1980s scientists found ozone over the South Pole falling sharply each spring: the ozone hole. Its cause is a special sequence of conditions. For most of the year chlorine is held in harmless reservoirs:

Cl O^{∙} + N O_{2} ClON O_{2} (iv)

C l^{∙} + C H_{4} C H_{3}^{∙} + HCl (v)

In the dark Antarctic winter, polar stratospheric clouds form. On their ice surfaces the reservoirs react, storing chlorine as HOCl and $C l_{2}$ (the $HN O_{3}$ stays in the ice):

ClON O_{2} + H_{2} O HOCl + HN O_{3} (vi)

ClON O_{2} + HCl C l_{2} + HN O_{3} (vii)

When sunlight returns in spring, the clouds break up and sunlight splits HOCl and $C l_{2}$ ; the chlorine radicals then run the chain of Section 6.2:

HOCl h ν H O^{∙} + C l^{∙} (viii)

C l_{2} h ν 2C l^{∙} (ix)

Figure 8: The ozone hole needs all three stages: harmless chlorine reservoirs, winter ice clouds that turn them into

C l_{2}

and HOCl, and spring sunlight that releases

C l^{∙}

. That is why the hole appears over Antarctica in September-October, not in winter.

6.4 Effects of ozone depletion

More UV reaches the troposphere, causing ageing of skin, cataract, sunburn and skin cancer; it kills many phytoplankton (which lowers fish productivity), damages plant proteins (harmful mutations), increases evaporation through the stomata of leaves so soil loses moisture, and makes paints and fibres fade faster.

Ozone up high (stratosphere)

Good: absorbs UV, protects life. Problem: too little, destroyed by $C l^{∙}$ from CFCs.

Ozone near the ground (troposphere)

Bad: toxic smog oxidant, eye irritant, cracks rubber. Problem: too much, made from $N O_{x}$ + hydrocarbons + sunlight.

Key idea

CFCs are dangerous because they are stable: they last long enough to reach the stratosphere, and there every chlorine atom they release works as a catalyst.

Quick Recall: tap to check

What is smog? How is classical smog different from photochemical smog? (NCERT 14.6)

Smog = smoke + fog. Classical smog (cool, humid; smoke, fog,

S O_{2}

) is reducing; photochemical smog (warm, dry, sunny;

O_{3}

N O_{x}

, PAN, aldehydes) is oxidising.

Write the reactions by which CFCs destroy ozone. (NCERT 14.9)

C F_{2} C l_{2} UV C l^{∙} + C F_{2} C l^{∙}

;

C l^{∙} + O_{3} \to Cl O^{∙} + O_{2}

;

Cl O^{∙} + O \to C l^{∙} + O_{2}

What is the ozone hole and what are its consequences? (NCERT 14.10)

The sharp spring thinning of ozone over Antarctica. More UV reaches the surface: skin cancer, cataract, sunburn, loss of phytoplankton and fish, plant mutations, drier soil, faded paints and fibres.

7. Water Pollution

Water pollution comes from human activity and reaches surface water or ground water by different routes.

Point source

Easily identified place of entry: municipal and industrial discharge pipes.

Non-point source

No single identifiable source: farm run-off, acid rain, storm-water drainage from streets, parking lots and lawns.

Pollutant	Source
Micro-organisms	domestic sewage
Organic wastes	sewage, animal excreta, decaying plants and animals, food-processing discharge
Plant nutrients	chemical fertilisers
Toxic heavy metals	industries and chemical factories
Sediments	soil erosion by agriculture and strip mining
Pesticides	chemicals used against insects, fungi and weeds
Radioactive substances	mining of uranium-containing minerals
Heat	cooling water from industries

7.1 Pathogens

The most serious water pollutants are disease-causing pathogens, bacteria and other organisms from domestic sewage and animal excreta. Human excreta carry Escherichia coli and Streptococcus faecalis, which cause gastrointestinal diseases.

