Why this chapter matters for UPSC: Acid rain (causes, effects on monuments, forests, aquatic ecosystems), soil pH and agriculture, water quality (pH testing), and industrial processes are GS3 topics. The neutralisation concept is foundational for understanding many environmental processes.

PART 1 — Quick Reference Tables

Common Acids

Acid Chemical Name Found In
Hydrochloric acid (HCl) Muriatic acid Stomach (gastric acid); cleaning agents
Sulphuric acid (H₂SO₄) Oil of vitriol Car batteries; fertiliser production; strongest common acid
Nitric acid (HNO₃) Aqua fortis Fertilisers; explosives production
Carbonic acid (H₂CO₃) — Fizzy drinks (CO₂ dissolved in water)
Acetic acid (CH₃COOH) Ethanoic acid Vinegar (5% solution)
Citric acid — Citrus fruits (lemon, orange)
Formic acid — Ant sting; nettle sting
Lactic acid — Sour milk, curd

Common Bases and Alkalis

Base Chemical Name Found In
Sodium hydroxide (NaOH) Caustic soda Soap making; paper industry; drain cleaners
Calcium hydroxide (Ca(OH)₂) Slaked lime Whitewash; construction; water treatment
Ammonium hydroxide (NH₄OH) Ammonia solution Cleaning products; smelling salts
Magnesium hydroxide (Mg(OH)₂) Milk of magnesia Antacid (neutralises stomach acid); laxative
Baking soda (NaHCO₃) Sodium bicarbonate Baking; antacid; fire extinguishers

Indicators

Indicator In Acid In Base In Neutral
Litmus Red Blue Purple
Turmeric Yellow Red-brown Yellow
Phenolphthalein Colourless Pink/magenta Colourless
Methyl orange Red/pink Yellow Orange

PART 2 — Detailed Notes

Acids, Bases, and pH

Key Term

pH scale:

  • Measures how acidic or basic (alkaline) a solution is
  • Scale: 0 to 14
    • 0–6: Acidic (pH 7 = neutral; lower = more acidic; pH 1 = very strong acid)
    • 7: Neutral (pure water)
    • 8–14: Basic/alkaline (higher = more basic; pH 14 = very strong base)
  • pH below 7 in rainwater = acid rain (normal rain is slightly acidic, pH ~5.6, due to dissolved CO₂; acid rain has pH < 5.6)

Common pH values:

  • Stomach acid (HCl): pH 1–2
  • Lemon juice: pH 2–3
  • Vinegar: pH 2–3
  • Tomato: pH 4
  • Coffee: pH 5
  • Rain (normal): pH 5.6
  • Pure water: pH 7
  • Baking soda: pH 8–9
  • Milk of magnesia: pH 10
  • Bleach: pH 12–13
  • NaOH: pH 14

Neutralisation reaction: Acid + Base → Salt + Water HCl + NaOH → NaCl + H₂O

Applications:

  • Antacids: Neutralise excess HCl in stomach (milk of magnesia, baking soda)
  • Lime (calcium hydroxide) applied to acidic soil → raises pH → better crop growth
  • Wastewater treatment: Industrial acidic effluents neutralised before discharge

Acid Rain

UPSC Connect

UPSC GS3 — Acid Rain:

Formation:

  1. Burning fossil fuels (coal, oil) releases SO₂ (sulphur dioxide) and NOₓ (nitrogen oxides)
  2. These gases react with water vapour and oxygen in the atmosphere
  3. SO₂ + H₂O → H₂SO₃ (sulphurous acid) → H₂SO₄ (sulphuric acid)
  4. NOₓ + H₂O → HNO₃ (nitric acid)
  5. These acids dissolve in rainwater → acid rain (pH < 5.6)

Effects:

  • Buildings and monuments: Marble and limestone react with acid → crumble and dissolve
    • Taj Mahal: White marble pitting and yellowing due to SO₂ from Mathura refinery (now relocated/filtered) and Agra's industry → landmark environmental case (M.C. Mehta vs Union of India)
    • Acid rain dissolves CaCO₃ (marble): CaCO₃ + H₂SO₄ → CaSO₄ (gypsum, soft and crumbly) + H₂O + CO₂ — this is called "marble cancer"
  • Forests: Damages leaves, leaches nutrients from soil, weakens trees → "forest dieback" in Germany's Black Forest and Scandinavian forests
  • Aquatic ecosystems: Acidifies lakes and rivers → kills fish, amphibians, aquatic insects (pH below 5 is lethal for most fish)
  • Soil: Makes acidic → inhibits plant growth → reduces crop yield; leaches essential minerals

India context:

  • Major industrial cities (Agra, Mumbai, Delhi, Surat) face acid rain issues
  • Power plants (thermal power) are the largest source of SO₂ in India
  • Environment Protection Act 1986: Emission standards for SO₂ and NOₓ; tall chimney requirement to disperse emissions

Everyday Applications

Explainer

Why baking soda makes cakes rise: Sodium bicarbonate (NaHCO₃) + acid (from buttermilk or cream of tartar) + heat → CO₂ released → gas bubbles expand batter → cake rises. This is a neutralisation reaction producing CO₂.

Soap making (saponification): Fat/oil + NaOH (caustic soda) → soap (sodium salt of fatty acid) + glycerol Industrial basis of the soap industry; NaOH is the key industrial base.

Chlorine bleach: NaOCl (sodium hypochlorite) in water; alkaline solution; kills bacteria; whitens by oxidation.

Soil pH and agriculture:

  • Most crops grow best at pH 6–7 (slightly acidic to neutral)
  • Acidic soils (pH < 6): Add lime (calcium hydroxide or calcium carbonate) to raise pH
  • Alkaline soils (pH > 7): Add sulphur or acidic organic matter to lower pH
  • India's laterite soils (red soils of Western Ghats, Northeast) are acidic → need liming for good crop yields

Exam Strategy

Prelims traps:

  • Acid rain pH < 5.6 (NOT just "below 7" — normal rain is already slightly acidic at pH 5.6 due to CO₂)
  • Taj Mahal damage = acid rain + SO₂ (from Mathura refinery → now addressed); "marble cancer"
  • Antacid = base (neutralises stomach acid); milk of magnesia, baking soda = alkaline
  • Litmus turns BLUE in base, RED in acid — frequently confused; mnemonic: Blue Base, Red acid
  • pH 7 = neutral; pH decreases = more acidic; pH increases = more basic (logarithmic scale)
  • Acid + Base → Salt + Water (neutralisation); this always happens, remember the products

Previous Year Questions

Prelims:

  1. "Acid rain" is caused by the reaction of atmospheric water with which gases released primarily by burning fossil fuels?
    (a) Carbon dioxide and carbon monoxide
    (b) Sulphur dioxide and nitrogen oxides
    (c) Hydrogen sulphide and ammonia
    (d) Chlorofluorocarbons and methane

  2. The "marble cancer" phenomenon observed in monuments like the Taj Mahal is caused by the reaction of marble (CaCO₃) with:
    (a) Nitrogen dioxide forming calcium nitrate
    (b) Sulphuric acid in acid rain forming calcium sulphate
    (c) Carbon dioxide forming calcium carbonate
    (d) Water vapour eroding the surface over time