Environmental Chemistry

• Primary pollutant — emitted directly from a source: CO, SO₂, NOx, hydrocarbons, particulates (PM).
• Secondary pollutant — formed in the air from primary ones: ozone & PAN in photochemical smog, H₂SO₄ in acid rain.

Each winter a thick smog settles over Lahore and parts of Karachi. Cool, still air sits trapped below a warmer layer above — a temperature inversion — so vehicle and factory smoke and crop-burning soot cannot rise and disperse. PM₂.₅ climbs far above WHO limits and the skyline disappears into a grey haze.

The biggest single source of urban air pollution is the vehicle exhaust. Incomplete combustion of petrol releases carbon monoxide (CO), while the high temperatures inside the engine make nitrogen oxides (NOx) from the air itself. Unburnt fuel adds hydrocarbons (VOCs).

Catalytic converters on the exhaust convert CO and NOx to CO₂ and N₂, which is why cleaner engines matter so much in a crowded city.

Sunlight warms the ground; the warm ground radiates infrared (IR) back out. Greenhouse gases — chiefly CO₂ and CH₄ — absorb that IR and send it back down, trapping heat. A little of this keeps Earth liveable; too much is global warming.

• BOD — Biochemical Oxygen Demand: the O₂ microbes use to break down organic waste. High BOD = heavily polluted, with little oxygen left for fish.

Eutrophication: nitrate and phosphate fertiliser runoff over-feeds the algae, so an algal bloom spreads across the surface. The algae die, microbes decompose them and strip the water of dissolved oxygen, and the fish suffocate. Add untreated sewage and heavy metals (Pb, Hg) and the river turns lifeless.

High in the stratosphere, the ozone layer (O₃) soaks up the Sun's harmful UV-B — it is Earth's natural sunscreen. Without it, UV reaches the ground and causes skin cancer, cataracts and crop damage.

CFCs from old fridges and ACs drift up, UV frees a Cl• radical, and one chlorine atom destroys tens of thousands of ozone molecules — tearing the ozone hole. The Montreal Protocol (1987) banned CFCs. Slide the control to thin the shield and watch how much UV reaches the skin below.

Normal rain is mildly acidic (pH ≈ 5.6) from dissolved CO₂. Acid rain (pH 4–4.5) forms when SO₂ and NOx dissolve and oxidise in cloud water into sulphuric and nitric acids.

It damages leaves and forests, acidifies lakes and kills fish, and eats marble buildings: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Control: scrub SO₂ with lime, remove sulphur from fuels, fit catalytic converters.

• Green chemistry — designing products and processes that reduce or eliminate hazardous substances and waste at the source, rather than cleaning up afterwards.

• Prevent waste rather than treat it.
• Maximise atom economy (most atoms end up in the product).
• Use safer solvents (water) and renewable feedstocks.
• Use catalysts, not stoichiometric reagents; design for energy efficiency.
• Design products that break down safely (biodegradable).

1. Air pollution: primary vs secondary; the winter smog & inversion.
2. Vehicle exhaust: CO & NOx; catalytic converters.
3. Greenhouse effect: CO₂ & CH₄ → global warming → melting glaciers.
4. Water pollution: BOD/COD, eutrophication, heavy metals.
5. Ozone: the CFC Cl• catalytic chain, the hole, Montreal Protocol.
6. Acid rain: SO₂/NOx → H₂SO₄/HNO₃; effects & control.
7. Green chemistry: prevent waste, atom economy, safer solvents, catalysts.