Why this chapter matters for UPSC: Acids, bases, and neutralisation are foundational chemistry concepts that underpin UPSC questions on soil health, water quality, industrial pollution, agricultural amendments (liming of soil), food safety, and medicine. Acid rain and its effects on ecosystems and monuments is a recurring Prelims theme.

PART 1 — Quick Reference Tables

Property Acids Bases (Alkalis) Neutral
Taste Sour (e.g., lemon) Bitter (e.g., baking soda) Neither
Touch Corrosive Soapy / slippery Normal
Effect on litmus Turns blue litmus red Turns red litmus blue No change
pH range Less than 7 More than 7 7
Examples HCl, H₂SO₄, vinegar, citric acid NaOH, Ca(OH)₂, ammonia, baking soda Water, common salt solution
Natural Indicator Colour in Acid Colour in Base
Litmus (lichen extract) Red Blue
Turmeric paper Yellow (no change) Reddish-brown
Red cabbage juice Red/pink Green/yellow
Red hibiscus extract Deep red Green
Onion (olfactory indicator) Retains smell Smell disappears
Common Acid/Base Chemical Name Everyday Source pH (approx.)
Hydrochloric acid (HCl) Muriatic acid Stomach acid 1–2
Acetic acid (CH₃COOH) Ethanoic acid Vinegar 2.9
Citric acid — Lemon, orange 2–3
Carbon dioxide + water → carbonic acid H₂CO₃ Soda water, rain 5.6
Sodium bicarbonate (NaHCO₃) Baking soda Baking 8.3
Sodium hydroxide (NaOH) Caustic soda Soap making 13–14
Calcium hydroxide Ca(OH)₂ Slaked lime Whitewash, soil amendment 12

PART 2 — Detailed Notes

Key Term

Acid: A substance that produces hydrogen ions (H⁺) in solution and turns blue litmus red. Acids are sour in taste and corrosive. Examples: hydrochloric acid (HCl) in the stomach, sulphuric acid (H₂SO₄) in car batteries, acetic acid in vinegar, citric acid in lemon.

Base: A substance that produces hydroxide ions (OH⁻) in solution and turns red litmus blue. Bases that dissolve in water are called alkalis. Bases are bitter in taste and feel soapy. Examples: sodium hydroxide (caustic soda), calcium hydroxide (lime water), ammonia solution (cleaning agents), sodium bicarbonate (baking soda).

Indicator: A substance that changes colour (or smell) in the presence of an acid or base. Used to identify whether a solution is acidic, basic, or neutral.

  • Natural indicators: Litmus (from lichen), turmeric, red cabbage juice, red hibiscus, red rose extract
  • Synthetic indicators: Phenolphthalein (colourless in acid, pink in base), methyl orange (red in acid, yellow in base)
  • Olfactory indicators: Substances whose smell changes — onion smell disappears in base; vanilla essence smell disappears in strong base

Neutralisation: A chemical reaction between an acid and a base that produces a salt and water. Acid + Base → Salt + Water. This is an exothermic reaction (releases heat).

pH Scale: A scale from 0–14 that measures the concentration of hydrogen ions in a solution. pH 7 = neutral; below 7 = acidic; above 7 = basic/alkaline.

UPSC Connect

Acid Rain — Environmental and Heritage Threat

Normal rain has a pH of about 5.6 (slightly acidic due to dissolved CO₂ forming carbonic acid). Acid rain has a pH below 5.6, caused by sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels reacting with atmospheric moisture to form sulphuric acid and nitric acid.

