Acid rain is due to

Correct — A, (a) Sulphur dioxide pollution.

Acid rain is a CHEMISTRY question, and the test that separates the four options is simple: does the pollutant dissolve in atmospheric water to give an ACID ?

Sulphur dioxide does, and by two routes:

SO₂ + H₂O → H₂SO₃ sulphurous acid, directly 2SO₂ + O₂ → 2SO₃ ; SO₃ + H₂O → H₂SO₄ sulphuric acid, after oxidation

The second route matters more, because sulphuric acid is the strong acid and it is the one that dominates the acidity of rain downwind of a coal-burning region. The oxidation happens in the atmosphere over hours to days, which is why the damage often falls hundreds of kilometres from the chimney that caused it.

WHERE THE SULPHUR DIOXIDE COMES FROM. Overwhelmingly from burning fossil fuels that contain sulphur — coal in thermal power stations above all, and heavy fuel oil — and from smelting sulphide ores of copper, lead and zinc, where the sulphur in the ore leaves as SO₂. Volcanoes are the main natural source.

OXIDES OF NITROGEN ARE THE OTHER HALF OF THE STORY, and a full answer names them: NO and NO₂ from high-temperature combustion in vehicle engines and furnaces oxidise and dissolve to nitric acid, HNO₃. Sulphur dioxide and the oxides of nitrogen together are what the phrase 'acid rain' refers to. The option set here offers only one of the two, so the choice is unambiguous.

WHAT COUNTS AS ACID RAIN. Clean rain is already mildly acidic — atmospheric carbon dioxide dissolves to carbonic acid and holds unpolluted rainwater at about pH 5·6 — so precipitation is conventionally called acid rain when its pH falls below that figure. The term itself is old: Robert Angus Smith showed the link between rain chemistry and air pollution at Manchester in 1852 and coined the phrase 'acid rain' in 1872.

The stem is printed without terminal punctuation, and each option begins with a capital, as this booklet sets its short definitional items throughout.

Why the others are wrong

• (b)Carbon monoxide pollution — The most tempting wrong answer, because carbon monoxide is a well-known combustion pollutant and because its close relative carbon DIOXIDE really does acidify rain. But carbon monoxide is not an acid-forming gas at all: it is barely soluble in water and forms no acid on dissolving. Its danger is physiological rather than chemical-atmospheric — it binds to haemoglobin roughly two hundred times more strongly than oxygen does, forming carboxyhaemoglobin and starving the tissues of oxygen, which is why it is lethal in enclosed spaces and why it is monitored at roadsides. The relative that does form an acid is carbon dioxide, giving carbonic acid, and that reaction is the natural background that puts clean rain at about pH 5·6 in the first place. It is the baseline acid rain is measured AGAINST, not the cause of it.
• (c)Pesticide pollution — A different environmental problem with a different mechanism. Pesticide pollution is about persistence and accumulation, not acidity: organochlorine compounds such as DDT resist breakdown, dissolve in fat rather than water, and concentrate up the food chain by biomagnification, so that top predators carry the highest burdens. Pesticides reach water bodies mainly by run-off and leaching from treated fields rather than through the atmosphere, and they do not dissolve in cloud droplets to give acids. The option is on the page because it names a real and serious form of pollution, which is what makes a distractor work — the candidate has to have the mechanism of acid rain in mind, not just a list of pollutants.
• (d)Dust particles in the atmosphere — Chemically the opposite of what is wanted. Dust is PARTICULATE MATTER — a category defined by physical size, measured as mass in suspension, and made up of mineral grains, carbon, metals and salts. Being particles rather than reactive gases, they neither dissolve to give acids nor generate them, and their harm is done mechanically and physiologically, by penetrating the respiratory tract. Mineral dust in fact tends to work AGAINST acidity: calcium-bearing dust is basic, and carbonates neutralise acids on contact, which is the same reaction that eats away limestone and marble buildings in polluted air. Confusing the two categories is the underlying error — gaseous pollutants are the acid-formers, and particulates are a separate parameter measured a separate way.

ACID RAIN — more precisely acid deposition — is precipitation made abnormally acidic by pollutant gases that dissolve in atmospheric water. Its two precursors are SULPHUR DIOXIDE and the OXIDES OF NITROGEN, and the sequence is always the same: emission, oxidation in the atmosphere, dissolution into cloud or rain droplets, deposition.

SO₂ → oxidised to SO₃ → dissolves as SULPHURIC ACID (H₂SO₄) NOₓ → oxidised → dissolves as NITRIC ACID (HNO₃)

Deposition comes in two forms. WET deposition is rain, snow, fog and mist carrying the dissolved acid. DRY deposition is acidic gases and particles settling directly on surfaces, later dissolving in dew or the next shower. Both are counted as acid deposition, which is why the technical term is preferred to the popular one.

