Acids, Bases & Salts – Comprehensive Revision Notes

Natural Indicators, Taste & Everyday Clues

• Sour taste ⇄ presence of acids; bitter taste ⇄ presence of bases.
• Household remedy for stomach acidity: baking-soda (basic) solution neutralises excess HCl; property used = mutual neutralisation.
• Litmus (purple dye from lichen)
  • Acid → blue litmus → red
  • Base → red litmus → blue
• Other natural indicators
  • Turmeric: curry stain + soap (base) → reddish-brown; rinsing with water → yellow again.
  • Red-cabbage leaves, petals of Hydrangea / Petunia / Geranium.
• Synthetic indicators: phenolphthalein, methyl-orange.
• Olfactory indicators (odour-changing): onion, vanilla, clove.
  • Strip test: dilute $\text{HCl}$ destroys onion/vanilla smell; dilute $\text{NaOH}$ retains / intensifies it.

Chemical Reactions of Acids & Bases

With Metals (Activity 2.3 & 2.4)

• General pattern:
  $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{H}_2\uparrow$
• Experimental set-up: Zn granules + dil. $\text{H}<em>2\text{SO}</em>4$; evolved gas collected through soap solution → bubbles ignite with a ‘pop’ ⇒ hydrogen identification.
• Bases can also liberate $\text{H}<em>2$ with some amphoteric metals: $2\text{NaOH}</em>{(aq)} + \text{Zn}<em>{(s)} \rightarrow \text{Na}</em>2\text{ZnO}<em>2 + \text{H}</em>2\uparrow$ (sodium-zincate)
• Not all metals react; reactivity depends on metal position in electrochemical series.

With Metal Carbonates / Hydrogencarbonates (Activity 2.5)

• Observations: effervescence; gas turns lime-water milky (formation of $\text{CaCO}_3$).
• Word & balanced equations:
  • $\text{Na}<em>2\text{CO}</em>3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}<em>2\text{O} + \text{CO}</em>2\uparrow$
  • $\text{NaHCO}<em>3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}</em>2\text{O} + \text{CO}_2\uparrow$
• Excess $\text{CO}<em>2$: $\text{CaCO}</em>3 + \text{CO}<em>2 + \text{H}</em>2\text{O} \rightarrow \text{Ca(HCO}<em>3)</em>2$ (soluble) ⇒ lime-water clears again.
• Summary rule:
  $\text{Metal (bi)carbonate} + \text{Acid} \rightarrow \text{Salt} + \text{CO}<em>2 + \text{H}</em>2\text{O}$

Neutralisation (Activity 2.6)

• Phenolphthalein: pink in base; colourless in acid.
• Reaction: $\text{NaOH}<em>{(aq)} + \text{HCl}</em>{(aq)} \rightarrow \text{NaCl}<em>{(aq)} + \text{H}</em>2\text{O}$
• General:
  $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$

Metallic Oxides vs. Acids (Activity 2.7)

• Example: $\text{CuO}<em>{(s)} + 2\text{HCl}</em>{(aq)} \rightarrow \text{CuCl}<em>2 + \text{H}</em>2\text{O}$ (blue-green solution).
• Metallic oxides behave as basic oxides.

Non-metallic Oxides vs. Bases

• Example already observed: $\text{Ca(OH)}<em>2 + \text{CO}</em>2 \rightarrow \text{CaCO}<em>3 + \text{H}</em>2\text{O}$.
• Conclusion: non-metal oxides are acidic in nature.

Ionisation in Water & Electrical Conductivity

Do all H-Compounds behave as acids?

• Electrical-conductivity experiment (Activity 2.8): bulb glows only with acid/alkali solutions, not with glucose or alcohol ⇨ mere presence of H atom ≠ acidity.
• Acidic behaviour requires ionisation → $\text{H}^+(aq)$ production.

