Acids, Bases and Salts

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

[Additional] Ocean Acidification — GS3 (Environment / Climate Change):

The mechanism (same carbonate chemistry as acid rain):

1. Burning fossil fuels releases CO₂ into the atmosphere
2. Oceans absorb ~25–30% of all CO₂ emitted by humans (acts as a carbon sink)
3. CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃⁻ (bicarbonate)
4. The extra H⁺ ions react with carbonate ions (CO₃²⁻) in seawater, reducing their availability
5. Corals and shellfish use CO₃²⁻ to build their calcium carbonate (CaCO₃) skeletons — less CO₃²⁻ available → shells and skeletons thin, weaken, and dissolve

pH data (IPCC AR6, 2021):

• Pre-industrial ocean surface pH: ~8.2
• Current ocean surface pH: ~8.1 (a drop of 0.1 pH units since industrial revolution)
• Since pH is a logarithmic scale, a drop of 0.1 pH units = ~30% increase in hydrogen ion (H⁺) concentration — the ocean is already 30% more acidic than pre-industrial levels
• Projected pH by 2100 (high-emissions scenario, IPCC AR6): ~7.8 — equivalent to a 150% increase in acidity compared to pre-industrial levels
• Rate: 0.016–0.020 pH units per decade in subtropical oceans since the 1980s

Important distinction — ocean is NOT acidic yet:

• Current pH ~8.1 is still alkaline (above 7) — the ocean has not turned acidic
• "Acidification" means becoming more acidic (lower pH), not necessarily reaching below pH 7
• UPSC frequently tests this subtle distinction

Impact on coral reefs:

• 90% of all coral reefs projected to experience severe annual bleaching events by 2055 under high-emissions scenarios (NOAA, 2024)
• Coral bleaching = stress response (expelling symbiotic algae zooxanthellae → coral turns white); if prolonged → coral death
• Both warming AND acidification stress corals simultaneously — a double threat

India's coral reefs at risk:

• Gulf of Kutch (Gujarat) — already under severe thermal stress
• Gulf of Mannar (Tamil Nadu) — declared Marine National Park/Biosphere Reserve
• Lakshadweep — shallow atoll reefs highly vulnerable
• Andaman & Nicobar Islands — diverse but threatened by warming + acidification

India's international position: India is a signatory to the Paris Agreement; committed to NDCs including renewable energy expansion, which reduces CO₂ emissions that drive ocean acidification.

[Additional] Chlor-Alkali Process — GS3 (Indian Industry / Science & Technology):

The reaction:

Electrolysis of brine (concentrated salt water, NaCl solution):

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

• At the anode (positive electrode): Cl₂ (chlorine gas) is released
• At the cathode (negative electrode): H₂ (hydrogen gas) is released
• In solution: NaOH (sodium hydroxide / caustic soda) remains

Three products, all valuable:

India's chlor-alkali industry (AMAI data, FY 2023-24):

• Installed caustic soda capacity: 57.85 lakh MTPA (metric tonnes per annum)
• Domestic demand: ~43.6 lakh MT (grew 5.3% year-on-year)
• India is now a net exporter of caustic soda (reversed from being an importer in earlier years)
• Largest new plant: Adani Enterprises (Mundra Petrochemical Ltd., Gujarat) — 2,200 TPD capacity, commissioned 2024; India's largest single chlor-alkali project

UPSC connection — "electrolysis" in industry: The chlor-alkali process is the textbook example of industrial electrolysis — applying electricity to drive a chemical reaction that would not happen spontaneously. The process requires large amounts of electrical energy → chlor-alkali plants are located near cheap power sources (thermal plants or hydro).

Alkali Manufacturers Association of India (AMAI): The industry body representing chlor-alkali producers; tracks production and demand data; works with government on policy for the chemical sector.