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: