VR

Acids, Bases and Salts – Key Vocabulary

Indicators and Detection of Acids & Bases

  • Taste vs. Laboratory Testing
    • Sour taste ↔ acids; bitter taste ↔ bases.
    • Practical remedy for acidity (overeating): suggest baking_soda \text{(basic)} solution because bases neutralise excess stomach acid.
  • Natural indicators
    • Litmus (purple dye from lichen)
    • Acid → blue litmus turns red.
    • Base → red litmus turns blue.
    • Turmeric
    • Turns reddish-brown with soap (basic); returns yellow after rinsing.
    • Red-cabbage extract, petals of Hydrangea, Petunia, Geranium, etc.
  • Synthetic indicators
    • Methyl orange: acid → red; base → yellow.
    • Phenolphthalein: acid → colourless; base → pink.
  • Olfactory indicators
    • Onion, vanilla, clove change/disappear odour in acidic or basic media.

Laboratory Identification Activity (Activity 2.1)

  • Provided solutions: \text{HCl},\;\text{H}_2\text{SO}_4,\;\text{HNO}_3,\;\text{CH}_3\text{COOH},\;\text{NaOH},\;\text{Ca(OH)}_2,\;\text{KOH},\;\text{Mg(OH)}_2,\;\text{NH}_4\text{OH}.
  • Test each with red/blue litmus, phenolphthalein, methyl orange; compile colour changes in Table 2.1.
  • Puzzle: Given red litmus only & 3 unknown test tubes (water, acid, base):
  1. Dip strip in each sample. 2. One that remains red → either acid or water; one that turns blue → base. 3. Use blue strip produced to test remaining two; differentiate further.

Reactions with Metals (Activity 2.3 & 2.4)

  • General reaction:

    \text{Acid} + \text{Metal} \;\to\; \text{Salt} + \text{H}_2\uparrow

  • Example with Zn + dilute \text{H}_2\text{SO}_4: bubbles of \text{H}_2 collected through soap → burning pop sound.

  • Bases (strong) with some metals (e.g., Zn + NaOH when warmed):

    2\text{NaOH}_\text{(aq)} + \text{Zn}_\text{(s)} \to \text{Na}_2\text{ZnO}_2 + \text{H}_2\uparrow

    (Sodium zincate formation.)

  • Not all metals respond similarly (e.g., Cu shows no reaction).

Reactions with Metal Carbonates / Hydrogencarbonates (Activity 2.5)

  • Summary:

    \text{Metal carbonate or hydrogencarbonate} + \text{Acid} \to \text{Salt} + \text{CO}_2\uparrow + \text{H}_2\text{O}

  • Example equations:

    • \text{Na}_2\text{CO}_3 + 2\text{HCl} \to 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2
    • \text{NaHCO}_3 + \text{HCl} \to \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2

  • \text{CO}_2 bubbled through lime water \big(\text{Ca(OH)}_2\big):

    \text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3\downarrow + \text{H}_2\text{O} (milky).

    Excess \text{CO}_2 dissolves precipitate: \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \to \text{Ca(HCO}_3)_2 (soluble).

Neutralisation (Activity 2.6)

  • Acid + Base → Salt + Water; demonstrated with phenolphthalein:

    \text{NaOH}_\text{(aq)} + \text{HCl}_\text{(aq)} \to \text{NaCl}_\text{(aq)} + \text{H}_2\text{O}_\text{(l)}

  • Colour cycle: basic (pink) → add acid (colourless) → add base (pink again).

Metallic & Non-metallic Oxide Reactions

  • Metallic oxides = basic oxides; behave like bases vs. acids:
    • \text{CuO} + 2\text{HCl} \to \text{CuCl}_2 + \text{H}_2\text{O} (solution turns blue-green).
  • Non-metallic oxides = acidic oxides; behave like acids vs. bases:
    • \text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3 + \text{H}_2\text{O} (lime water test again).

Common Features of Acids & Bases (Section 2.2)

  • Acids produce \text{H}^+ (actually \text{H}_3\text{O}^+) in aqueous solution.

  • Bases/alkalis produce \text{OH}^- in aqueous solution.

  • Demonstration with conductivity apparatus (6 V battery, bulb):

    • Bulb glows for \text{HCl}, \text{H}_2\text{SO}_4 solutions (ions present).
    • No glow for glucose or ethanol though they contain hydrogen → hydrogen must be ionisable.

  • Dry \text{HCl} gas does not change dry litmus: ionisation requires water.

  • In presence of water: \text{HCl} + \text{H}_2\text{O} \to \text{H}_3\text{O}^+ + \text{Cl}^-.

  • Bases in water:

    \text{NaOH}_\text{(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+ + \text{OH}^- etc.

Dilution & Exothermic Mixing (Activity 2.10)

  • Adding concentrated acid/base to water releases heat (exothermic).
  • Safety rule: "Add acid to water, never water to acid" to avoid splashes & glass breakage.
  • Dilution lowers [\text{H}_3\text{O}^+] or [\text{OH}^-] per unit volume.

pH Scale & Universal Indicator (Section 2.3)

  • \text{pH} = -\log_{10}[\text{H}^+] (conceptually).
  • Range 0–14:
    • <7 acidic (0 extremely strong).
    • neutral.>7 basic (14 extremely strong).
  • Universal indicator colours (approx.): red (1), orange (3), yellow-green (5–6), green (7), blue (9), violet (12+).
  • Strength depends on degree of ionisation:
    • Strong acid: fully ionises (e.g., \text{HCl}).
    • Weak acid: partial (e.g., \text{CH}_3\text{COOH}).
Everyday Relevance of pH
  • Human blood/stomach: optimum pH 7.0–7.8; deviation harms metabolism.
  • Acid rain: pH <5.6; damages aquatic life, monuments.
  • Soil pH: affects crop suitability; farmers add quicklime \text{(CaO)}/slaked lime \text{(Ca(OH)}_2) or chalk \text{(CaCO}_3) to neutralise acidic soil. Digestion & antacids: excess stomach acid neutralised by \text{Mg(OH)}_2 (milk of magnesia) baking soda. Tooth decay: enamel dissolves below pH 5.5; bacteria generate acids; toothpastes (basic) counteract. Chemical defence: bee sting nettle hair inject methanoic acid; relief via mild base (baking soda) dock leaf sap (basic).