THE pH BASELINE is what makes the definition precise. Carbon dioxide is always present and always dissolves a little, giving carbonic acid, so unpolluted rain is naturally about pH 5·6. Precipitation is called acidic below that figure; industrial-region rain has been recorded well below pH 4.

WHAT IT DAMAGES. Lakes and streams acidify, and fish stocks fail once the pH falls far enough, particularly where the surrounding rock has little buffering capacity. Acidified soil loses calcium and magnesium and mobilises ALUMINIUM, which is toxic to fine roots and to fish gills. Forests decline through this root damage combined with direct injury to foliage. Buildings and monuments in limestone and marble are dissolved outright, because calcium carbonate reacts with sulphuric acid to give calcium sulphate, which is soluble and washes away — the effect commonly called stone leprosy, and the reason emissions around the Taj Mahal have been regulated for decades.

IT IS A TRANSBOUNDARY PROBLEM. Because the oxidation step takes time, the acid falls far downwind of the source, often across a national border — which is what forced the earliest international air-pollution agreements in Europe and North America.

CONTROL works at three points: use low-sulphur fuel, remove sulphur from the flue gas with FLUE GAS DESULPHURISATION (scrubbing with lime or limestone), and cut the oxides of nitrogen with combustion controls and catalytic converters on vehicles.

Key facts

• Acid rain is caused by SULPHUR DIOXIDE and the OXIDES OF NITROGEN, which oxidise in the atmosphere and dissolve to give sulphuric acid and nitric acid respectively.
• Sulphur dioxide comes chiefly from burning sulphur-bearing coal and heavy fuel oil and from smelting sulphide ores; volcanoes are the main natural source.
• Clean rain is already about pH 5·6 because atmospheric carbon dioxide dissolves as carbonic acid, so precipitation is called acidic only below that figure.
• Acid deposition is both WET — rain, snow, fog — and DRY, where acidic gases and particles settle directly and dissolve later.
• Robert Angus Smith first demonstrated the link between rain chemistry and air pollution at Manchester in 1852 and coined the term 'acid rain' in 1872.
• Damage includes acidified lakes and soils, mobilisation of toxic aluminium, forest decline, and dissolution of limestone and marble buildings by reaction with sulphuric acid.
• Because the oxidation step takes hours to days, the acid falls far downwind of its source, making acid rain a transboundary problem.
• Control measures are low-sulphur fuel, flue gas desulphurisation with lime or limestone, and catalytic converters and combustion controls for the oxides of nitrogen.

Common traps

• Confusing carbon monoxide with carbon dioxide. Carbon dioxide gives carbonic acid and sets the natural pH 5·6 baseline; carbon monoxide forms no acid at all.
• Naming only sulphur dioxide when a question asks for all the precursors. The oxides of nitrogen contribute nitric acid and belong in any full answer.
• Treating dust as an acidifier. Particulate matter is a size-defined category, and calcium-bearing mineral dust in fact neutralises acids.
• Assuming acid rain falls where the pollutant was emitted. Atmospheric oxidation takes time, so the deposition is characteristically downwind and often across a border.
• Mixing up mechanisms — attributing acid rain to the greenhouse gases, or the ozone hole to sulphur dioxide. Each phenomenon has one chemical story and they do not overlap.

• practice — not a real PYQ

  Besides sulphur dioxide, which one of the following is the principal precursor of acid rain ?

  • (a)Carbon monoxide
  • (b)Oxides of nitrogen
  • (c)Methane
  • (d)Chlorofluorocarbons

  Answer(b) Oxides of nitrogen — nitric oxide and nitrogen dioxide from high-temperature combustion oxidise in the atmosphere and dissolve to give nitric acid. Carbon monoxide forms no acid; methane is a greenhouse gas; chlorofluorocarbons destroy stratospheric ozone. Only the oxides of nitrogen share sulphur dioxide's acid-forming chemistry.

• practice — not a real PYQ

  Precipitation is conventionally described as acid rain when its pH falls below approximately

  • (a)7·0
  • (b)6·5
  • (c)5·6
  • (d)4·0

  Answer(c) 5·6 — dissolved atmospheric carbon dioxide forms carbonic acid and holds even unpolluted rain at about pH 5·6, so that value, and not neutrality at pH 7, is the baseline against which acidity is judged. Rain in heavily industrialised regions has been recorded below pH 4, but that is a measure of severity, not the threshold.