Acids, Bases and Salts
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1. Core Definitions
- Arrhenius theory: An acid is a substance that releases H⁺ (hydrogen) ions in aqueous solution; a base releases OH⁻ (hydroxide) ions.
- Bronsted-Lowry theory: An acid is a proton (H⁺) donor; a base is a proton acceptor. This explains substances like ammonia (NH₃) that behave as bases without containing OH⁻.
- Lewis theory: An acid is an electron-pair acceptor; a base is an electron-pair donor. This is the broadest definition and covers species with no protons at all (e.g., BF₃ as a Lewis acid).
- Salt: The ionic compound formed when an acid reacts with a base (neutralisation), consisting of a cation from the base and an anion from the acid, e.g., NaOH + HCl → NaCl + H₂O.
2. The pH Scale
- pH = −log₁₀[H⁺]. Scale runs 0–14 at 25°C.
- pH 7 = neutral (pure water). pH < 7 = acidic. pH > 7 = basic/alkaline.
- Each whole-number step is a 10-fold change in H⁺ ion concentration.
- Invented by Danish chemist Søren Sørensen in 1909.
- Universal indicator and pH paper are used to estimate pH by colour change.
Common substance pH values (approximate, frequently tested)
| Substance | pH |
|---|---|
| Gastric/stomach acid (HCl) | 1.5–2 |
| Lemon juice | 2–2.5 |
| Vinegar | 2.5–3 |
| Tomato juice | 4 |
| Black coffee | 5 |
| Human saliva | 6.5–7.5 |
| Pure water | 7 |
| Human blood | 7.35–7.45 |
| Sea water | 8 |
| Baking soda solution | 9 |
| Milk of magnesia | 10.5 |
| Ammonia solution | 11 |
| Soap solution | 9.5–10 |
| Bleach | 12.5 |
| Caustic soda (NaOH) solution | 13–14 |
3. Types of Acids
By source: - Organic acids (from living things): Citric acid (citrus fruits), Lactic acid (sour milk, muscles during exercise), Acetic acid (vinegar), Tartaric acid (tamarind, grapes), Oxalic acid (tomato, spinach), Formic acid (ant/bee sting, nettle sting), Malic acid (apples), Ascorbic acid = Vitamin C. - Mineral/inorganic acids: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃), Carbonic acid (H₂CO₃), Phosphoric acid (H₃PO₄).
By strength: Strong acids ionise completely in water (HCl, H₂SO₄, HNO₃). Weak acids ionise partially (acetic acid, carbonic acid, citric acid).
By basicity (number of replaceable H⁺): Monobasic (HCl, HNO₃), Dibasic (H₂SO₄, H₂CO₃), Tribasic (H₃PO₄).
4. Types of Bases
- A base that dissolves in water is called an alkali (all alkalis are bases, not all bases are alkalis — e.g., Cu(OH)₂ is a base but insoluble, hence not an alkali).
- Strong bases: NaOH (caustic soda), KOH (caustic potash) — completely ionise.
- Weak bases: NH₄OH (ammonium hydroxide), Mg(OH)₂.
- Common alkalis: Sodium hydroxide, Potassium hydroxide, Calcium hydroxide (limewater), Ammonium hydroxide.
5. Neutralisation and Salts
General reaction: Acid + Base → Salt + Water
Classification of salts by strength of parent acid/base
| Salt Type | Parent Acid | Parent Base | Nature | Example |
|---|---|---|---|---|
| Neutral salt | Strong | Strong | pH ≈ 7 | NaCl (from HCl + NaOH) |
| Acidic salt | Strong | Weak | pH < 7 | NH₄Cl (from HCl + NH₄OH) |
| Basic salt | Weak | Strong | pH > 7 | Na₂CO₃ (from H₂CO₃ + NaOH), CH₃COONa |
Other salt categories
- Acid salts: contain a replaceable H atom, e.g., NaHCO₃ (sodium bicarbonate/baking soda), NaHSO₄.