7.2 Organic wastes, DO and BOD

Leaves, grass, trash (carried in run-off) and excess phytoplankton are biodegradable organic wastes. Bacteria that decompose them consume the dissolved oxygen (DO), and water holds very little oxygen: about 9-10 ppm at ordinary temperatures (NCERT quotes up to 10 ppm in cold water), against about 200,000 ppm of oxygen in air. So even moderate organic matter can empty water of its oxygen.

DO below 6 ppm: growth of fish is inhibited.
Oxygen enters from the air and from photosynthesis by day; at night plants only respire, so DO falls.
If all DO is used, aerobic life dies and anaerobic bacteria take over, producing foul-smelling chemicals harmful to health.

Biochemical Oxygen Demand (BOD): the amount of oxygen needed by bacteria to break down the organic matter in a given volume of water. It measures the organic load. Clean water: BOD below 5 ppm; highly polluted water: 17 ppm or more.

Figure 9: Oxygen-sag curve (Streeter-Phelps model,

k_{1} = 0.35

and

k_{2} = 0.60

per day,

20^{\circ} C

): as bacteria eat the organic load, dissolved

O_{2}

falls to 3.4 ppm and stays below 6 ppm from day 0.5 to day 5.5. Excess phosphate does the same job indirectly, through dead algae.

7.3 Chemical pollutants and eutrophication

Water dissolves many inorganic chemicals. Heavy metals (Cd, Hg, Ni and others) are dangerous because the body cannot excrete them: they accumulate past the tolerance limit and damage kidneys, the central nervous system and the liver. Acids from mine drainage and salts such as NaCl and $CaC l_{2}$ (used to melt snow) are other soluble pollutants.

Organic chemicals: petroleum (oil spills), pesticides drifting from sprays or run-off, and industrial chemicals such as polychlorinated biphenyls (PCBs), used as cleansing solvents and suspected carcinogens. Even biodegradable detergents cause harm: bacteria feeding on them multiply and use up the dissolved oxygen, killing fish and plants.

Eutrophication: phosphates from fertilisers and detergents feed a dense growth of algae (algal bloom). It covers the surface and cuts the oxygen supply, decaying matter produces anaerobic conditions with foul smell and animal deaths, and biodiversity is lost.

7.4 International standards for drinking water

Substance	Limit or level	Why it matters
Fluoride, $F^{-}$	about 1 ppm (1 mg dm $^{- 3}$ ) is added	too little: tooth decay; above 2 ppm: brown mottling of teeth; above 10 ppm: bones and teeth harmed (reported from parts of Rajasthan)
Lead	about 50 ppb (NCERT)	from lead pipes; damages kidney, liver, reproductive system
Sulphate, $S O_{4}^{2 -}$	above 500 ppm harmful	laxative effect; harmless at moderate levels
Nitrate, $N O_{3}^{-}$	50 ppm maximum	methemoglobinemia (blue baby syndrome)

Fluoride hardens teeth by converting the enamel, hydroxyapatite, into much harder fluorapatite:

C a_{10} (P O_{4})_{6} (OH)_{2} + 2 F^{-} C a_{10} (P O_{4})_{6} F_{2} + 2O H^{-}

NCERT writes the same two minerals as $[3 C a_{3} (P O_{4})_{2} \cdot Ca (OH)_{2}]$ (hydroxyapatite) and $[3 C a_{3} (P O_{4})_{2} \cdot Ca F_{2}]$ (fluorapatite). Current WHO and BIS guidelines set lead lower, at 10 ppb.

Metal	Fe	Mn	Al	Cu	Zn	Cd
Maximum (ppm or mg dm $^{- 3}$ )	0.2	0.05	0.2	3.0	5.0	0.005

Figure 10: Fluoride has a narrow useful window (about 1 ppm). On the log scale the limits span five powers of ten: cadmium (

0.005

ppm) is allowed 1000 times less than zinc (5 ppm) because the body cannot excrete heavy metals. Red bars are the most toxic metals.

Exam Trick

5-6-17 and 1-2-10. Oxygen: BOD below 5 is clean, DO below 6 starves fish, BOD of 17 or more is highly polluted. Fluoride: 1 ppm protects teeth, 2 stains them, 10 harms bones.

