UGC NET Questions (Paper – 1)

Acid rain forms when sulphur dioxide and nitrogen oxides released from combustion processes react with water vapour and oxidants in the atmosphere. These reactions produce sulphuric and nitric acids that are deposited as wet or dry precipitation. The resulting acidic deposition damages forests, aquatic ecosystems, soils and built structures. Therefore, the pairing of sulphur dioxide and nitrogen oxides is recognised as the principal cause of acid rain.

Option A:
Option A is incorrect because carbon monoxide and ozone play different roles in air pollution. Carbon monoxide is a toxic gas affecting human health, and ground-level ozone is a component of photochemical smog, but they are not the main precursors of strong acids in rainwater. Their chemistry does not primarily create acidic precipitation.

Option B:
Option B correctly names the gases that transform into sulphuric and nitric acids when they dissolve in cloud droplets. These gases are emitted largely from coal-based power plants, industrial boilers and vehicles, and controlling them is central to acid rain mitigation strategies.

Option C:
Option C is incorrect since methane is more associated with greenhouse effects, and chlorofluorocarbons are linked to ozone depletion and climate forcing. Neither group is directly responsible for the formation of the strong acids that cause acid rain in most regions.

Option D:
Option D is incorrect because hydrogen and helium are light gases that do not form acidic compounds in the atmosphere under normal environmental conditions. They are not significant contributors to rain chemistry or ecosystem acidification.

Power plants, industries and vehicles emit sulphur dioxide and nitrogen oxides into the atmosphere. These gases are oxidised to sulphuric and nitric acids, which can be deposited with rain, snow or dust. Acidic deposition lowers the pH of soils and water bodies, damaging forests, aquatic life and buildings. Hence, control of SO₂ and NOx is central to acid rain mitigation.

Option A:
Option A is incorrect because carbon monoxide and ozone are associated more with urban smog and health impacts than with acid deposition.

Option B:
Option B is correct since SO₂ and NOx are the main precursors that form strong acids responsible for acid rain.

Option C:
Option C is incorrect as methane and CFCs are important greenhouse and ozone-depleting gases respectively, but they are not primary acid rain precursors.

Option D:
Option D is incorrect because argon and helium are inert gases that do not form such acids.

Acid rain forms when sulphur dioxide and nitrogen oxides react with water vapour and other chemicals in the atmosphere to produce sulphuric and nitric acids. These acids then fall to the ground with rain, snow or dust. Because the stem mentions burning fossil fuels and industrial processes,the combination of sulphur dioxide and nitrogen oxides best explains the phenomenon.

Option A:
Carbon monoxide and ozone are important air pollutants, but they do not directly form strong acids responsible for acid rain. Carbon monoxide mainly affects human health, and tropospheric ozone is a component of photochemical smog. Therefore this pair does not match the primary chemical cause described.

Option B:
Sulphur dioxide and nitrogen oxides are released in large quantities from coal based power plants, industrial boilers and vehicles. Once in the atmosphere, they undergo oxidation and acid formation, leading to acid deposition. This clearly aligns with the explanation of acid rain in the question.

Option C:
Methane and ammonia can participate in atmospheric chemistry and contribute to other environmental issues such as eutrophication and greenhouse effects. However, they are not the principal precursors of acid rain in most contexts. Hence this option is not correct.

Option D:
Chlorofluorocarbons are associated mainly with ozone layer depletion, and helium is a chemically inert gas. Neither plays a significant role in generating acid rain, so this pair is inappropriate for the stem.