Environmental Chemistry (Class 11): Pollution, Causes and Control

Tropospheric pollution

Acid rain

Oxides of sulphur and nitrogen from combustion dissolve in atmospheric moisture and form sulphuric and nitric acids, lowering rainwater pH well below its natural value.

Note that unpolluted rain is already slightly acidic, at about pH 5.6, because dissolved carbon dioxide forms carbonic acid. Acid rain means a pH substantially below that, and stating the natural baseline is often part of the expected answer.

Its effects are the corrosion of stone containing carbonates, acidification of lakes harming aquatic life, and damage to soil chemistry.

Photochemical smog

Formed in warm sunny conditions from nitrogen oxides and unburnt hydrocarbons. Sunlight splits nitrogen dioxide, generating oxygen atoms that produce ozone, which then reacts further with hydrocarbons to give oxidising products.

The greenhouse effect

Certain gases absorb infrared radiation emitted by the earth and re-radiate it, warming the lower atmosphere. Carbon dioxide, methane, water vapour, nitrous oxide and chlorofluorocarbons all contribute, and they differ in how strongly they absorb and how long they persist.

The effect itself is natural and necessary — without it the planet would be far colder. The concern is its enhancement by increased concentrations, and phrasing it that way is what a careful answer does.

Stratospheric ozone depletion

Ozone in the stratosphere absorbs harmful ultraviolet radiation. Chlorofluorocarbons drift upward, are broken by ultraviolet light to release chlorine atoms, and each chlorine atom then destroys many ozone molecules in a catalytic cycle before being removed.

The catalytic nature is the crucial point: one chlorine atom does not destroy one ozone molecule but thousands, which is why small quantities of these compounds have a large effect. A question asking why CFCs are so damaging is asking for that catalytic argument.

Water pollution

COD is always greater than or equal to BOD, because chemical oxidation attacks substances that microorganisms cannot degrade. Explaining that relationship is a standard question.

Eutrophication

Excess nutrients, mainly phosphates and nitrates from fertiliser runoff and detergents, cause rapid algal growth. When the algae die, their decomposition consumes dissolved oxygen, and fish and other aquatic organisms suffocate. The sequence — nutrients, growth, death, decomposition, oxygen depletion — is what the answer should trace.

Green chemistry

The principle is to design processes that do not generate pollution rather than treating it afterwards. In practice this means using safer solvents, improving atom economy so more of the reactants end up in the product, using catalysts rather than stoichiometric reagents, and choosing renewable feedstocks.

Atom economy is the concept most often asked: it measures the proportion of reactant mass that appears in the desired product, so a reaction can have high yield yet poor atom economy if it produces large byproducts.

Frequently asked questions

Why is rainwater naturally acidic?

Because atmospheric carbon dioxide dissolves to form carbonic acid, giving a pH of about 5.6. Acid rain refers to values significantly below this.

Why is ozone harmful in one layer and helpful in another?

In the stratosphere it absorbs ultraviolet radiation before it reaches the surface. In the troposphere, where we breathe, it is a powerful oxidant that irritates tissue and damages plants.

Why is COD always at least as high as BOD?

Because chemical oxidation degrades everything oxidisable, including substances microorganisms cannot break down. Biological demand is therefore a subset of chemical demand.

What is the difference between yield and atom economy?

Yield measures how much of the theoretical product was actually obtained. Atom economy measures how much of the reactant mass is present in the product at all. A reaction can score well on one and poorly on the other.

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