Role of Water (Activity 2.9)

• Dry $\text{HCl}$ gas does NOT turn dry litmus red; moist litmus does change.
• Reaction with water:
  $\text{HCl} + \text{H}<em>2\text{O} \rightarrow \text{H}</em>3\text{O}^+ + \text{Cl}^-$
• $\text{H}^+$ never exists free; represented as $\text{H}_3\text{O}^+$(aq).
• Bases ionise similarly:
  $\text{NaOH} \xrightarrow{H_2O} \text{Na}^+ + \text{OH}^-$

Exothermic Dilution (Activity 2.10)

• Adding conc. $\text{H}<em>2\text{SO}</em>4$ or $\text{NaOH}$ pellets to water raises temperature (exothermic).
• Safety rule: ALWAYS add acid to water slowly with stirring (never reverse).
• Dilution ↓ ion concentration ⇒ ‘dilute’ acid/base.

Strength of Acids & Bases — The pH Scale

• Universal indicator: mixture giving colour spectrum 0–14.
• Definition: $\text{pH} = -\log_{10}[\text{H}^+]$ (German potenz = power).
• Benchmarks:
  • $\text{pH}=7$ → neutral (pure water).
  • $\text{pH}<7$ → acidic; lower value ⇒ stronger acid. • $\text{pH}>7$ → basic; higher value ⇒ stronger base.
• Relation of ion concentrations (Fig 2.6 concept): $[\text{H}^+][\text{OH}^-] = 10^{-14}\,(\text{at }25^{\circ}\text{C})$.
• Strong vs. weak: 1 M $\text{HCl}$ gives more $\text{H}^+$ than 1 M $\text{CH}_3\text{COOH}$.

pH in Everyday Life

• Biological window: human blood & cells function at pH 7.0–7.8.
• Acid rain: rainwater pH < 5.6 ⇒ aquatic life threatened.
• Soil pH: dictates crop suitability; acidic soils treated with quick-lime $\text{CaO}$, slaked-lime $\text{Ca(OH)}<em>2$ or chalk $\text{CaCO}</em>3$.
• Digestion: stomach secretes $\text{HCl}$; excess acid ⇒ indigestion → antacids (e.g. $\text{Mg(OH)}_2$, baking-soda).
• Tooth decay: enamel corrodes below pH 5.5; bacterial acids from sugar; basic toothpaste neutralises.
• Chemical warfare in nature: bee-stings/nettle-stings inject methanoic acid; relief by mild bases (baking-soda, dock-leaf juice).

Salts — Classification & pH

Families (Activity 2.13)

• Same cation or same anion ⇒ common family.
  • Sodium family: $\text{NaCl},\; \text{Na}<em>2\text{SO}</em>4$.
  • Chloride family: $\text{NaCl},\; \text{KCl},\; \text{NH}_4\text{Cl}$ etc.

Acidic / Basic / Neutral Salts (Activity 2.14)

• Salt from strong acid + strong base → neutral (pH ≈ 7) e.g. $\text{NaCl},\; \text{KNO}_3$.
• Strong acid + weak base → acidic (pH < 7) e.g. $\text{NH}<em>4\text{Cl},\; \text{CuSO}</em>4$.
• Weak acid + strong base → basic (pH > 7) e.g. $\text{Na}<em>2\text{CO}</em>3,\; \text{NaCH}_3\text{COO}$.

Common Salt and Allied Chemicals

Rock-salt & History

• Deposits from evaporated ancient seas; brown due to impurities; mined like coal.
• Symbol of Indian freedom struggle: Gandhi’s 1930 Dandi March against salt tax.

Chlor-Alkali Process

• Electrolysis of brine:
  $2\text{NaCl}<em>{(aq)} + 2\text{H}</em>2\text{O} \xrightarrow{\text{Electricity}} 2\text{NaOH}<em>{(aq)} + \text{Cl}</em>2\uparrow + \text{H}_2\uparrow$
• Products & their major uses (Fig 2.8):
  • $\text{Cl}<em>2$ ⇒ PVC, disinfectants, CFCs, bleaching-agents. • $\text{H}</em>2$ ⇒ fuels, margarine, ammonia synthesis.
  • $\text{NaOH}$ ⇒ soaps, paper, rayon, degreasing metals.

Bleaching Powder

• Manufacture: $\text{Ca(OH)}<em>2 + \text{Cl}</em>2 \rightarrow \text{Ca(OCl)}<em>2 + \text{CaCl}</em>2 + \text{H}_2\text{O}$
• Uses: textile & paper bleaching, germicidal water treatment, oxidising agent.

