Chemistry · Ch 06

🧪 Acid, Base & Salt

BPSC / BSSC · Exam Fusion Prep  |  pH Scale · Indicators · Aqua Regia · Salt Types · Acid Sources · Base Uses

🧪 Chapter At a Glance
🔴 ACID
  • pH < 7
  • Sour taste
  • Blue litmus → Red
  • Releases H⁺ (Arrhenius)
  • Proton donor (Brønsted)
  • Accepts e⁻ pair (Lewis)
🔵 BASE
  • pH > 7
  • Bitter taste
  • Red litmus → Blue
  • Releases OH⁻ (Arrhenius)
  • Proton acceptor (Brønsted)
  • Donates e⁻ pair (Lewis)
← ACIDS (pH 0–6) →
7
Neutral
← BASES (pH 8–14) →
📊 Key Facts — Quick Reference
Aqua Regia
3:1 HCl:HNO₃ (dissolves Au, Pt)
Sørensen
Discoverer of pH concept
7.35–7.45
Normal Blood pH
±0.2
Blood pH change → Death
🔴 Acids — Concepts & Properties
Arrhenius Concept
Releases H⁺ in aqueous solution

e.g., HCl → H⁺ + Cl⁻ in water | H₂SO₄ → 2H⁺ + SO₄²⁻

Brønsted-Lowry Concept
Proton (H⁺) donor

e.g., HCl, H₂SO₄ donate proton to base

Lewis Concept
Accepts electron pair

Lewis acids: BF₃, AlCl₃ (electron deficient)

  • Nature: Corrosive
  • Taste: Sour
  • Blue litmus → Red litmus
  • pH < 7
  • Aqueous solution = Good conductor of electricity
⚗️ Acid Reactions
Acid + Metal → Salt + H₂↑Zn + 2HCl → ZnCl₂ + H₂↑
Acid + Carbonate → Salt + H₂O + CO₂↑CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑
Acid + Alkali (Base) → Salt + Water (Neutralisation)HCl + KOH → KCl + H₂O
👑 Aqua Regia
📘 Aqua Regia

Fresh mixture of 3 parts concentrated HCl + 1 part concentrated HNO₃. Called "Royal Water". Dissolves Gold (Au) and Platinum (Pt) — noble metals that resist ordinary acids.

Ratio
3:1 (HCl:HNO₃)
Dissolves
Gold (Au) & Platinum (Pt)
Nature
Must be freshly prepared
⚠️ Exam Trap — Special Acids
  • HCl (Hydrochloric acid) = helps in digestion of food (stomach acid)
  • HF (Hydrofluoric acid) = dissolves glass (only acid that does)
  • Benzoic acid = used in medicine
  • Glacial acetic acid = 99–100% pure acetic acid (ethanoic acid)
  • Aqua Regia ratio = 3:1 (HCl:HNO₃) NOT 1:3
🍋 Natural Sources & Uses of Major Acids
Acid Natural Source Industrial Formation Key Uses
Citric Acid Citrus fruit, lemon, orange, tomato Fermentation of raw sugar Food, medicine, cleaning, textile
Acetic Acid Fruit juice, milk, vinegar, oil Fermentation of vinegar; synthesis from acetylene Sour foods; solvent (acetone)
Formic Acid Red ants, scorpions Carbonylation of methanol Fruit preservation, antibacterial, leather & rubber
Sulphuric Acid Acid rain, volcanic activity Lead chamber process & Contact method Rechargeable batteries, medicines, explosives
Nitric Acid Alum and saltpeter Saltpeter method & Workland extract Photography, batteries, medicines, fertilisers
Benzoic Acid Grass, leaves, urine of herbivores Hydrolysis of benzoyl chloride Medicine, food preservation
Oxalic Acid Sorrel tree, spinach, beetroot From sodium formate Photography, printing, leather bleaching
Maleic Acid Apple, banana, orange peel, potato, carrot Hydrolysis of maleic anhydride Cosmetics, skin care products
⚠️ Exam Trap — Acid Sources
  • Formic acid = Red ants & Scorpions (also causes ant bite sting)
  • Citric acid = Lemon, Orange, Tomato
  • Oxalic acid = Spinach, Beetroot
  • Acetic acid = Vinegar (glacial acetic = 99–100% pure)
  • Sulphuric acid (Contact method): catalyst = V₂O₅
🔵 Bases — Concepts & Properties
Arrhenius Concept
Gives OH⁻ ions in aqueous solution

e.g., NaOH → Na⁺ + OH⁻ | KOH → K⁺ + OH⁻

Brønsted-Lowry Concept
Accepts protons (H⁺)

e.g., NH₃, H₂O act as bases by accepting H⁺

Lewis Concept
Donates (loses) electron pair

e.g., Cl⁻, F⁻, OH⁻ (electron pair donors)

