Acids, Bases and Salts – Chapter 2 Comprehensive Notes

Taste, Indicators & First Observations

• Characteristic tast

  es

  • Acids: sour (e.g.

  • lemon juice

  • vinegar)

  • Bases: bitter (e.g.

  • baking soda solution)

• Practical remedy for acidity: recommend base (baking soda) because acids & bases neutralise each other.

• Classical colour indicators

  • Litmus (natural, from lichen)

  • Acid → blue → red

  • Base → red → blue

  • Turmeric

  • Base turns yellow stains reddish-brown; acid/water restores yellow.

  • Synthetic : phenolphthalein (colourless → pink in base), methyl orange (red in acid → yellow in base).

• Olfactory (smell) indicators

  • Onion, vanilla, clove

  • Acid/base environment changes or removes characteristic odour.

  • Used when colour change cannot be observed.

Laboratory Identification of Acids & Bases

• Given only red litmus

  1. Place strip in each unknown test tube.
     • Remains red → either acid or water.
     • Turns blue → base (identified).

  2. Transfer blue litmus (formed) to remaining tubes to separate water vs. acid.

• Standard solutions for testing (HCl, $H<em>2SO</em>4$, $HNO<em>3$, $CH</em>3COOH$, NaOH, $Ca(OH)<em>2$, KOH, $Mg(OH)</em>2$, $NH_4OH$).

• Table 2.1 observations record colour with four indicators for every solution.

Characteristic Reactions of Acids & Bases

Acid + Metal (Activity 2.3)

• General: $\text{Acid} + \text{Metal} \to \text{Salt} + H_2\uparrow$

• Example: \ce{Zn(s) + H2SO4(aq) -> ZnSO_4(aq) + H2(g) }

• Hydrogen confirmed by burning bubble → ‘pop’ sound.

• Same behaviour for HCl, $HNO<em>3$ (dil.), $CH</em>3COOH$.

Base + Metal (Activity 2.4)

• Not universal but some metals (Zn, Al) react with strong bases.

  • $2NaOH(aq) + Zn(s) \rightarrow Na<em>2ZnO</em>2(s) + H_2(g)$

Metal Carbonate / Hydrogencarbonate + Acid (Activity 2.5)

• $\text{Metal CO}<em>3 / \text{HCO}</em>3 + \text{Acid} \to \text{Salt} + CO<em>2\uparrow + H</em>2O$

• Test: $CO<em>2$ turns lime water $Ca(OH)</em>2$ milky via $CaCO<em>3$ ppt; excess $CO</em>2$ redissolves to $Ca(HCO<em>3)</em>2$ (clear).

• Equations:

  • $Na<em>2CO</em>3 + 2HCl \rightarrow 2NaCl + H<em>2O + CO</em>2$

  • $NaHCO<em>3 + HCl \rightarrow NaCl + H</em>2O + CO_2$

Acid + Base (Neutralisation) (Activity 2.6)

• $\text{Base} + \text{Acid} \to \text{Salt} + H_2O$

• Ionic core: $H^+<em>{(aq)} + OH^-</em>{(aq)} \rightarrow H_2O(l)$

• Phenolphthalein: pink in base, colourless after neutralisation; pink reappears when base is added again.

Metallic Oxide + Acid (Activity 2.7)

• $\text{Metal Oxide (basic)} + \text{Acid} \to \text{Salt} + H_2O$

• Example: $CuO(s)+2HCl(aq)\rightarrow CuCl<em>2(aq)+H</em>2O(l)$ (solution turns blue-green).

Non-metallic Oxide + Base

• Non-metal oxides behave as acids.

  • $Ca(OH)<em>2(aq)+CO</em>2(g)\rightarrow CaCO<em>3(s)+H</em>2O(l)$

Ions in Aqueous Solution

• Acids produce $H^+$ (actually $H_3O^+$) only in presence of water.

  • Dry $HCl$ gas shows no colour change on dry litmus; moist litmus turns red.

  • $HCl + H<em>2O \rightarrow H</em>3O^+ + Cl^-$

• Bases dissociate to give $OH^-$ ions:

  • $NaOH(s) \xrightarrow{H_2O} Na^+ + OH^-$

• Electric conductivity observed with bulb set-up (Fig 2.3): acid/base solutions conduct; glucose & alcohol (contain H but covalent) do not.

• Alkali: base that is water-soluble.

Dilution & Safety (Activity 2.10)

• Dissolving concentrated acid/base in water is highly exothermic.

  • Always add acid to water, stir constantly; reverse may splash.

• Dilution lowers $[H_3O^+]$ or $[OH^-]$ per unit volume.

Quantifying Strength – pH Scale

• Universal indicator: colour spectrum 0-14.

  • pH<7 acidic; $pH=7$ neutral; pH>7 basic.

• $pH=-\log_{10}[H^+]$ (not given but conceptually ‘power of H’).

• Strong acids (e.g.
  $HCl$) ionise almost completely → more $H^+$ → lower pH.

• Weak acids (e.g.
  $CH_3COOH$) partially ionise.

Biological & Environmental Relevance

• Human blood/stomach enzymes work within $7.0-7.8$.

• Acid rain: rainwater pH<5.6 harms aquatic life, heritage.

