Q.Define - Acid rain.

Acid rain is precipitation made abnormally acidic (pH well below the natural ~5.6) by dissolved SO2 and NOx from fossil-fuel combustion.

Acid rain is defined as rain (or snow, fog, etc.) with a pH noticeably lower than that of normal, unpolluted rain water (which itself is mildly acidic, around pH 5.6, due to dissolved atmospheric CO2). It is caused primarily by air pollutants — sulfur dioxide (SO2) and various nitrogen oxides (NOx) — released from burning fossil fuels in vehicles, power plants, and industries.

In the atmosphere, these oxides react with water vapour and oxygen to form strong acids:

SO2 + H2O → H2SO3, further oxidised to H2SO4 (sulfuric acid)

2NO2 + H2O → HNO3 + HNO2 (forming nitric acid)

…