  • Taste: Bitter
  • Red litmus → Blue litmus
  • pH > 7
  • Insoluble bases (ZnO, FeO) — react with acids but do NOT have basic properties in water
💊 Uses of Important Bases
Base Common Name Key Uses
Ca(OH)₂ Slaked Lime Treating acid burns
NaOH Caustic Soda Soap making, petroleum refining
Mg(OH)₂ Milk of Magnesia Removing stomach acidity (antacid)
CaO Quicklime Making bleaching powder & sodium carbonate
MgO Magnesia Pharmaceuticals, rubber fillers, boilers
KOH Caustic Potash Lab reagents, CO₂ & SO₂ absorber
⚠️ Exam Trap — Base Uses
  • Stomach acidity = Mg(OH)₂ (Milk of Magnesia) — NOT NaOH
  • Soap making = NaOH (Caustic Soda)
  • Acid burns treatment = Ca(OH)₂ (Slaked Lime)
  • KOH absorbs CO₂ and SO₂ gases in lab
🧂 Salt — Definition & Types
📘 Definition

Product of neutralisation reaction of an acid and a base.
Acid + Base → Salt + Water

HNO₃ + KOH → KNO₃ + H₂OAcid + Base → Salt + Water (Neutralisation)
Type of Salt Description Example
Common Salt Sodium Chloride NaCl
Acidic Salt Partial neutralisation of polyprotic acid NaHCO₃
Basic Salt Partial neutralisation of weak base with strong acid Mg(OH)Cl
Double Salt Mixing two or more salts Alum: K₂SO₄·Al₂(SO₄)₃·24H₂O
Mixed Salt One acid + two bases OR two acids + one base CaOCl₂ (Bleaching Powder)
💊 Important Salts & Their Uses
Salt Formula Common Name Key Uses
NaCl NaCl Common Salt / Table Salt Food, pickle preservation; making baking soda & NaOH
NaOH NaOH Caustic Soda Soap, detergents, wood pulp (paper), purification of bauxite
Na₂CO₃ Na₂CO₃ Washing Soda Glass, paper, soap, borax making; treats permanent hardness of water
NaHCO₃ NaHCO₃ Baking Soda Fire extinguisher; yeast agent in baking; making fried food
Ca(OCl)₂ Ca(OCl)₂ Bleaching Powder Swimming pool cleaning; water disinfection; making chloroform
CaSO₄·½H₂O CaSO₄·½H₂O Plaster of Paris (P.O.P.) Plastering broken bones; chalk, toys, decorative items
C₆H₅COONa C₆H₅COONa Sodium Benzoate Preservative; making jam, tomato sauce
🔬 Important Salt Formulas (Crystalline Salts)
Substance Formula Common Name
Epsom Salt MgSO₄·7H₂O Magnesium Sulphate heptahydrate
Mohr's Salt FeSO₄·(NH₄)₂SO₄·6H₂O Iron Ammonium Sulphate
Alum K₂SO₄·Al₂(SO₄)₃·24H₂O Potassium Alum
Blue Vitriol CuSO₄·5H₂O Copper Sulphate pentahydrate
White Vitriol ZnSO₄·7H₂O Zinc Sulphate heptahydrate
⚠️ Exam Trap — Salt Uses
  • Baking Soda (NaHCO₃) = fire extinguisher + yeast agent
  • Washing Soda (Na₂CO₃) = treats permanent hardness of water
  • Bleaching Powder = Ca(OCl)₂ (Mixed salt) = pool + water disinfection
  • Plaster of Paris = CaSO₄·½H₂O (NOT CaSO₄·2H₂O which is Gypsum)
  • Blue Vitriol = CuSO₄·5H₂O (Copper sulphate — blue colour)
  • Epsom salt = MgSO₄·7H₂O (laxative, bath salt)
🎨 Indicators
📘 Definition

Substances used to test acidity or basicity of a solution by changing colour.