At home: do not dump waste into drains that reach rivers or ponds, use compost instead of chemical fertiliser, use dried neem leaves instead of DDT or malathion, and add a few crystals of $KMn O_{4}$ or bleaching powder to the water tank.

Key idea

Organic waste, detergents and phosphates all kill fish the same way: they feed bacteria or algae whose decay uses up dissolved oxygen. High BOD means low DO.

8. Soil Pollution and Industrial Waste

Insecticides, pesticides and herbicides protect crops and stored food from insects, rodents, weeds and disease, but they also pollute the soil, so they must be used judiciously.

8.1 Pesticides

Before World War II: natural chemicals such as nicotine (tobacco planted in the field).
DDT: first used to control malaria and other insect-borne diseases, then in farming. Insects became resistant, and it is now banned for agricultural use in India.
Aldrin, dieldrin (organochlorines): water-insoluble and non-biodegradable, so they pass from lower to higher trophic levels and build up (biomagnification).
Organophosphates and carbamates: less persistent and more biodegradable, but severe nerve toxins, more harmful to humans (deaths of farm workers). Insects became resistant to these too.

Figure 11: Each step up the food chain concentrates a persistent, fat-soluble toxin about 10 times, so after six transfers the top consumer holds

1 0^{6}

times the level in water. This is why DDT and aldrin were replaced by less persistent pesticides.

8.2 Herbicides

The industry has shifted to herbicides such as sodium chlorate ( $NaCl O_{3}$ ) and sodium arsenite ( $N a_{3} As O_{3}$ ). Most are toxic to mammals but less persistent than organochlorines, decomposing in a few months; they still concentrate in the food web, some cause birth defects, and herbicide-sprayed corn fields suffer more insect attack and disease than hand-weeded ones.

8.3 Industrial solid waste

Type	Produced by
Biodegradable	cotton mills, food processing units, paper mills, textile factories
Non-biodegradable	thermal power plants (fly ash); integrated iron and steel plants (blast furnace slag, steel melting slag); Al, Zn and Cu industries (mud and tailings); fertiliser industries (gypsum)
Hazardous	inflammables, composite explosives, highly reactive substances from metal, chemical, drug, pharmaceutical, dye, pesticide and rubber industries

Poorly disposed non-degradable waste threatens the environment. Better uses now exist: fly ash and slag are used by the cement industry. Large amounts of toxic waste are destroyed by controlled incineration (small amounts burnt with factory garbage in open bins pollute the air).

9. Controlling Pollution and Green Chemistry

9.1 Waste management

Besides household waste there are medical, agricultural, industrial and mining wastes; improper disposal is a major cause of environmental damage.

Domestic waste goes into small bins, then community bins (private or municipal workers).
It is carried to the disposal site and sorted into biodegradable and non-biodegradable parts.
Non-biodegradable waste (plastic, glass, metal scrap) is sent for recycling.
Biodegradable waste is placed in landfills and turned into compost.

Uncollected waste enters sewers or is eaten by cattle: polythene bags choke sewers and can kill animals, and contaminated ground water spreads epidemics. Rag pickers and waste workers need gloves, water-proof boots and gas masks. Recycling ideas: fuel from plastic waste (high octane, lead-free, a "green fuel"), clothes from recycled plastic, and garbage turned into biogas (methane from bacterial culture) for electricity, with the residue used as manure. The Swachh Bharat Abhiyan (launched 2014) works through SBM-Urban (open-defecation-free cities, scientific solid-waste management) and SBM-Gramin (rural sanitation), with its first target set for 2 October 2019, the 150th birth anniversary of Mahatma Gandhi.

9.2 What green chemistry means

Green chemistry uses existing knowledge of chemistry to stop pollution at its source: production processes that give minimum pollution, energy use and waste. By-products that are not used add to pollution and to cost, so they are both environmentally and economically unsound.

Choose starting materials and conditions that convert reactants into useful products with yield close to 100%.
Use an eco-friendly medium: water has a high specific heat and low volatility, and is cheap, non-flammable and non-carcinogenic.
Avoid toxic organic solvents such as benzene, toluene and carbon tetrachloride.
Control temperature, pressure and catalysts, since they decide how far a reaction goes.

