CA08 — Environmental Chemistry

✈ Chemistry – CA08 · AFCAT General Awareness AFCAT Level

Environmental chemistry links chemistry to topical issues — global warming, ozone depletion, and air/water pollution. These questions appear in AFCAT frequently because they connect chemistry with current affairs. High scoring potential with focused preparation.

📌 AFCAT Focus: Acid rain caused by SO₂ + NO₂; greenhouse gases (CO₂ most significant, CH₄, N₂O, CFCs); ozone layer depletion by CFCs (Montreal Protocol 1987); BOD as water pollution measure; eutrophication (excess fertiliser → algae bloom → fish die); photochemical smog = NO₂ + hydrocarbons + sunlight.

1. Air Pollution — Key Pollutants

Fig. 1 — Major Air Pollutants: Source, Effect and Control Method

2. Greenhouse Effect & Ozone Layer

🌡️ Greenhouse Effect & Global Warming

Greenhouse gases trap infrared radiation from Earth's surface
Key gases (by significance): CO₂ (most impactful), CH₄ (methane), N₂O, H₂O vapour, CFCs
Enhanced greenhouse effect → global warming → climate change
Consequences: glaciers melting, sea level rise, extreme weather events
CO₂ level pre-industrial: 280 ppm; now over 420 ppm
Kyoto Protocol (1997) and Paris Agreement (2015) = international treaties

🌍 Ozone Layer Depletion

Ozone (O₃) layer in stratosphere (15–30 km) absorbs UV-B radiation
CFCs release Cl· radicals which destroy ozone: Cl + O₃ → ClO + O₂
One Cl atom destroys up to 100,000 O₃ molecules
Ozone hole mainly over Antarctica
Montreal Protocol 1987: International agreement to phase out CFCs
HFCs (hydrofluorocarbons) used as substitutes

3. Water Pollution

Key Water Pollution Terms for AFCAT:

● BOD (Biochemical Oxygen Demand): O₂ consumed by microorganisms to decompose organic matter. High BOD = heavily polluted water. Clean water BOD < 5 mg/L; sewage BOD > 200 mg/L.

● Eutrophication: Excess fertiliser runoff causes massive algae growth → algae consumes O₂ → fish die (dead zones in water bodies).

● Acid rain formation: SO₂ + H₂O → H₂SO₃ (sulfurous acid); NO₂ + H₂O → HNO₃ (nitric acid). pH of acid rain < 5.6 (normal rain is ≈5.6 due to dissolved CO₂).

● Photochemical smog: NO₂ + unburnt hydrocarbons + sunlight → ozone (O₃) + PAN (peroxyacetyl nitrate). Causes eye irritation, respiratory problems. Common in sunny cities with heavy traffic.

4. Primary vs Secondary Pollutants

This distinction is increasingly tested in AFCAT as it connects chemistry with environmental current affairs. Primary pollutants are emitted directly; secondary pollutants form through atmospheric reactions.

Fig. 2 — Primary vs Secondary Pollutants: Definition, Examples and Formation

📌 AFCAT Key Point: Ozone (O₃) is a secondary pollutant at ground level (formed by NO₂ + VOCs + sunlight) — harmful to lungs and plants — but a natural shield in the stratosphere where it blocks UV-B radiation. The same molecule is beneficial high up and harmful near the ground. This contrast is directly tested in AFCAT environmental questions.

📝 AFCAT PYQs — Environmental Chemistry

Q1. Acid rain is primarily caused by: AFCAT PYQ

(a) CO₂ and CO(b) SO₂ and NO₂(c) CFCs and O₃(d) CH₄ and N₂O

✔ Answer: (b) SO₂ and NO₂

SO₂ (from coal burning) and NO₂ (from vehicle exhaust) react with atmospheric water to form H₂SO₄ and HNO₃ — causing acid rain. Acid rain (pH < 5.6) corrodes limestone buildings (like the Taj Mahal), kills aquatic life, and damages forests. CO₂ causes normal rain to be slightly acidic (pH 5.6) but not acid rain.

Q2. The Montreal Protocol was designed to protect the: AFCAT PYQ

(a) Coral reefs(b) Ozone layer(c) Polar ice caps(d) Rainforests

✔ Answer: (b) Ozone layer

The Montreal Protocol (1987) is the international treaty to phase out the production and use of ozone-depleting substances, primarily CFCs. It is considered the most successful international environmental treaty. CFCs were used in refrigerators, air conditioners, and aerosol sprays. They release chlorine atoms in the stratosphere that catalytically destroy ozone molecules.

Q3. High BOD value in a water body indicates: AFCAT PYQ

(a) Clean water with high oxygen(b) High organic pollution(c) Presence of minerals(d) High pH

✔ Answer: (b) High organic pollution

BOD (Biochemical Oxygen Demand) measures oxygen consumed by microbes decomposing organic matter. High BOD = high organic pollution. Microorganisms consume all available dissolved oxygen while decomposing organic waste, leaving none for fish and aquatic life. Clean rivers: BOD < 5 mg/L; heavily polluted: >200 mg/L. This is a direct AFCAT environmental chemistry question.

Q4. Which greenhouse gas has the highest contribution to global warming? AFCAT PYQ

(a) CH₄(b) N₂O(c) CO₂(d) CFC

✔ Answer: (c) CO₂

While CH₄ is 80× more potent per molecule than CO₂ as a greenhouse gas over 20 years, CO₂ has the highest overall contribution to global warming because of its massive quantity in the atmosphere (~76% of GHG emissions). CO₂ lasts 100+ years in the atmosphere. AFCAT asks "highest contribution" = CO₂ due to volume, not per-molecule potency.

🧠 Quick Memory Chart — CA08

🌡 Greenhouse & Climate

Key GHGs: CO₂, CH₄, N₂O, CFCs
CO₂ = highest contribution (volume)
Kyoto Protocol (1997); Paris (2015)
Pre-industrial CO₂: 280 ppm
Current CO₂: >420 ppm

🌊 Water Pollution

BOD high = heavily polluted
Eutrophication: fertiliser → algae → O₂↓
Acid rain: SO₂ + NO₂ in atmosphere
Acid rain pH < 5.6
Taj Mahal affected by acid rain

🚨 Ozone & Smog

Ozone: stratosphere, absorbs UV-B
CFCs deplete ozone
Montreal Protocol 1987: banned CFCs
Photochem smog: NO₂+HC+sunlight
Ozone hole: mainly over Antarctica

📝 Practice Exercise

E1. The main cause of depletion of the ozone layer is:

(a) CO₂(b) SO₂(c) CFCs(d) NO₂

E2. Eutrophication of water bodies is caused by:

(a) Oil spills(b) Excess nitrogen and phosphorus from fertilisers(c) Heavy metal pollution(d) Plastic waste

E3. Carbon monoxide (CO) is dangerous because it:

(a) Is corrosive(b) Binds to haemoglobin preventing oxygen transport(c) Depletes ozone(d) Causes acid rain

E4. Which international protocol addresses ozone layer depletion?

(a) Kyoto Protocol(b) Paris Agreement(c) Montreal Protocol(d) Stockholm Convention

Answers:
E1 → (c) CFCs [chlorine radicals from CFCs catalytically destroy ozone molecules in stratosphere] | E2 → (b) Excess N and P from fertilisers [cause algal blooms; algae consumes O₂; fish die; dead zones form] | E3 → (b) Binds haemoglobin [CO has 200× higher affinity for Hb than O₂; forms carboxyhaemoglobin; no O₂ transport → suffocation] | E4 → (c) Montreal Protocol [1987; phase-out of CFCs; most successful environmental treaty]

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