Key Concepts & Self-Assessment19 Key Facts
Review key Acid Rain exam facts and rate your mastery to track revision.
Progress: 0/19 Rated 0 Mastered 0 Review Later
#1
Scottish chemist Robert Angus Smith coined the term "Acid Rain" in 1852 after studying the link between coal-burning industrial soot in Manchester and acidic rainwater; he is recognized as the "Father of Acid Rain Studies."
#2
Clean, unpolluted rainwater has a natural pH of approximately 5.6 (NOT 7.0) because atmospheric Carbon Dioxide () dissolves reversibly in water droplets to form weak Carbonic Acid ().
#3
Rainwater is scientifically classified as Acid Rain only when its pH drops below 5.6 due to strong mineral acids—specifically Sulfuric Acid () and Nitric Acid ().
#4
Because the pH scale is logarithmic (), rainwater with a pH of 4.6 is 10 times more acidic than clean rain (pH 5.6), and rain with a pH of 3.6 is 100 times more acidic.
#5
The two primary gaseous precursors of acid rain are Sulfur Dioxide (, contributing roughly two-thirds of total acidity) and Nitrogen Oxides ( and , collectively , contributing roughly one-third).
#6
Sulfur Dioxide () converts to Sulfuric Acid in the atmosphere via two steps: catalytic/photochemical oxidation () followed by hydration ().
#7
Nitrogen Dioxide () reacts with atmospheric water and oxygen (and hydroxyl radicals) to form Nitric Acid ().
#8
Acid Deposition occurs in two forms: Wet Deposition (acidic rain, snow, sleet, and acid fog) and Dry Deposition (acidic sulfate/nitrate aerosols and gases that settle onto dry leaves, soil, and buildings within days and turn into acid upon meeting morning dew).
#9
Major anthropogenic sources of are coal-fired thermal power plants, petroleum refineries (such as the Mathura Refinery), and copper/lead/zinc sulfide ore smelting; major sources of are vehicular exhaust engines and high-temperature industrial boilers.
#10
"Stone Leprosy" (or "Marble Cancer") occurs when sulfuric acid in acid rain reacts chemically with the white Calcium Carbonate (, Makrana marble) of monuments like the Taj Mahal: .
#11
The resulting Calcium Sulfate (, gypsum) is roughly 100 times more water-soluble than marble and has a larger molar volume, causing the marble surface to swell, turn yellow-brown, crack, and peel off.
#12
In the landmark environmental judgment M.C. Mehta v. Union of India (Taj Trapezium Case, 1996), the Supreme Court of India established the 10,400-square-kilometer Taj Trapezium Zone (TTZ) around Agra, ordering 292 coal-based brick kilns and iron foundries to switch to natural gas (CNG/PNG) or relocate.
#13
To clean yellow soot and insoluble particulate crusts from the Taj Mahal without using harsh acids, the Archaeological Survey of India (ASI) periodically applies "Multani Mitti" (Fuller’s Earth)—a traditional lime-rich clay mud-pack treatment that absorbs grease and grime as it dries.
#14
In aquatic ecosystems, when lake water pH drops below 5.0, acid rain leaches toxic trivalent Aluminum ions () out of surrounding watershed silicate clays into the lake water; precipitates on fish gills, causing mucus suffocation and reproductive failure.
#15
Paradoxically, acid-dead lakes often appear crystal clear and turquoise-blue ("oligotrophic clarity") because the high acidity and aluminum toxicity kill nearly all phytoplankton, zooplankton, frogs, and fish, leaving only acid-tolerant Sphagnum moss on the lakebed.
#16
In temperate and boreal forests (such as Germany’s Black Forest, Waldsterben), acid rain strips calcium () and magnesium () ions from soil humus and damages waxy cuticles on conifer needles, starving trees of nutrients required for chlorophyll.
#17
Agricultural soils and acidified lakes are neutralized via "Liming"—spreading powdered agricultural limestone () or slaked lime () by helicopter or tractor to raise pH.
#18
Coal thermal power plants reduce emissions by 90% to 95% by installing Flue Gas Desulfurization (FGD) wet scrubbers, which spray a slurry of limestone () or lime () into the smokestack to capture as synthetic gypsum ().
#19
Internationally, transboundary acid rain across Europe and North America was curbed by the 1979 UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP, Geneva), the 1985 Helsinki Protocol (mandating a 30% cut in sulfur emissions), and the 1999 Gothenburg Protocol.
Subject Specialist Commentary
Analytical perspective & practical exam advice from the Master10 academic board
Here is the #1 trick question in UPSC Prelims and SSC Chemistry exams: 'Is rainwater with a pH of 6.2 considered Acid Rain?' The answer is NO! Even on a pristine island with zero factories or cars, natural rainwater has a pH of 5.6 because carbon dioxide in the air dissolves in clouds to form weak carbonic acid. Rain is only classified as Acid Rain when its pH drops below 5.6 due to strong Sulfuric Acid (, from ) and Nitric Acid (, from ).
For environmental law and Art & Culture questions, connect Acid Rain directly to 'Marble Cancer' ( gypsum scaling on Makrana marble), the Supreme Court's 1996 M.C. Mehta (Taj Trapezium Zone) judgment covering 10,400 sq. km around Agra, and ASI's non-abrasive Multani Mitti (Fuller's Earth) clay-pack restoration of the Taj Mahal. For UPSC CSE, State PCS, CDS, and SSC CGL aspirants, examiners frequently construct multi-statement elimination questions around What Is Acid Rain? Sulfur Dioxide, Nitrogen Oxides, pH 5.6 Threshold & Taj Mahal Stone Leprosy by swapping primary statutory nodal agencies, constitutional or international treaty timelines, and underlying physical or institutional parameters. Mastering both the foundational mechanism and its real-world Indian policy application ensures 100% accuracy in analytical Prelims and Mains questions.
Related Knowledge Topics to Discover
Looking for more GK practice?
Explore 52,789+ questions across 65 General Knowledge categories.