Salts: Types, pH & Families (Section 2.4)

  • Salt = ionic compound formed from acid + base reaction.
  • Family concept: same cation or anion.
  • Example: \text{NaCl}, \text{KCl} → chloride family; \text{Na}_2\text{SO}_4, \text{K}_2\text{SO}_4 → sulphate family.
  • Salt pH depends on parent acid/base strength:
    • Strong acid + strong base → neutral (pH 7) e.g., \text{NaCl}.
    • Strong acid + weak base → acidic (pH <7) e.g., \text{NH}_4\text{Cl} .
    • Weak acid + strong base → basic (pH>7) e.g., \text{Na}_2\text{CO}_3 .
Common Salt (NaCl) & the Chlor-Alkali Process
  • Brine electrolysis:
    2\text{NaCl}_\text{(aq)} + 2\text{H}_2\text{O}_\text{(l)} \xrightarrow{\text{electricity}} 2\text{NaOH}_\text{(aq)} + \text{Cl}_2\uparrow + \text{H}_2\uparrow.
  • Co-products & their major uses (Fig 2.8):
    • \text{NaOH}: soaps, paper, rayon, cleaning agents.
    • \text{Cl}_2: PVC, disinfectants, CFCs, pesticides.
    • \text{H}_2: fuels, margarine, ammonia synthesis.
Bleaching Powder (Ca(OCl)₂)
  • Manufacture: \text{Ca(OH)}_2 + \text{Cl}_2 \to \text{Ca(OCl)}_2 + \text{H}_2\text{O} + \text{CaCl}_2.
  • Uses: textile/paper bleaching, oxidising agent, water sterilisation.
Baking Soda (NaHCO₃)
  • Made via Solvay route: \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \to \text{NaHCO}_3 + \text{NH}_4\text{Cl}.
  • Properties: mild basic; pH >7.
  • Uses:
  1. Baking powder (mixture with tartaric acid) → \text{CO}_2 leavens dough.
  2. Antacid for indigestion.
  3. Component of soda-acid fire extinguishers.
  • Heating: 2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2.
Washing Soda (Na₂CO₃·10H₂O)
  • Obtain by recrystallising \text{Na}_2\text{CO}_3 from hot water:
    \text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \to \text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}.
  • Uses: glass, soap, paper manufacture; domestic cleaning; water-softening; precursor to borax.
Water of Crystallisation (Activity 2.15)
  • Defined: fixed no. of water molecules incorporated in crystal lattice.
  • Example 1: Blue \text{CuSO}_4\cdot5\text{H}_2\text{O} → on heating gives white \text{CuSO}_4; colour returns when re-hydrated.
  • Example 2: Gypsum \text{CaSO}_4\cdot2\text{H}_2\text{O}.
  • Plaster of Paris (POP): heating gypsum to 373 K: \text{CaSO}_4\cdot2\text{H}_2\text{O} \xrightarrow{373\,\text{K}} \text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O}.
    • Reacts with water to reset into hard gypsum → casts for fractured bones, decorative items.

Ethical / Historical Notes & Real-World Connections

  • Rock salt mining & Gandhi’s 1930 Dandi March: protest against British salt tax.
  • Environmental note: Venus’ clouds contain \text{H}_2\text{SO}_4 → hostile to life.
  • Safety symbols: concentrated acids/bases labelled with corrosive sign (Fig 2.5).
  • Traditional remedies: dock plant (basic juice) neutralises nettle acid sting.

Numerical / Stoichiometric Highlights

  • Carbonate tests (Activity 2.5): gas volume proportional to amount of carbonate.
  • Titration example: 10 mL NaOH neutralised by 8 mL HCl ⇒ doubling volume to 20 mL needs 16 mL HCl (exercise 3).

Formulae / Equations Compendium

  • Metal + Acid: \text{Zn} + 2\text{HCl} \to \text{ZnCl}_2 + \text{H}_2.
  • Metal Carbonate + Acid: \text{Na}_2\text{CO}_3 + 2\text{CH}_3\text{COOH} \to 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2.
  • Neutralisation (ionic): \text{H}^+ + \text{OH}^- \to \text{H}_2\text{O}.
  • POP setting: \text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O} + \tfrac32\text{H}_2\text{O} \to \text{CaSO}_4\cdot2\text{H}_2\text{O}.

Key Terms & Definitions

  • Indicator, olfactory indicator, neutralisation, alkali, pH, universal indicator, water of crystallisation, strong/weak acid/base.

Practical / Exam Tips

  • Always write state symbols (aq, s, l, g) and balance equations.
  • For salt pH predictions, memorise rule: strong vs. weak parent acid/base.
  • In viva/MCQ, recall: acid + metal → \text{H}_2 (pop sound test); carbonate test gives milky lime water.
  • Safety: