Acids, Bases and Salts

Why this chapter matters for UPSC: The pH scale is the lens through which we understand ocean acidification (a direct consequence of rising CO₂), acid rain (industrial and vehicular pollution), soil health management, and water treatment. The salts produced in this chapter — NaOH, NaCl, NaHCO₃, bleaching powder — are the raw materials for entire industrial sectors. India's salt production (world's 3rd largest), the chlor-alkali industry, and the use of lime in agriculture and water treatment are recurring themes in GS3 and GS2 environment questions.

PART 1 — Quick Reference Tables

Common Acids: Properties and Uses

Common Bases: Properties and Uses

Important Salts and Their Uses

PART 2 — Detailed Notes

1. Acids

Acids are substances that donate protons (H⁺ ions) to water, forming hydronium ions (H₃O⁺). This is the Arrhenius definition: acids produce H⁺ ions in aqueous solution.

Properties of acids:

• Sour taste (citric acid in lemons; acetic acid in vinegar — never test with taste in lab)
• Turn blue litmus paper red
• React with metals (above hydrogen in reactivity series) to produce hydrogen gas: Zn + H₂SO₄ → ZnSO₄ + H₂↑
• React with metal carbonates and bicarbonates to produce CO₂: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ (used as a test for carbonates; bubbles of CO₂ turn lime water milky)
• React with bases (neutralisation): HCl + NaOH → NaCl + H₂O (always exothermic)

Strong vs weak acids: Strong acids (HCl, H₂SO₄, HNO₃) fully dissociate in water. Weak acids (CH₃COOH, H₂CO₃, H₃PO₄) partially dissociate — equilibrium lies towards undissociated form. At the same concentration, strong acid has a lower pH.

2. Bases and Alkalis

Bases produce OH⁻ (hydroxide) ions in solution. An alkali is a base that is soluble in water — all alkalis are bases but not all bases are alkalis (e.g., Cu(OH)₂ is insoluble, so it is a base but not an alkali).

Properties of bases:

• Bitter taste; soapy/slippery feel
• Turn red litmus paper blue
• React with acids (neutralisation)
• React with certain metals (amphoteric metals like Al and Zn react with both acids and bases)

3. The pH Scale

pH in everyday life:

4. Acid Rain

Acid rain forms when sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels and industrial processes react with atmospheric moisture:

• SO₂ + H₂O → H₂SO₃ (then further oxidised to H₂SO₄)
• 4NO₂ + 2H₂O + O₂ → 4HNO₃

Effects of acid rain:

• Buildings and monuments: Marble (CaCO₃) and limestone react with acid rain — "marble cancer." The Taj Mahal in Agra is under threat from SO₂ emissions from the Mathura oil refinery and brick kilns in the surrounding area. Supreme Court ordered relocation of polluting industries from the Taj Trapezium Zone (TTZ — 10,400 sq km area around Taj Mahal).
• Forests: Acidifies soil, leaches nutrients (Ca²⁺, Mg²⁺), releases toxic aluminium ions — kills roots and microorganisms; damages leaves
• Freshwater ecosystems: Lakes and rivers acidify, killing fish and aquatic invertebrates (pH below 5.5 is lethal for most fish)
• Soil microbiome: Kills nitrogen-fixing bacteria, harming agriculture

5. Salts

Salts form when an acid reacts with a base (neutralisation). The nature of the salt (acidic, basic, or neutral) depends on the strength of the parent acid and base.

Baking soda (NaHCO₃): Used in cooking because it reacts with acids in the batter (yoghurt, buttermilk, lemon juice) to produce CO₂ bubbles — making the batter light and porous. Also used in fire extinguishers: at high temperatures NaHCO₃ → Na₂CO₃ + H₂O + CO₂; the CO₂ smothers the fire.

Bleaching powder (CaOCl₂): Made by passing Cl₂ over dry slaked lime. Releases Cl₂ when exposed to atmospheric CO₂ or when mixed with dilute acids. Used extensively for disinfecting public water supplies and sewage.

Plaster of Paris: When gypsum (CaSO₄·2H₂O) is heated to ~120°C, it loses 1.5 molecules of water to become Plaster of Paris (CaSO₄·½H₂O). When mixed with water, PoP sets hard by reabsorbing water and converting back to gypsum — the reaction is slightly exothermic (that is why a freshly applied cast feels warm).

6. Water of Crystallisation

Some salts incorporate specific numbers of water molecules into their crystal structure — water of crystallisation. The water molecules are not loosely held — they are an integral part of the crystal lattice and give the salt its characteristic shape and sometimes its colour.

7. Buffer Solutions

A buffer resists changes in pH when small amounts of acid or base are added. The most important buffer in the body is the bicarbonate buffer system in blood: HCO₃⁻ / H₂CO₃. When excess acid enters blood, HCO₃⁻ neutralises it; when excess base is added, H₂CO₃ neutralises it. This keeps blood pH firmly at 7.35–7.45 — deviations beyond this range are medical emergencies (acidosis or alkalosis). Buffer systems are also critical in industrial fermentation (maintaining optimal pH for microbial activity) and environmental chemistry (ocean's buffering capacity is being overwhelmed by CO₂).

Exam Strategy

Prelims traps:

• All alkalis are bases but not all bases are alkalis — only water-soluble bases are alkalis. Cu(OH)₂ and Fe(OH)₃ are insoluble bases, not alkalis.
• Acid rain pH is defined as below 5.6 — not below 7. Normal rainwater is already slightly acidic (pH ~5.6) due to dissolved CO₂.
• Plaster of Paris sets by absorbing water (converting back to gypsum) — not by drying out. This is why it generates warmth when setting.
• Bleaching powder is not pure CaOCl₂ — it is a mixture of Ca(OCl)Cl, Ca(OH)₂ and CaCl₂; the question may test its active ingredient (hypochlorite).
• Baking powder ≠ baking soda: baking powder = baking soda + a dry acid (cream of tartar/tartaric acid) + starch; baking soda alone needs an acid in the recipe.
• The Taj Mahal threat is from SO₂ + acid rain — NOT from directly from CO₂ or NOₓ alone (though NOₓ also contributes to acid rain).

Mains frameworks:

• Ocean acidification: atmospheric CO₂ → ocean chemistry → coral reef degradation → marine biodiversity loss → fisherfolk livelihoods → climate finance (loss and damage)
• Acid rain: industrial SO₂/NOₓ → acid rain → Taj Trapezium Zone → SC orders → clean energy transition
• Soil pH and agriculture: acidic soils (NE India, tea plantations) → lime application (Ca(OH)₂) → soil health → crop yield → PM-KISAN, soil health card scheme

Previous Year Questions

Prelims:

1. With reference to ocean acidification, which of the following statements is/are correct?
   (a) Ocean acidification is caused by the absorption of CO₂ from the atmosphere
   (b) Ocean acidification threatens the survival of coral reefs
   (c) Ocean pH has decreased by about 0.1 units since industrialisation
   (d) All of the above

2. Which of the following is used as an antacid to neutralise excess stomach acid?
   (a) Baking soda (NaHCO₃) only
   (b) Milk of magnesia (Mg(OH)₂) only
   (c) Aluminium hydroxide (Al(OH)₃) only
   (d) All of the above are used as antacids

Mains:

1. What is ocean acidification and how is it linked to climate change? Discuss its impact on Indian marine ecosystems and the policy responses needed. (CSE Mains 2022, GS Paper 3, 15 marks)

2. Discuss the environmental and cultural impact of acid rain on India's heritage monuments, with specific reference to the Taj Mahal. What legal and administrative measures have been taken to protect the monument? (CSE Mains 2018, GS Paper 3, 10 marks)