Previous Year Question
Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A): CO₂, SO₂ and NO₂ gases get dissolved in rain water and produce acid rain.
Reason (R): Atmospheric air is polluted due to high concentration of gases like carbon dioxide, sulphur dioxide and oxides of nitrogen.
Select the correct answer from the codes given below:
Both (A) and (R) are true and (R) is the correct explanation of (A).
(A) is false, but (R) is true.
Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(A) is true, but (R) is false.
Correct Answer: A
Detailed Solution
Assertion (A) is true. Acid Rain refers to precipitation that is more acidic than normal rainwater, generally having a pH below 5.6. Sulphur Dioxide (SO₂) and Nitrogen Oxides (NOₓ) present in the atmosphere undergo chemical reactions with water vapour and other atmospheric constituents to form Sulphuric Acid (H₂SO₄) and Nitric Acid (HNO₃), respectively. These acids may reach the Earth's surface through rain, snowfall, or fog. Carbon Dioxide (CO₂) also dissolves in water to form weak Carbonic Acid (H₂CO₃), but SO₂ and NOₓ are the principal pollution-related contributors to acid rain.
Reason (R) is also true. Excessive atmospheric concentrations of gases such as Sulphur Dioxide, Nitrogen Oxides, and Carbon Dioxide can cause environmental problems. SO₂ is primarily released from the combustion of coal and sulphur-containing fossil fuels and industrial processes, whereas NOₓ is emitted from vehicles, thermal power plants, and high-temperature combustion. CO₂ is a major Greenhouse Gas whose concentration increases through fossil-fuel combustion and other human activities. Higher atmospheric availability of SO₂ and NOₓ, in particular, promotes acid-forming reactions, thereby increasing the likelihood of acid rain.
The Chemical Reactions involved in acid rain formation explain the relationship between atmospheric gases and moisture. CO₂ reacts with water to form carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. The overall oxidation and reaction of sulphur dioxide with water can be represented as 2SO₂ + O₂ + 2H₂O → 2H₂SO₄. Similarly, the overall reaction representing nitric acid formation from nitrogen dioxide is 4NO₂ + O₂ + 2H₂O → 4HNO₃. Normal rainwater has a pH of approximately 5.6, mainly because of carbonic acid formed from naturally occurring CO₂. The additional presence of sulphuric and nitric acids can further decrease the pH of rainwater.
Acid rain affects Soil, Aquatic Ecosystems, Vegetation, and Historical Structures. It can increase Nutrient Leaching from soils and reduce the pH of lakes and rivers, thereby affecting aquatic organisms. Acid Deposition can also accelerate the deterioration of marble and limestone structures containing calcium carbonate. Acidic substances reach the Earth's surface through Wet Deposition associated with rain and snowfall or through Dry Deposition in the form of gases and particles. Controlling SO₂ and NOₓ emissions is a major measure for reducing these effects.