A useful number for comparing routes (beyond NCERT, often used in problems) is the atom economy:

atom economy = \frac{mass of desired product}{total mass of reactants} \times 100%

9.3 Green chemistry in daily life

Dry cleaning: tetrachloroethene, $C l_{2} C = CC l_{2}$ , contaminates ground water and is a suspected carcinogen; it is being replaced by liquid $C O_{2}$ with a suitable detergent.
Bleaching: $H_{2} O_{2}$ now bleaches clothes (better results, less water) and, with a catalyst, paper; earlier paper was bleached with chlorine gas.
Ethanal: made in one step by oxidising ethene in water with an ionic catalyst, 90% yield:

2C H_{2} = C H_{2} + O_{2} PdC l_{2} / CuC l_{2}, water 2C H_{3} CHO

Turbid water: powder of tamarind-seed kernel (an agricultural waste) cleans municipal and industrial waste water; it is non-toxic, biodegradable and cheap, unlike alum, which adds toxic ions.

Figure 12: Each green swap removes a hazardous substance at the source instead of treating it later: fewer by-products, safer solvents, higher yield. The Wacker route puts every atom of the reactants into ethanal (100% atom economy).

The 2005 Nobel Prize in chemistry (Yves Chauvin, Robert Grubbs, Richard Schrock) honoured metathesis, a way to swap groups of atoms between molecules ("couples changing partners in a dance") that cuts hazardous waste in making drugs, food products and greener polymers.

Personal steps help too: keep a compost tin for kitchen waste, carry a cloth bag instead of plastic, and send newspapers, glass and aluminium for recycling.

Quick Recall: tap to check

What are pesticides and herbicides? Give examples. (NCERT 14.15)

Pesticides kill pests: DDT, aldrin, dieldrin (organochlorines), organophosphates such as malathion, carbamates. Herbicides kill weeds:

NaCl O_{3}

N a_{3} As O_{3}

What is green chemistry, and how does it reduce pollution? (NCERT 14.16)

Designing processes that give maximum useful product with safe reagents and media, so fewer pollutants are made at all: liquid

C O_{2}

dry cleaning,

H_{2} O_{2}

bleaching, one-step ethanal in water.

How would you control water and soil pollution in your area? (NCERT 14.12, 14.14)

Treat sewage before discharge; do not dump waste in drains; use compost instead of fertilisers and neem instead of DDT; use pesticides sparingly; segregate and recycle waste; report polluters to the Pollution Control Board.

10. Quick Revision: Flowchart and Mind Map

Most questions give a clue and ask for the phenomenon, its species or its reactions. The flowchart below sorts every air-pollution clue; the mind map collects the whole topic.

Figure 13: Ask the four questions in this order and every air-pollution question on this page falls into one box, together with the species you are expected to write.

Figure 14: The whole topic on one page: eight branches, each with the reaction or number most often asked.

11. Solved Examples

Solved Example 1

Which statement about photochemical smog is correct?
(A) It forms in cool, humid weather and is reducing in nature.
(B) Its main oxidants are

O_{3}

and PAN, and it forms in warm, dry, sunny weather.
(C) It is caused mainly by

S O_{2}

and smoke from coal.
(D) Its ozone is emitted directly by vehicle exhausts.

Solution:

Answer: (B). (A) and (C) describe classical smog. (D) is wrong because ozone is a secondary pollutant: it forms in air from $N O_{2}$ photolysis and builds up only when hydrocarbons turn NO into $N O_{2}$ .

Solved Example 2

Four water samples were tested: P (BOD 3 ppm, DO 8 ppm), Q (BOD 8 ppm, DO 6.5 ppm), R (BOD 12 ppm, DO 5 ppm) and S (BOD 20 ppm, DO 2 ppm). Which option is correct?
(A) P is highly polluted
(B) Q cannot support fish
(C) S is highly polluted and fish cannot grow in it
(D) all four are clean

Solution:

Answer: (C). BOD of 17 ppm or more means highly polluted water, and DO below 6 ppm inhibits fish growth: S has BOD 20 ppm and DO 2 ppm. P (BOD below 5 ppm) is clean; Q still has DO above 6 ppm.