Effects:

  • Damages forests and aquatic ecosystems (lakes become too acidic for fish)
  • Corrodes buildings and historical monuments — the Taj Mahal has suffered marble cancer (kharna) due to acid rain from the Mathura Refinery and vehicle emissions. The Supreme Court's 1996 order (Taj Trapezium Zone case) restricted polluting industries within 10,400 km² around the Taj.
  • Leaches nutrients from soil, reducing agricultural fertility

Soil pH and Agriculture:

  • Most crops grow best at pH 6–7
  • Acidic soils are treated with lime (calcium hydroxide or calcium carbonate) — a process called liming
  • Basic/alkaline soils are treated with gypsum (calcium sulphate) or acidifying agents
  • ICAR and state agriculture departments monitor soil health under the Soil Health Card Scheme (launched 2015) — tests soil pH, macro-nutrients (N, P, K), and micro-nutrients

Neutralisation in Daily Life:

  • Ant bite (formic acid) → treated with calamine lotion (zinc carbonate, slightly basic)
  • Indigestion (excess HCl in stomach) → antacids like milk of magnesia Mg(OH)₂
  • Factory effluents (acidic) → neutralised with lime before discharge
  • Swimming pool water → pH maintained at 7.2–7.8 using chemicals
Explainer

Acid Rain and the Taj Mahal — A UPSC Classic

The Taj Mahal is made of white marble (calcium carbonate, CaCO₃). Sulphur dioxide from Mathura Refinery (established 1972, ~40 km from Agra) and vehicle emissions reacts with moisture to form sulphuric acid. This acid reacts with marble:

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Calcium sulphate (gypsum) is soft and soluble, causing the marble to crumble — a phenomenon called marble cancer or stone leprosy. The Supreme Court in MC Mehta v. Union of India (1996) directed 292 industries to shut or relocate from the Taj Trapezium Zone (TTZ), and mandated the use of CNG for vehicles in Agra.

Indicators — UPSC MCQ Trap:

  • Litmus is obtained from lichens (a symbiotic association of fungi and algae)
  • Turmeric does NOT change colour with acids — only with bases (turns reddish-brown)
  • Red cabbage juice is a universal indicator — shows a range of colours from red (strong acid) to yellow-green (strong base)
  • Olfactory indicators (onion, clove oil, vanilla) change smell, not colour

Neutralisation Applications Tested in Prelims:

  • Sodium bicarbonate (baking soda) releases CO₂ when heated → used in baking to make bread rise
  • Chlorine (a non-metal, slightly acidic oxide) is used to purify water — but excess chlorine causes health issues
  • Calcium carbide + water → acetylene gas (used in ripening fruits — a controversial practice regulated by FSSAI)

Exam Strategy

  • The Taj Mahal + acid rain + marble cancer connection is a favourite Prelims topic. Also know the SC case: MC Mehta v. Union of India (1996).
  • Remember: Acids turn blue litmus RED (A before B in alphabet; Acid = Red). Bases turn red litmus BLUE.
  • Turmeric gives a reddish-brown colour only with bases, not acids — a common trap in MCQs.
  • The Soil Health Card Scheme (2015) distributes cards to farmers showing 12 soil parameters including pH. Know which ministry runs it: Ministry of Agriculture and Farmers Welfare.
  • Antacids contain mild bases (Mg(OH)₂, NaHCO₃, CaCO₃) — they neutralise excess stomach acid (HCl). Prelims sometimes asks "which compound is present in antacids."
  • Acid rain forms from SO₂ + NOₓ emissions; pH below 5.6. Normal rain is pH 5.6 (not 7) due to dissolved CO₂.

Previous Year Questions

Q1. With reference to acid rain, consider the following statements:

  1. Acid rain is caused mainly by emissions of sulphur dioxide and nitrogen oxides.
  2. Normal rainfall is neutral with pH 7.
  3. Acid rain can damage historical monuments made of marble.

Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 1 and 3 only
(c) 2 and 3 only
(d) 1, 2 and 3

(b) 1 and 3 only

Q2. The Soil Health Card scheme was launched in which year?
(a) 2013
(b) 2015
(c) 2017
(d) 2019

(b) 2015

Q3. Which of the following is an olfactory indicator?
(a) Litmus
(b) Turmeric
(c) Onion
(d) Red hibiscus

(c) Onion