Q.a) What is meant by acid rain?

Acid rain (pH < ~5.6) is caused by SO2/NOx pollutants oxidising in the atmosphere to sulfuric/nitric acid, and it damages monuments, ecosystems, crops, and infrastructure.

a) What is meant by acid rain?

Normal rainwater is naturally slightly acidic (pH ≈ 5.6) because atmospheric CO2 dissolves in it to form weak carbonic acid. Acid rain refers to rain (or other precipitation such as snow, fog, or dew) whose pH has dropped significantly below this natural value (typically below ~5.6, often around pH 4–4.5 or lower in badly polluted regions) due to the presence of stronger acids formed from air pollutants — chiefly sulfuric acid (H2SO4) and nitric acid (HNO3).

b) Chemistry behind the formation of acid rain:

Acid rain originates from oxides of sulfur and nitrogen released into the atmosphere, mainly from burning fossil fuels (coal, petroleum) in power plants, industries, and vehicles, and from volcanic activity:

1. Sulfur present in fossil fuels burns to form sulfur dioxide: S + O2 → SO2
2. SO2 is further oxidised in the atmosphere (by O2 or by reaction with atmospheric oxidants) to sulfur trioxide: 2SO2 + O2 → 2SO3
3. SO3 dissolves in atmospheric water vapour/rain droplets to form sulfuric acid: SO3 + H2O → H2SO4

Similarly, nitrogen oxides (formed from N2 + O2 reacting at the high temperatures of combustion engines/lightning: N2 + O2 → 2NO, then 2NO + O2 → 2NO2) dissolve in water:

3NO2 + H2O → 2HNO3 + NO

These acids (H2SO4, HNO3) then fall to the ground dissolved in rain, snow, or fog as acid rain, or are deposited directly as acidic gases/particles ('dry deposition').

c) Harmful effects of acid rain:

• Corrodes buildings and monuments made of marble/limestone (calcium carbonate) — reacting with CaCO3 to form soluble calcium salts, causing pitting and disintegration of stone (famously threatening monuments like the Taj Mahal — sometimes called 'marble cancer'). …