Solved Example 3

Show that unpolluted rain has pH about 5.6. Air contains 420 ppm

C O_{2}

, so

p_{C O_{2}} = 4.2 \times 1 0^{- 4}

atm. Henry's constant of

C O_{2}

3.4 \times 1 0^{- 2}

mol L

^{- 1}

atm

^{- 1}

and

K_{a 1}

H_{2} C O_{3}

4.3 \times 1 0^{- 7}

Solution:

Dissolved $C O_{2}$ : $c = K_{H} p = 3.4 \times 1 0^{- 2} \times 4.2 \times 1 0^{- 4} = 1.43 \times 1 0^{- 5}$ mol L $^{- 1}$ .
Only the first ionisation matters ( $K_{a 2}$ is far smaller): $[H^{+}] = K_{a 1} c$ .
$[H^{+}] = 4.3 \times 1 0^{- 7} \times 1.43 \times 1 0^{- 5} = 2.48 \times 1 0^{- 6}$ mol L $^{- 1}$ .
$pH = - lo g (2.48 \times 1 0^{- 6}) = 5.61$ , so about 5.6. With NCERT's 0.03% $C O_{2}$ the answer is 5.68; only rain below 5.6 is acid rain.

Solved Example 4

Carbon monoxide is more dangerous than carbon dioxide. Why? (NCERT 14.3) Using

\frac{[ HbCO ]}{[ Hb O _{2} ]} = 300 \times \frac{p _{CO}}{p _{O_{2}}}

, find the CO level in air (21%

O_{2}

) that ties up 4% of haemoglobin.

Solution:

Reason: CO binds haemoglobin about 300 times more strongly than $O_{2}$ , so it blocks oxygen transport even at trace levels and is colourless and odourless. $C O_{2}$ does not block haemoglobin; it is harmful only at very high levels (and through global warming).

4% HbCO means $\frac{[ HbCO ]}{[ Hb O _{2} ]} = \frac{0.04}{0.96} = 0.0417$ .
$p_{CO} = \frac{0.0417 \times 0.21}{300} = 2.92 \times 1 0^{- 5}$ atm.
That is 29 ppm, only 0.003% of the air (compare 420 ppm of harmless $C O_{2}$ ).

Solved Example 5

Which statements about stratospheric ozone depletion are correct?
(A)

C l^{∙}

acts as a catalyst.
(B) The net result of the chlorine cycle is

O_{3} + O \to 2 O_{2}

.
(C)

ClON O_{2}

and HCl are chlorine reservoirs that do not destroy ozone themselves.
(D) The ozone hole is deepest in the Antarctic mid-winter (June-July).

Solution:

Answer: (A), (B) and (C). Adding $C l^{∙} + O_{3} \to Cl O^{∙} + O_{2}$ and $Cl O^{∙} + O \to C l^{∙} + O_{2}$ cancels the chlorine species, leaving $O_{3} + O \to 2 O_{2}$ , so (A) and (B) hold. (C) holds: chlorine is inactive while stored in reservoirs. (D) is wrong: winter clouds only convert the reservoirs into $C l_{2}$ and HOCl; ozone loss starts when sunlight returns in spring (September-October).

Solved Example 6

What is BOD? (NCERT 14.13) In a test, 10 mL of sewage is diluted to 300 mL with aerated water. Its DO is 8.8 ppm at the start and 4.3 ppm after 5 days at 20 °C in the dark. Find the 5-day BOD and judge the sample.

Solution:

BOD is the oxygen needed by bacteria to break down the organic matter in a given volume of water; the standard test measures the DO used in 5 days at 20 °C.

Oxygen used in the diluted bottle: $8.8 - 4.3 = 4.5$ ppm.
Fraction of sewage in the bottle: $P = \frac{10}{300}$ .
$BOD_{5} = \frac{4.5}{P} = 4.5 \times 30 = 135$ ppm.
Far above 17 ppm: raw, heavily polluted sewage. It must be treated before it reaches a river, or it will strip the river of oxygen (Figure 9).

