Environmental Chemistry — Revision Notes

⚡ 30-Second Revision

Air Pollutants — $SO_x, NO_x, CO, CO_2$, Hydrocarbons, Particulates.
Acid Rain — Caused by $SO_2, NO_x \rightarrow H_2SO_4, HNO_3$. pH < 5.6.
Smog — Classical (smoke, fog, $SO_2$, reducing); Photochemical (sunlight, $NO_x$, VOCs, $O_3$, PAN, oxidizing).
Ozone Depletion — Stratospheric $O_3$ destroyed by CFCs ($dot{Cl}$ radicals).
Greenhouse Gases — $CO_2, CH_4, N_2O$, CFCs, $O_3$ (tropospheric).
Water Pollutants — Pathogens, Organic waste, Chemical pollutants (pesticides, heavy metals).
BOD — Biochemical Oxygen Demand. High BOD = high organic pollution.
Eutrophication — Nutrient enrichment $ightarrow$ algal bloom $ightarrow$ oxygen depletion $ightarrow$ aquatic death.
Fluoride in water — Optimal $1,\text{ppm}$, $> 2,\text{ppm}$ causes fluorosis.
Green Chemistry — 12 principles for sustainable chemical processes.

2-Minute Revision

Environmental Chemistry focuses on the chemical processes in our surroundings and human impact. Air pollution is categorized into tropospheric (lower atmosphere) and stratospheric (upper atmosphere). Tropospheric pollutants like sulfur oxides ($SO_x$) and nitrogen oxides ($NO_x$) cause acid rain, while carbon monoxide (CO) is toxic due to its affinity for hemoglobin.

Photochemical smog, prevalent in sunny climates, involves $NO_x$, hydrocarbons, and forms harmful ozone and PAN. Stratospheric pollution primarily concerns ozone layer depletion by chlorofluorocarbons (CFCs), leading to increased UV radiation.

Water pollution is assessed by BOD (Biochemical Oxygen Demand), where high values indicate organic contamination. Eutrophication, caused by nutrient runoff, leads to algal blooms and oxygen depletion, harming aquatic life.

Soil pollution involves pesticides and industrial wastes. Green chemistry offers a sustainable approach to minimize hazardous substances in chemical processes, emphasizing prevention over remediation.

Remember key examples, sources, effects, and permissible limits for NEET.

5-Minute Revision

Environmental Chemistry is the study of chemical phenomena in the environment and the impact of human activities. It's broadly divided into atmospheric, water, and soil pollution.

Atmospheric Pollution:

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Tropospheric Pollution — Occurs in the lowest layer of the atmosphere.

* Gaseous Pollutants: $SO_2$ (from fossil fuels) causes acid rain ($SO_2 + H_2O \rightarrow H_2SO_3$). $NO_x$ (from high-temp combustion) also causes acid rain and contributes to smog. CO (incomplete combustion) is highly toxic, binding to hemoglobin.

$CO_2$ is a major greenhouse gas. Hydrocarbons are carcinogenic and contribute to smog. * Smog: * Classical Smog: Smoke + fog + $SO_2$. Reducing. Occurs in cool, humid conditions. * Photochemical Smog: Sunlight + $NO_x$ + VOCs.

Oxidizing. Components: $O_3$, PAN, acrolein. Causes eye irritation, respiratory problems.

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Stratospheric Pollution — Concerns the ozone layer ($O_3$) at 15-50 km altitude, which absorbs UV radiation.

* Ozone Depletion: Primarily caused by CFCs. UV breaks CFCs to release $dot{Cl}$ radicals: $CF_2Cl_2 xrightarrow{hv} dot{C}F_2Cl + dot{Cl}$. $dot{Cl}$ catalytically destroys $O_3$: $dot{Cl} + O_3 \rightarrow Cldot{O} + O_2$; $Cldot{O} + O \rightarrow dot{Cl} + O_2$. Leads to 'ozone hole', increased UV, skin cancer.

Water Pollution: Contamination of water bodies.

Sources — Sewage, industrial effluents, agricultural runoff.
Parameters

* BOD (Biochemical Oxygen Demand): Oxygen required by microbes to decompose organic matter. High BOD (e.g., $>17,\text{ppm}$) indicates high pollution; low BOD (e.g., $<5,\text{ppm}$) indicates clean water. * Eutrophication: Nutrient enrichment (N, P) $ightarrow$ algal bloom $ightarrow$ oxygen depletion $ightarrow$ death of aquatic life.

Chemical Pollutants — Pesticides (DDT), heavy metals (Pb, Hg, Cd), fluorides (optimal $1,\text{ppm}$, $>2,\text{ppm}$ causes fluorosis).

Soil Pollution: Contamination by industrial waste, pesticides, solid waste.

Green Chemistry: Design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Focus on prevention, atom economy, safer solvents, etc.