- Double salts: combination of two simple salts that dissociate into all constituent ions in water, e.g., Potash alum KAl(SO₄)₂·12H₂O, Mohr's salt FeSO₄(NH₄)₂SO₄·6H₂O.
- Complex salts: do not dissociate fully into simple ions, e.g., K₄[Fe(CN)₆] (potassium ferrocyanide).
- Mixed salts: contain more than one basic or acidic radical, e.g., bleaching powder Ca(OCl)Cl.
6. Indicators
| Indicator | Colour in Acid | Colour in Base | Colour in Neutral |
|---|---|---|---|
| Litmus (blue) | Turns red | Stays blue | Purple |
| Litmus (red) | Stays red | Turns blue | Purple |
| Phenolphthalein | Colourless | Pink/magenta | Colourless |
| Methyl orange | Red/pink | Yellow | Orange |
| Turmeric | Yellow (unchanged) | Turns red/brown | Yellow |
| Red cabbage extract | Red/pink | Green/yellow | Purple |
Litmus is extracted from lichens. Natural indicators asked about in exams: litmus, turmeric, red cabbage, china rose (Gudhal) petal extract, grape juice.
7. Important Reactions and Properties
- Acids react with active metals to release hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂↑. This is tested with a burning splint giving a "pop" sound.
- Acids react with metal carbonates/bicarbonates to release CO₂: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ (tested by turning limewater milky).
- Concentrated sulphuric acid is a strong dehydrating agent (chars sugar and wood by removing water).
- Aqua regia (3:1 HCl:HNO₃) is one of the few things that dissolves gold and platinum.
- "King of chemicals": Sulphuric acid, due to its vast industrial use (fertilisers, batteries, detergents).
- Antacids (e.g., milk of magnesia, sodium bicarbonate tablets) neutralise excess stomach acid — a classic application of neutralisation in daily life.
- Bee/ant stings are acidic (formic acid) — treated with a mild base like baking soda. Wasp stings are alkaline — treated with a mild acid like vinegar.
- Tooth decay begins when mouth pH falls below ~5.5, dissolving tooth enamel — hence toothpaste is basic to neutralise acid formed by bacteria.
- Soil pH: most crops prefer near-neutral soil (pH 6–7.5); acidic soil is treated with quicklime/slaked lime, alkaline soil with gypsum or organic matter.
8. Buffers and Ionic Equilibrium (Higher-Level Additions)
- A buffer solution resists changes in pH when small amounts of acid or base are added — made of a weak acid and its conjugate base (or weak base and its conjugate acid), e.g., acetic acid/sodium acetate.
- Human blood is buffered by the carbonic acid–bicarbonate system (H₂CO₃/HCO₃⁻), keeping blood pH tightly regulated at 7.35–7.45; a fall below 7.35 is acidosis, a rise above 7.45 is alkalosis — both are medical emergencies, a fact that bridges chemistry with the physiology topics in these notes.
- Ostwald's dilution law and the concept of degree of ionisation (α) explain why weak acids/bases only partially ionise — background for why acetic acid is "weak" despite being a common acid.
- Common ion effect: addition of an ion common to a weak electrolyte suppresses its ionisation — relevant to why adding sodium acetate to acetic acid reduces the acid's ionisation.
9. Industrial and Applied Chemistry (UPSC/Higher-Level Additions)
- Contact process: industrial manufacture of sulphuric acid using vanadium pentoxide (V₂O₅) as catalyst — sulphuric acid is the single most-produced industrial chemical worldwide, used in fertilisers (superphosphate), batteries (lead-acid), and detergents.
- Haber process: industrial synthesis of ammonia (N₂ + 3H₂ → 2NH₃) using an iron catalyst under high pressure/temperature — ammonia is the base for nitrogenous fertilisers (urea), directly linking to India's fertiliser-subsidy and Green Revolution policy discussions in Mains GS3.
- Solvay process: industrial production of sodium carbonate (washing soda) from brine and limestone.
- Chlor-alkali process: electrolysis of brine (NaCl solution) to produce chlorine, hydrogen, and sodium hydroxide simultaneously — chlorine is used for water disinfection and PVC manufacture, linking to public health/sanitation policy (water treatment plants under Jal Jeevan Mission use chlorination).