Solved Example 7

A large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping, but phytoplankton is abundant. Suggest a reason. (NCERT 14.18)

Solution:

The lake is eutrophic. Phosphate and nitrate run-off (fertilisers, detergents) fed an algal bloom. The bloom blocks light, algae die, and bacteria decomposing them (plus night-time respiration) use up the dissolved oxygen. Once DO falls well below 6 ppm, fish suffocate. No poison is needed: the oxygen demand does the killing.

Solved Example 8

Compare the atom economy of two routes to ethanal (molar masses:

C H_{3} CHO

44.05,

C_{2} H_{4}

28.05,

O_{2}

32.00,

C_{2} H_{5} OH

46.07,

K_{2} C r_{2} O_{7}

294.18,

H_{2} S O_{4}

98.08 g mol

^{- 1}

). Route 1:

2C H_{2} = C H_{2} + O_{2} \to 2C H_{3} CHO

. Route 2:

3C H_{3} C H_{2} OH + K_{2} C r_{2} O_{7} + 4 H_{2} S O_{4} \to 3C H_{3} CHO + K_{2} S O_{4} + C r_{2} (S O_{4})_{3} + 7 H_{2} O

Solution:

Route 1: $\frac{2 \times 44.05}{2 \times 28.05 + 32.00} \times 100 = 100%$ . Every atom ends up in the product.
Route 2: reactant mass $= 3 (46.07) + 294.18 + 4 (98.08) = 824.7$ g; product $= 3 (44.05) = 132.15$ g.
Atom economy $= \frac{132.15}{824.7} \times 100 = 16%$ : 84% of the mass becomes chromium and sulphate waste.
Route 1 (the Wacker-type process of NCERT, 90% yield in water) is the green route.

Practice Questions

Explain tropospheric pollution in about 100 words. (NCERT 14.2)Answer: undesirable gases ( $S O_{2}$ , $N O_{x}$ , CO, $C O_{2}$ , hydrocarbons, $O_{3}$ ) and particulates (dust, smoke, mist, fumes, smog) in the lowest 10 km of air, mostly from burning fuel; they cause respiratory disease, acid rain, smog and global warming.
Statues and monuments in India are affected by acid rain. How? (NCERT 14.5)Answer: $CaC O_{3} + H_{2} S O_{4} \to CaS O_{4} + H_{2} O + C O_{2}$ ; marble (Taj Mahal) turns to gypsum, loses lustre and is disfigured.
Write the reactions involved in the formation of photochemical smog. (NCERT 14.7)Answer: $N O_{2} h ν NO + O$ ; $O + O_{2} \to O_{3}$ ; $NO + O_{3} \to N O_{2} + O_{2}$ ; then $O_{3}$ and $N O_{2}$ oxidise hydrocarbons to HCHO, acrolein and PAN.
What are the harmful effects of photochemical smog, and how can they be controlled? (NCERT 14.8)Answer: eye irritation ( $O_{3}$ , PAN), nose and throat irritation, headache, chest pain, cough; cracked rubber, plant damage, corrosion. Control: catalytic converters, less NO and hydrocarbon emission, plants such as Pinus and Quercus.
What are the major causes of water pollution? (NCERT 14.11)Answer: pathogens from sewage, biodegradable organic wastes (high BOD), chemical pollutants (heavy metals, acids, salts, petroleum, pesticides, PCBs, detergents, fertilisers), sediments, radioactive substances and heat.
What would happen if greenhouse gases were totally missing from the atmosphere? (NCERT 14.17)Answer: no infrared would be trapped; the energy balance gives about 255 K ( $- 18$ °C) instead of about 15 °C, so Earth would be frozen and unfit for most life, with extreme day-night swings.
How can domestic waste be used as manure? How should a compost pit be run to avoid bad odour and flies? (NCERT 14.19, 14.20)Answer: separate biodegradable kitchen and garden waste into the pit; keep it covered with a soil layer, moist and turned for air (aerobic decay does not smell), keep plastics and glass out for recycling; the product is manure that replaces chemical fertiliser.