• Soil pH affects crop growth; liming (quick/slaked lime, chalk) corrects acidity.

• Digestion: excess stomach acid (pain) relieved by antacids (Mg(OH)₂, baking soda).

• Tooth decay: enamel corrodes below $pH=5.5$ → brush, basic toothpaste.

• Stings: bee/ant inject methanoic acid → relieve with base (baking soda); nettle–dock plant pairing is natural neutralisation.

Salts – Classification & pH

• Salt ‘family’ determined by common cation or anion.

  • NaCl & Na₂SO₄ – sodium family; NaCl & KCl – chloride family.

• Nature of salt solution

  • Strong acid + strong base → neutral (pH≈7) e.g.
    NaCl, KNO₃.

  • Strong acid + weak base → acidic (pH<7) e.g.
    $NH<em>4Cl$, $CuSO</em>4$.

  • Weak acid + strong base → basic (pH>7) e.g.
    $Na<em>2CO</em>3$, $CH_3COONa$.

Common Salt (NaCl) – Source & Central Hub

• Obtained from sea-water evaporation or mined as rock salt (impure, brown).

• Historic importance: Gandhi’s Dandi March symbolised salt tax protest.

Chlor-Alkali Process (Electrolysis of Brine)

• $2NaCl(aq) + 2H<em>2O(l) \xrightarrow{electricity} 2NaOH(aq)+Cl</em>2(g)+H_2(g)$

  • Products & key uses (Fig 2.8):

  • $NaOH$: soaps, paper, rayon, detergents.

  • $Cl_2$: PVC, disinfectants, CFCs, solvents.

  • $H_2$: fuels, ammonia syntheses, margarine.

Bleaching Powder – $Ca(OCl)_2$

• Manufacture: $Ca(OH)<em>2 + Cl</em>2 \rightarrow Ca(OCl)<em>2 + CaCl</em>2 + H_2O$

• Uses: textile/paper bleaching, water disinfection, oxidising agent.

Baking Soda – $NaHCO_3$

• Prepared via Solvay-type reaction:
  $NaCl + NH<em>3 + CO</em>2 + H<em>2O \rightarrow NH</em>4Cl + NaHCO_3$

• Mild base (see pH>7 in Activity 2.14); neutralises acid.

• Thermal decomposition for baking: $2NaHCO<em>3 \xrightarrow{\Delta} Na</em>2CO<em>3 + H</em>2O + CO_2\uparrow$ → leavening action.

• Other uses: antacid, component of soda-acid fire extinguisher.

Washing Soda – $Na<em>2CO</em>3·10H_2O$

• From heating $NaHCO<em>3$ → $Na</em>2CO_3$, followed by recrystallisation.

• Applications: glass manufacture, domestic cleaning, water softening, raw material for borax.

Water of Crystallisation

• Certain salts include fixed water molecules in lattice.

  • $CuSO<em>4·5H</em>2O$ (blue).
    • Heating → white anhydrous $CuSO_4$ + water droplets (Activity 2.15).
    • Adding water restores colour.

  • $Na<em>2CO</em>3·10H_2O$ (washing soda) – crystals look dry yet contain water.

  • Gypsum $CaSO<em>4·2H</em>2O$ when heated to $373\,K$ loses $1\tfrac12$ water → Plaster of Paris (POP) $CaSO<em>4·\tfrac12H</em>2O$.

  • POP + water → solid gypsum again (setting casts, immobilising fractures, decorative pieces).

Safety, Disposal & Practical Notes

• Warning symbol (Fig 2.5) on concentrated acids/bases; corrosive.

• Always wear goggles, gloves; neutralise spills with suitable base/acid.

• Store Plaster of Paris in moisture-proof container to avoid premature setting.

Key Equations & Formulas Snapshot

• Acid + Metal: $Zn + H<em>2SO</em>4 \rightarrow ZnSO<em>4 + H</em>2$

• Base + Metal: $2NaOH + Zn \rightarrow Na<em>2ZnO</em>2 + H_2$

• Carbonate + Acid: $Na<em>2CO</em>3 + 2HCl \rightarrow 2NaCl + H<em>2O + CO</em>2$

• Neutralisation (ionic): $H^+ + OH^- \rightarrow H_2O$

• Brine electrolysis: $2NaCl + 2H<em>2O \rightarrow 2NaOH + Cl</em>2 + H_2$

• POP formation: $CaSO<em>4·2H</em>2O \xrightarrow{373\,K} CaSO<em>4·\tfrac12H</em>2O + 1\tfrac12H_2O$

Quick Concept Checks

• Why glucose not acidic? – no $H^+$ generation in water.

• Dry $HCl$ no colour change – needs water to ionise.

• Fizz stronger with $HCl$ than $CH<em>3COOH$ on Mg – $HCl$ is a strong acid (faster $H</em>2$ production).

• Milkman adds baking soda – makes milk slightly basic, delays souring (bacterial acid neutralised), hence longer time to curdle.

Exam-Style Prompts to Practise

• Write balanced equations for metals with dilute $H<em>2SO</em>4$, $HCl$, etc.
• Arrange pH 1,4,7,9,11 in order of $[H^+]$.
• Explain role of POP in medicine with relevant reaction.
• Describe chlor-alkali cell and list three industrial applications of each product.