- Acid rain (formed from SO₂/NOx emissions reacting with atmospheric moisture to form dilute H₂SO₄/HNO₃) is a recurring environment-chemistry crossover theme — the Taj Mahal's marble (CaCO₃) discolouration/corrosion from acid rain and industrial pollution (the "Taj Trapezium Zone" pollution-control area) is a classic UPSC-relevant case study connecting chemistry to heritage conservation and environmental law.
- Ocean acidification (absorption of atmospheric CO₂ by seawater, forming carbonic acid and lowering ocean pH) is a live climate-change topic threatening coral reefs and shellfish (which rely on calcium carbonate shells) — a frequent GS3 environment theme.
10. UPSC Angle: Static-Dynamic Linkages
- Antacids, ORS (oral rehydration solution, mildly alkaline/neutral electrolyte balance), and pH-based water quality standards (BIS drinking water standards specify pH 6.5–8.5) connect this static chemistry to public health schemes (Jal Jeevan Mission, National Health Mission).
- Fertiliser chemistry (ammonia, sulphuric acid, phosphoric acid as feedstocks for urea/DAP/NPK) is the static base for the recurring Mains GS3 theme of fertiliser subsidy reform, nutrient-based subsidy (NBS) policy, and soil health.
- Aqua regia's ability to dissolve gold/platinum links to questions on gold recycling, e-waste management (extracting precious metals from electronic waste using acid treatments), and India's gold-import/monetisation policy discussions.
11. PYQ-Style Practice Pointers
- SSC/RRB pattern: "pH of human blood is approximately ___" (7.35–7.45) — frequently tested exact range, easy to mix up with the "neutral = 7" fact.
- SSC/RRB pattern: matching common substances to approximate pH (lemon juice, milk, blood, soap) — memorise the ordering, not just individual values.
- UPSC Prelims pattern: statement-based question on "which of the following processes is used to manufacture X" (Haber for ammonia, Contact for sulphuric acid, Solvay for sodium carbonate) — these process names are static facts but are usually wrapped inside an applied/industrial-context question stem.
12. Practice Questions (With Answers)
SSC/RRB style (direct recall):
The pH value of a neutral solution at 25°C is: (a) 0 (b) 5 (c) 7 (d) 14 Answer: (c) 7
Which acid is present in the human stomach? (a) Sulphuric acid (b) Acetic acid (c) Hydrochloric acid (d) Nitric acid Answer: (c) Hydrochloric acid
Phenolphthalein turns which colour in a basic solution? (a) Colourless (b) Pink/magenta (c) Yellow (d) Blue Answer: (b) Pink/magenta
Which of the following is used to treat acidic soil? (a) Gypsum (b) Slaked lime (c) Ammonium sulphate (d) Sulphuric acid Answer: (b) Slaked lime — a base is needed to neutralise acidic soil.
Baking soda is chemically known as: (a) Sodium carbonate (b) Sodium bicarbonate (c) Sodium hydroxide (d) Calcium carbonate Answer: (b) Sodium bicarbonate (NaHCO₃)
UPSC Prelims style (statement-based/applied):
- Consider the following statements regarding buffer solutions in the human body:
- Blood pH is maintained by the carbonic acid–bicarbonate buffer system.
A blood pH below 7.35 is termed alkalosis. Which of the statements given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Answer: (a) 1 only — a pH below 7.35 is acidosis, not alkalosis (statement 2 is reversed/incorrect).
The Contact Process is associated with the industrial manufacture of which of the following? (a) Ammonia (b) Sulphuric acid (c) Sodium carbonate (d) Nitric acid Answer: (b) Sulphuric acid — using vanadium pentoxide as catalyst.
With reference to ocean acidification, consider the following statements:
- It results from the absorption of atmospheric CO₂ by seawater.
- It leads to a rise in ocean pH. Which of the statements given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Answer: (a) 1 only — absorbed CO₂ forms carbonic acid, which lowers ocean pH (statement 2 is incorrect).