Mini-Example: If a water sample has a BOD of $25,\text{ppm}$, it signifies a high level of organic pollution, meaning a large amount of oxygen will be consumed by microorganisms, making the water unsuitable for most aquatic life. This is a critical indicator for NEET questions.

Prelims Revision Notes

Environmental Chemistry is a high-yield chapter for NEET, primarily testing factual recall and conceptual understanding.

I. Atmospheric Pollution

Tropospheric Pollution (Lower Atmosphere)

* Gaseous Pollutants: * **Oxides of Sulfur ($SO_2, SO_3$)**: Sources: Burning fossil fuels (coal, oil). Effects: Acid rain ($H_2SO_4$), respiratory diseases. * **Oxides of Nitrogen ($NO, NO_2$)**: Sources: High-temperature combustion (vehicles, power plants).

Effects: Acid rain ($HNO_3$), respiratory problems, photochemical smog precursor. $NO_2$ is reddish-brown. * Carbon Monoxide (CO): Source: Incomplete combustion. Effect: Highly toxic, forms carboxyhemoglobin (300x affinity for Hb than $O_2$).

* **Carbon Dioxide ($CO_2$)**: Source: Combustion of fossil fuels, deforestation. Effect: Major greenhouse gas, global warming. * Hydrocarbons: Source: Unburnt fuels. Effects: Carcinogenic, photochemical smog.

* Smog: * Classical Smog (London Smog): Cool, humid. Smoke + fog + $SO_2$. Reducing agent. * Photochemical Smog (Los Angeles Smog): Warm, sunny. $NO_x$ + VOCs + sunlight. Oxidizing agent.

Components: $O_3$, PAN (Peroxyacetyl Nitrate), acrolein, formaldehyde. Effects: Eye irritation, respiratory issues, plant damage. * Particulate Pollutants: Dust, mist, smoke, fumes, soot. Effects: Respiratory diseases, reduced visibility.

Stratospheric Pollution (Upper Atmosphere)

* **Ozone Layer ($O_3$)**: Located 15-50 km, absorbs UV radiation. * Ozone Depletion: Caused by Chlorofluorocarbons (CFCs). CFCs release $dot{Cl}$ radicals under UV: $CF_2Cl_2 xrightarrow{hv} dot{C}F_2Cl + dot{Cl}$.

$dot{Cl}$ catalytically destroys $O_3$: $dot{Cl} + O_3 \rightarrow Cldot{O} + O_2$; $Cldot{O} + O \rightarrow dot{Cl} + O_2$. Effects: Increased UV radiation, skin cancer, cataracts, damage to crops/plankton.

* Greenhouse Effect/Global Warming: Greenhouse gases ($CO_2, CH_4, N_2O$, CFCs, tropospheric $O_3$) trap heat. Enhanced effect leads to global warming.

II. Water Pollution

Sources — Domestic sewage, industrial effluents, agricultural runoff.
Key Parameters/Concepts

* BOD (Biochemical Oxygen Demand): Amount of $O_2$ required by microbes to decompose organic matter. High BOD $ightarrow$ high organic pollution $ightarrow$ low dissolved $O_2$. Clean water BOD: $<5,\text{ppm}$.

Highly polluted water BOD: $>17,\text{ppm}$. * Eutrophication: Nutrient enrichment (N, P) $ightarrow$ algal bloom $ightarrow$ decomposition $ightarrow$ $O_2$ depletion $ightarrow$ death of aquatic life.

* Pathogens: Bacteria from sewage (e.g., typhoid, cholera). * Chemical Pollutants: * Pesticides: DDT, Aldrin (non-biodegradable, biomagnification). * Heavy Metals: Pb, Hg, Cd (toxic, from industrial waste).

Permissible limits: Pb ($50,\text{ppb}$), Cd ($5,\text{ppb}$), Hg ($1,\text{ppb}$). * Fluoride: Optimal $1,\text{ppm}$ (prevents tooth decay). $>2,\text{ppm}$ causes brown mottling (fluorosis). $>10,\text{ppm}$ causes bone damage.

III. Soil Pollution

Sources — Pesticides, industrial waste, municipal solid waste.

IV. Green Chemistry: Sustainable approach to minimize hazardous substances. 12 principles (e.g., prevention, atom economy, safer solvents, catalysis, design for degradation).

Vyyuha Quick Recall

To remember the major Greenhouse Gases, think of 'Water N' CO-CH-CFC':

Water — vapor ($H_2O$) - though not typically listed in MCQs as a primary anthropogenic concern, it's the most abundant natural GHG.
Nitrous oxide ($N_2O$)
CO2 (Carbon Dioxide)
CH4 (Methane)
CFCs (Chlorofluorocarbons)

This mnemonic covers the main anthropogenic greenhouse gases frequently tested in NEET.