Environmental Chemistry
The study of chemical processes occurring in the environment and the impact of human activities on air, water, and the atmosphere.
A. Air Pollution
Major Air Pollutants
| Pollutant | Main Source | Effect |
|---|---|---|
| CO₂ (Carbon Dioxide) | Combustion of fossil fuels, deforestation | Main greenhouse gas — global warming, ocean acidification |
| CO (Carbon Monoxide) | Incomplete combustion (car engines, gas heaters) | Toxic — binds to haemoglobin (250× stronger than O₂) → prevents O₂ transport → death |
| SO₂ (Sulfur Dioxide) | Burning sulfur-containing fuels (coal, oil), volcanic eruptions | Acid rain (SO₂ + H₂O → H₂SO₃), respiratory damage, corrodes buildings |
| NOₓ (Nitrogen Oxides) | Vehicle exhausts, power plants (high temperature combustion) | Acid rain, photochemical smog, ozone depletion in lower atmosphere |
| CH₄ (Methane) | Livestock digestion, landfills, rice paddies, natural gas leaks | Potent greenhouse gas (80× more powerful than CO₂ over 20 years) |
| CFCs (Chlorofluorocarbons) | Old refrigerants (fridges, AC), aerosol sprays, foam packaging | Ozone layer depletion — Cl radicals destroy O₃ molecules in stratosphere |
| Particulates (PM2.5) | Industry, vehicle exhaust, burning biomass | Respiratory disease, reduced visibility, lung cancer |
⚡ MCQ Tip CO binds haemoglobin — toxic even in small amounts. SO₂ = acid rain. CFCs = ozone depletion. CO₂ = main greenhouse gas. NOₓ = smog + acid rain. Methane is more potent per molecule than CO₂ as a greenhouse gas.
B. Greenhouse Effect & Global Warming
The Greenhouse Mechanism
- Short-wave solar radiation passes through the atmosphere and warms the Earth's surface
- Earth re-emits long-wave infrared (heat) radiation back upward
- Greenhouse gases absorb and re-emit this infrared radiation — trapping heat in the lower atmosphere
- The natural greenhouse effect is essential for life — without it, average Earth temp = −18°C
- Enhanced greenhouse effect: human activities increase greenhouse gas concentrations → extra warming → global warming
| Greenhouse Gas | Main Human Source |
|---|---|
| CO₂ | Burning fossil fuels, cement production, deforestation |
| CH₄ (Methane) | Agriculture (livestock), landfills, natural gas |
| N₂O (Nitrous oxide) | Fertilisers, vehicle exhausts |
| CFCs | Old refrigerants, aerosols (also destroys ozone) |
| H₂O vapour | Natural amplifier — increases with warming |
⚡ Effects of Global Warming Melting ice caps and glaciers → rising sea levels → coastal flooding. More extreme weather (droughts, floods, hurricanes). Coral bleaching. Species extinction. Shifts in agriculture zones.
C. Ozone Layer
Ozone Layer — Protection & Depletion
- Ozone (O₃) layer is located in the stratosphere (15–35 km altitude)
- Absorbs and blocks harmful UV-B and UV-C radiation from reaching Earth's surface
- Without ozone layer: increased skin cancer, cataracts, damage to marine ecosystems, crop failure
- CFCs (Chlorofluorocarbons) are the main cause of ozone depletion
- Mechanism: CFCs rise to stratosphere → UV breaks them down → Cl radicals released → Cl + O₃ → ClO + O₂ (each Cl atom can destroy ~100,000 O₃ molecules)
- Ozone hole discovered over Antarctica in 1985
- Montreal Protocol (1987): international treaty to phase out CFC production — most successful environmental agreement
D. Water Pollution
Water Pollutants & Effects
| Pollutant | Source | Effect |
|---|---|---|
| Sewage | Domestic/industrial waste water | Spreads waterborne diseases (cholera, typhoid); depletes dissolved oxygen |
| Fertilisers (Nitrates/Phosphates) | Agricultural runoff | Eutrophication — algal bloom → algae block sunlight → plants die → bacteria decompose them → oxygen depletion → aquatic life dies |
| Heavy metals (Pb, Hg, Cd) | Industrial discharge, mining | Bioaccumulation up food chain; nervous system damage; Minamata disease (Hg poisoning) |
| Oil spills | Tanker accidents, offshore drilling | Kills marine life, coats seabirds' feathers (prevents flight/insulation), destroys coastal ecosystems |
| Acid rain | SO₂ + NOₓ from industry/vehicles + H₂O | Acidifies lakes and rivers → kills fish; damages buildings, statues, forests |
| Plastic waste | Improper disposal | Microplastics enter food chain; strangles/poisons marine animals |
Acid Rain Formation
SO₂ + H₂O → H₂SO₃ (sulfurous acid)
Also: 2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄ (sulfuric acid — stronger acid rain)
⚡ Eutrophication sequence (MCQ favourite): Excess fertiliser → algal bloom → algae block sunlight → aquatic plants die → bacteria decompose dead matter → bacteria use up dissolved O₂ → fish and aquatic life die.
Quick MCQ Revision
| Fact | Answer |
|---|---|
| CO is toxic because | Binds haemoglobin 250× stronger than O₂ → prevents oxygen transport |
| SO₂ causes | Acid rain and respiratory problems |
| CFCs cause | Ozone layer depletion in the stratosphere |
| Montreal Protocol (1987) | International treaty to phase out CFCs |
| Main greenhouse gas (human) | CO₂ from burning fossil fuels |
| Eutrophication caused by | Excess nitrates/phosphates from fertilisers → algal bloom → O₂ depletion |
| Ozone layer location | Stratosphere (15–35 km) — protects from UV radiation |
| Acid rain formula | SO₂ + H₂O → H₂SO₃ (also NOₓ contributes) |