Common Mistakes to Avoid

Watch out

Calling any rain with pH below 7 acid rain. Natural rain is already about 5.6 because of $C O_{2}$ ; acid rain is below 5.6.
Swapping the smogs: classical smog is $S O_{2}$ + smoke + fog and reducing; photochemical smog is $N O_{x}$ + hydrocarbons + sunlight and oxidising ( $O_{3}$ , PAN).
Treating ozone as always good. Stratospheric ozone shields UV; ground-level ozone is a toxic smog pollutant.
Writing $C l^{∙}$ as used up in ozone depletion. It is regenerated each cycle (a catalyst); $ClON O_{2}$ and HCl are harmless reservoirs.
Reading high BOD as good water. High BOD means much organic matter and therefore low dissolved oxygen.
Saying the ozone hole forms in winter. Winter clouds only prepare $C l_{2}$ and HOCl; ozone is destroyed in spring, when sunlight returns.
Thinking all fluoride is harmful (or all harmless): about 1 ppm prevents decay, above 2 ppm mottles teeth, above 10 ppm harms bones.
Assuming biodegradable detergents are safe in water: the bacteria that eat them use up dissolved oxygen, and phosphate additives cause eutrophication.

Frequently Asked Questions

What is the difference between classical smog and photochemical smog?

Classical smog forms in cool, humid weather from smoke, fog and sulphur dioxide, and is reducing. Photochemical smog forms in warm, dry, sunny weather when sunlight acts on nitrogen oxides and hydrocarbons from vehicles; it is oxidising, rich in ozone, nitrogen dioxide and PAN, and peaks in the afternoon.

Why is normal rain slightly acidic, with pH about 5.6?

Rain dissolves carbon dioxide from the air, forming carbonic acid, which ionises to give hydrogen ions and bicarbonate. At today's carbon dioxide level this gives a pH of about 5.6. Only rain with pH below 5.6, made more acidic by sulphuric and nitric acids, is called acid rain.

How do CFCs destroy the ozone layer?

CFCs are so stable that they reach the stratosphere, where ultraviolet light breaks them into chlorine radicals. A chlorine radical turns ozone into chlorine monoxide and oxygen, and chlorine monoxide reacts with an oxygen atom to give back the chlorine radical. Being regenerated, one chlorine radical destroys about a hundred thousand ozone molecules.

What is BOD and what value shows that water is polluted?

Biochemical oxygen demand is the amount of oxygen bacteria need to break down the organic matter in a given volume of water. It measures organic pollution. Clean water has a BOD below 5 ppm, while highly polluted water has 17 ppm or more. High BOD leaves little dissolved oxygen for fish.

Why is carbon monoxide more dangerous than carbon dioxide?

Carbon monoxide binds haemoglobin about 300 times more firmly than oxygen, forming carboxyhaemoglobin, so blood cannot carry oxygen. About 3 to 4 percent carboxyhaemoglobin already causes headache and heart strain, and CO is colourless and odourless. Carbon dioxide does not block haemoglobin in this way.

What is green chemistry? Give two everyday examples.

Green chemistry designs chemical processes that give maximum useful product with safe reagents and solvents, so pollutants are not made at all. Examples: liquid carbon dioxide with a detergent replacing tetrachloroethene in dry cleaning, and hydrogen peroxide replacing chlorine for bleaching paper and clothes.

Is environmental chemistry in the JEE Advanced syllabus?

Yes. The JEE Advanced chemistry syllabus lists environmental chemistry: atmospheric, water and soil pollution, industrial waste, strategies to control pollution and green chemistry. It was removed from JEE Main in 2024. Expect statement-based, multiple-correct and small numerical questions on pH, BOD, ppm limits and reactions.

Is environmental chemistry asked in NEET?

No. Environmental chemistry was dropped from the NEET (UG) chemistry syllabus from 2024, after NCERT rationalised the Class 11 book. NEET aspirants can skip it for chemistry, although BOD still appears in biology under sewage treatment in Microbes in Human Welfare.

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