Indicator Colour in Acid Colour in Base
Litmus Red Blue
Phenolphthalein Colourless Pink
Methyl Orange Red Yellow
Methyl Red Red Yellow
Phenol Red Yellow Red/Pink
Olfactory Indicators (Smell-based)
Lose smell in Base; Retain smell in Acid
Indicator In Base In Acid
Clove Oil Smell lost ✗ Smell retained ✓
Vanilla Smell lost ✗ Smell retained ✓
Onion Smell lost ✗ Smell retained ✓
⚠️ Exam Trap — Indicators
  • Phenolphthalein: Acid = Colourless; Base = Pink
  • Methyl Orange: Acid = Red; Base = Yellow
  • Phenol Red: Acid = Yellow; Base = Red/Pink (opposite of Methyl Orange!)
  • Olfactory indicators: smell lost in Base, retained in Acid
📊 pH Scale
📘 pH Definition

Measure of H⁺ ion concentration in a solution. Discovered by Sørensen.
pH = "Power of Hydrogen"

pH = −log[H⁺]Or [H⁺] = 10^(−pH) | Discovered by Sørensen
← ACIDS (pH 0–6) →
7
Neutral
← BASES (pH 8–14) →
Substance pH Value
Lemon Juice 2.0–2.5
Vinegar 2.4–3.4
Milk (cow) 6.4–6.8
Saliva 6.2–7.6
Neutral (Pure Water) 7.0
Tears 7.0–7.5
Blood Plasma 7.35–7.45
Sea water 7.8–8.4
Beverages 2–4
⚠️ Exam Trap — pH
  • pH discovered by Sørensen (NOT Arrhenius or Brønsted)
  • Blood pH = 7.35–7.45 (slightly basic)
  • Blood pH change of ±0.2 units = death
  • Lemon pH = 2.0–2.5 (most acidic commonly asked)
  • Sea water pH = 7.8–8.4 (basic)
  • Higher pH = more basic; Lower pH = more acidic
  • pH = 0 → strongest acid | pH = 14 → strongest base
🎯 High-Frequency BPSC/BSSC Exam Points
  • Acid: pH < 7, sour, Blue litmus → Red
  • Base: pH > 7, bitter, Red litmus → Blue
  • Arrhenius Acid = H⁺ donor | Arrhenius Base = OH⁻ donor
  • Brønsted: Acid = proton donor; Base = proton acceptor
  • Lewis: Acid = accepts e⁻ pair (BF₃, AlCl₃); Base = donates e⁻ pair
  • Aqua Regia = 3:1 HCl:HNO₃; dissolves Gold & Platinum
  • HCl = stomach acid (digestion) | HF = dissolves glass
  • Glacial acetic acid = 99–100% pure acetic acid
  • Formic acid found in Red ants & Scorpions
  • Citric acid in lemon, orange, tomato
  • Oxalic acid in spinach, beetroot
  • NaOH = Caustic Soda = soap making, paper (wood pulp)
  • Mg(OH)₂ = stomach antacid
  • Baking Soda = NaHCO₃ = fire extinguisher + baking
  • Washing Soda = Na₂CO₃ = treats permanent hardness of water
  • Bleaching Powder = Ca(OCl)₂ = pool cleaning, water disinfection
  • Plaster of Paris = CaSO₄·½H₂O
  • Blue Vitriol = CuSO₄·5H₂O | Epsom Salt = MgSO₄·7H₂O
  • Phenolphthalein: Acid = Colourless; Base = Pink
  • Methyl Orange: Acid = Red; Base = Yellow
  • Olfactory indicators: smell lost in Base, retained in Acid
  • pH = −log[H⁺]; discovered by Sørensen
  • Blood pH = 7.35–7.45; change of ±0.2 = death
  • Lemon pH = 2.0–2.5 | Sea water pH = 7.8–8.4
📋 Acid vs Base Quick Table
Property Acid Base
pH < 7 > 7
Taste Sour Bitter
Litmus Blue → Red Red → Blue
Arrhenius Gives H⁺ Gives OH⁻
Brønsted Proton donor Proton acceptor
Lewis Accepts e⁻ pair Donates e⁻ pair
Phenolphthalein Colourless Pink
Methyl Orange Red Yellow
⚠️ Most Common Exam Traps
  • Aqua Regia = HCl:HNO₃ = 3:1 (NOT 1:3)
  • HF dissolves glass (NOT HCl)
  • Phenolphthalein in acid = Colourless (NOT red)
  • Phenol Red: acid = Yellow (opposite of Methyl Orange)
  • Olfactory indicators lose smell in Base
  • POP = CaSO₄·½H₂O (NOT 2H₂O — that's Gypsum)
  • Washing Soda = Na₂CO₃ (NOT NaHCO₃ — that's Baking Soda)
  • Bleaching powder = Mixed salt (NOT double salt)
  • Lewis ACID accepts e⁻; Lewis BASE donates e⁻
  • Blood pH ±0.2 = death (very small tolerance)
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