Baking Soda (Sodium hydrogencarbonate, $\text{NaHCO}_3$)

• Industrial route (Solvay step):
  $\text{NaCl} + \text{NH}<em>3 + \text{CO}</em>2 + \text{H}<em>2\text{O} \rightarrow \text{NaHCO}</em>3 + \text{NH}_4\text{Cl}$
• Thermal decomposition in cooking:
  $2\text{NaHCO}<em>3 \xrightarrow{\Delta} \text{Na}</em>2\text{CO}<em>3 + \text{H}</em>2\text{O} + \text{CO}_2\uparrow$ (leavening).
• Applications: baking powder (with tartaric acid), antacid, soda-acid fire extinguishers.

Washing Soda (Sodium carbonate decahydrate, $\text{Na}<em>2\text{CO}</em>3·10\text{H}_2\text{O}$)

• Obtained via heating $\text{NaHCO}<em>3$ → $\text{Na}</em>2\text{CO}_3$ then recrystallisation.
• Uses: glass, soap, paper industries; household cleaning; softening hard water; precursor for borax.

Water of Crystallisation

• Definition: fixed number of water molecules integral to crystal lattice.
• Copper(II) sulphate:
  $\text{CuSO}<em>4·5\text{H}</em>2\text{O}$ (blue) ⇄ (heat) $\text{CuSO}<em>4$ (white) + $5\text{H}</em>2\text{O}$.
• Washing soda contains 10 waters; crystals appear dry yet hold water.
• Gypsum: $\text{CaSO}<em>4·2\text{H}</em>2\text{O}$.

Plaster of Paris (POP)

• Preparation:
  $\text{CaSO}<em>4·2\text{H}</em>2\text{O} \xrightarrow{373\,K} \text{CaSO}<em>4·\tfrac{1}{2}\text{H}</em>2\text{O} + 1.5\text{H}_2\text{O}$
• Setting reaction with water:
  $\text{CaSO}<em>4·\tfrac{1}{2}\text{H}</em>2\text{O} + \tfrac{3}{2}\text{H}<em>2\text{O} \rightarrow \text{CaSO}</em>4·2\text{H}_2\text{O}$ (hard gypsum)
• Uses: surgical casts for fractures, toys, decorative items, wall smoothing.
  (Named ‘Paris’ because early large-scale calcination of gypsum occurred near Paris’s Montmartre quarries.)

Safety, Ethical & Practical Implications

• Highly exothermic dilution demands PPE, slow acid-to-water addition, labelled hazard pictograms (Fig 2.5).
• Industrial chlorine handling requires containment due to toxicity & ozone-depletion potential of derivatives (CFCs).
• Over-liming agricultural soil disturbs micro-flora; balanced pH management essential for sustainability.
• Neutralisation in effluent treatment prevents ecological damage from acidic/basic discharges.

Numerical / Statistical References & Formulae

• Ionic product of water at 25 °C: $K_w = 1\times10^{-14}$.
• pH definition formula already provided.
• Stoichiometric ratios highlighted in balanced equations throughout.

Connections to Prior Knowledge & Wider Context

• Builds on Class-7/8 concept of acids/bases indicators.
• Links electrolysis (covered in electricity chapter) with industrial chemical production (chlor-alkali).
• Integrates environmental science (acid rain, soil pH) and biology (digestive system, tooth enamel) demonstrating interdisciplinary relevance.

Sample Examination-Style Questions Embedded in Text

• Reasoning: why curd shouldn’t be stored in copper/brass (acid corrodes metal, forms toxic salts).
• Gas tests: pop-test for $\text{H}<em>2$, lime-water for $\text{CO}</em>2$.
• Calculation: neutralisation volume ratio (Ex 3) based on molarity concept.

Traditional & Real-World Extensions

• Dock-plant base neutralises nettle acid — ethnobotanical remedy.
• Home-made indicators from beetroot / petals (Group Activity) show citizen-science potential.
• Soda-acid fire extinguisher DIY illustrates practical chemistry in safety equipment.