Chemistry of the Environment

Water — testing and treatment

Water shows up in almost every practical procedure — as reactant, solvent, coolant or product. Two hydrated-salt colour changes tell you whether water is present at all; the boiling and melting points tell you whether it is pure.

Two colour tests for the presence of water

Both tests exploit reversible hydration reactions: CuSO₄ (white) + 5H₂O ⇌ CuSO₄·5H₂O (blue), and CoCl₂ (blue) + 6H₂O ⇌ CoCl₂·6H₂O (pink). Cobalt chloride is often supplied on absorbent paper — a blue paper turning pink is an unmistakable indicator that liquid water has touched it.

Checking purity

The presence of water is one question; whether it is pure water is another. Pure water has a single sharp boiling point of 100 °C and a single sharp melting point of 0 °C. Dissolved impurities raise the boiling point, lower the melting point, and make the transitions broad rather than sharp. For lab work, distilled water is preferred: distillation boils water to vapour and condenses it back, leaving dissolved solids behind. Tap water contains ions (Ca²⁺, Cl⁻) and dissolved gases that would interfere with sensitive reactions.

Domestic water treatment

Fertilisers — feeding the soil

Plants remove nitrogen, phosphorus and potassium from the soil every time they grow — and the soil doesn’t automatically replace them. Fertilisers put those three elements back in a form the plants can absorb.

Why N, P and K

Nitrogen builds proteins — without it, leaves yellow and growth stalls. Phosphorus is needed for healthy roots and energy transfer inside the cell. Potassium is needed for water regulation, disease resistance and the formation of flowers and fruit. A crop grown repeatedly in the same field removes these elements faster than natural weathering and decay put them back, so farmers add them directly.

Common fertiliser formulae

Ammonium nitrate is the workhorse of world agriculture — cheap, 35% nitrogen by mass, and easily dissolved.

The environmental catch

Fertilisers are so soluble that heavy rain washes them off fields into rivers before plants absorb them. This nutrient run-off triggers algal blooms: algae grow explosively on the free fertiliser, block sunlight, and use up dissolved oxygen when they die. Fish suffocate; the lake becomes a dead zone.

Air — composition and pollutants

The atmosphere is mostly two gases and a very small everything else. It’s in that “everything else” that all the pollution and climate chemistry hides — and it’s what humans have been adding to for two centuries.

Composition of clean, dry air

Clean air is roughly 78% nitrogen and 21% oxygen, with the remaining 1% made up mostly of argon, a smaller share of other noble gases, and about 0.04% carbon dioxide. Water vapour is present too, though the amount varies with weather. The composition was stable for millions of years until industrialisation began shifting the trace fraction.

The six main pollutants

Combustion, in symbols

Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion, which makes carbon monoxide: 2CH₄ + 3O₂ → 2CO + 4H₂O.

Reducing air pollution

Two industrial technologies handle the two most stubborn categories of air pollution: catalytic converters in the exhausts of vehicles, and flue-gas desulfurisation on the smokestacks of power stations. Together with a switch to renewable energy, they are the chemistry side of the fix.

Catalytic converters

A modern petrol car’s exhaust contains a honeycomb of ceramic coated with a thin layer of platinum and rhodium. As hot exhaust gases flow through, two pollutants react on the catalyst surface and cancel each other out: 2CO + 2NO → N₂ + 2CO₂. A toxic gas (CO) is oxidised to CO₂ and an acid-rain gas (NO) is reduced to inert N₂ in a single redox reaction. The converter also breaks down unburnt hydrocarbons — but it does not remove the CO₂, which is the greenhouse pollutant.

Flue-gas desulfurisation

Coal-fired power stations produce enormous quantities of SO₂, which can be scrubbed out of the chimney gases before they reach the atmosphere. Hot flue gases are passed through a slurry of calcium carbonate (limestone): CaCO₃ + SO₂ → CaSO₃ + CO₂. The calcium sulfite by-product can be oxidised further to calcium sulfate (gypsum), sold for plasterboard — a rare case of pollution control producing something useful.

Renewable energy

Catalytic converters and scrubbers are patches on fossil-fuel technology — they clean the exhaust but do nothing about CO₂. The only route that reduces CO₂ is to burn less fossil fuel: wind, solar, hydro and geothermal each produce no combustion products at all.

Climate change & the greenhouse effect

The natural greenhouse effect makes Earth habitable — without it the surface would average around −18 °C. The enhanced greenhouse effect, driven by the extra CO₂ and CH₄ humans release, is warming the planet faster than any period on record.

How the greenhouse effect works

The mechanism is a four-step relay: (1) the Sun emits energy as a broad spectrum of radiation, most of which passes through the atmosphere and reaches the surface; (2) the surface absorbs it, warms, and re-emits energy as infrared (thermal) radiation; (3) molecules of CO₂ and CH₄ in the atmosphere absorb that outgoing infrared and re-emit it in all directions — some back down to the surface; (4) less thermal energy escapes to space than would without these gases, so the surface stays warm. Since the Industrial Revolution, CO₂ has risen by more than 50% and methane has more than doubled, so the trapping is intensifying.

Consequences of the enhanced effect

Reducing the effect

The chemistry-side solutions are three: switch to renewable energy (no CO₂ from generation), reduce methane leaks from agriculture and landfill, and remove CO₂ from the atmosphere through reforestation — trees are natural CO₂ absorbers, running the reverse chemistry of combustion.