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Class 10 Science Notes — Chapter 2: Acids, Bases and Salts

Properties and reactions of acids and bases, pH, and salts used in daily life.

Detailed NCERT notes

  • Acids taste sour, turn blue litmus red, release H⁺ ions in water. Bases taste bitter, feel soapy, turn red litmus blue, release OH⁻ in water.
  • Common indicators: litmus (blue↔red), methyl orange (red in acid, yellow in base), phenolphthalein (colourless in acid, pink in base). Olfactory indicators: onion, vanilla, clove.
  • Acid + Metal → Salt + H₂↑ (except Cu, Ag with dilute acids).
  • Acid + Metal carbonate/bicarbonate → Salt + CO₂↑ + H₂O.
  • Base + Metal (amphoteric like Zn, Al) → Salt + H₂↑ (e.g. 2NaOH + Zn → Na₂ZnO₂ + H₂).
  • Acid + Base → Salt + H₂O (neutralisation, exothermic).
  • Metal oxides are basic (react with acids); non-metal oxides are acidic (react with bases).
  • Strong acid/base: ionises completely (HCl, H₂SO₄, NaOH, KOH). Weak: partial (CH₃COOH, NH₄OH).
  • pH = −log₁₀[H⁺]. pH < 7 acidic, = 7 neutral, > 7 basic. Universal indicator gives approximate pH by colour.
  • Importance of pH in daily life: soil pH for plants, digestion (HCl in stomach), tooth decay (below pH 5.5), self-defence in animals (nettles, bee sting), antacids (mild base).
  • Salts: pH depends on parent acid/base. Strong acid + strong base → neutral; strong acid + weak base → acidic; weak acid + strong base → basic.
  • Important salts:
  • Common salt (NaCl): from seawater/rock salt; raw material for many chemicals.
  • Sodium hydroxide (NaOH) via chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electrolysis of brine).
  • Bleaching powder Ca(OCl)Cl from slaked lime + chlorine; used to bleach cotton, disinfect water.
  • Baking soda NaHCO₃: mild base, used in antacids, baking (with tartaric acid ⇒ CO₂ makes cake rise).
  • Washing soda Na₂CO₃·10H₂O: from baking soda; softens hard water, used in glass/paper industry.
  • Plaster of Paris CaSO₄·½H₂O from gypsum by heating at 373 K; used in casts, sculptures.
  • Water of crystallisation: fixed number of water molecules in a salt's crystal (e.g., CuSO₄·5H₂O — blue; on heating turns white anhydrous).

Formulas & key results

  • pH = −log[H⁺]
  • Acid + Metal → Salt + H₂
  • Acid + Carbonate → Salt + CO₂ + H₂O
  • Acid + Base → Salt + H₂O (neutralisation)

Mind map

  • Indicators → properties → strength → pH
  • Salts: preparation, uses (NaOH, bleaching powder, baking soda, washing soda, POP)
  • Water of crystallisation

Tricks & shortcuts

  • Always add acid to water, never water to acid (highly exothermic).
  • 'Strong' ≠ 'concentrated'; strong = fully ionised, concentrated = high amount per volume.

Common mistakes to avoid

  • Confusing bleaching powder Ca(OCl)Cl with slaked lime Ca(OH)₂.
  • Assuming all salts are neutral.

Competency-based questions & answers

  1. Q. Why does dry HCl gas not turn blue litmus red?
    A. No water → HCl cannot dissociate → no H⁺ ions → no acidic behaviour.
  2. Q. Why is Plaster of Paris stored in moisture-proof containers?
    A. It absorbs water and turns back into hard gypsum (CaSO₄·2H₂O), losing its usefulness.
  3. Q. Fresh milk (pH ~6) becomes curd (pH ~4). Explain.
    A. Lactic acid formed by bacteria during fermentation → pH decreases → milk curdles.