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Acids, Bases, and Salts Overview

Sep 13, 2025

Overview

This lecture covers the chapter "Acids, Bases and Salts," explaining their properties, reactions, identification, and uses, with key focus on chemical processes and exam-style questions.

Acids: Properties and Examples

  • Acids are substances that donate H+ ions (protons).
  • Acids taste sour, turn blue litmus red, and are dangerous (red is danger).
  • Examples: Hydrochloric acid (HCl), sulfuric acid (H2SO4), citric acid (found in lemon), acetic acid (CH3COOH, in vinegar).
  • Naturally occurring acids: acetic (vinegar), lactic (curd), citric (lemon/orange), tartaric (tamarind), oxalic (tomato), methanoic (ant/bee sting).

Bases: Properties and Examples

  • Bases accept H+ ions or release OH– ions (hydroxide ions).
  • Bases taste bitter, feel slippery/soapy, and turn red litmus blue.
  • Examples: Sodium hydroxide (NaOH), ammonia (NH3).
  • Natural bases: baking soda, limewater.

Indicators: Types and Use

  • Indicators change color/smell in acids or bases to help identification.
  • Litmus: purple in neutral, red in acid, blue in base.
  • Natural indicators: litmus, hydrangea flower, turmeric (yellow in acid, red in base), onion, vanilla, clove oil (smell disappears with base).
  • Synthetic indicators: phenolphthalein (colorless in acid, pink in base), methyl orange (red in acid, yellow in base).
  • Universal indicator: shows pH by color (uses VIBGYOR colors).

Reactions of Acids and Bases

  • Acid + Metal → Salt + Hydrogen gas (test: pop sound with burning splint).
  • Acid + Metal carbonate/hydrogen carbonate → Salt + Water + Carbon dioxide (test: limewater turns milky then colorless with excess CO2).
  • Acid + Base → Salt + Water (Neutralization reaction).
  • Acid + Metal oxide → Salt + Water (metal oxides are basic).
  • Base + Metal (strong/alkaline bases) → Salt + Hydrogen gas (e.g., NaOH + Zn → sodium zincate + H2).
  • Base + Non-metal oxide → Salt + Water (non-metal oxides act as acids).

Strength, Dilution, and pH

  • Strong acids/bases completely dissociate in water; weak ones only partly.
  • Acid/base properties only show in presence of water (due to ionization).
  • pH scale: 0-14 (below 7: acidic, 7: neutral, above 7: basic); lower pH means higher H+ ion concentration.
  • pH affects digestion, soil, teeth health, blood, and aquatic life.

Salts: Types and Formation

  • Salts are ionic compounds formed from acid-base reactions.
  • Types: Neutral (strong acid + strong base), Acidic (strong acid + weak base), Basic (weak acid + strong base).
  • Common salt (NaCl) formed by neutralization; used in food, preservation, and as raw material.

Important Compounds and Processes

  • Sodium hydroxide (NaOH, caustic soda) made by chlor-alkali process (brine electrolysis)—produces NaOH, Cl2, H2.
  • Uses: NaOH (soap, paper), Cl2 (bleaching powder, water treatment), H2 (fuel, ammonia).
  • Bleaching powder (CaOCl2) made by passing Cl2 over dry slaked lime; used for bleaching, disinfection.
  • Baking soda (NaHCO3): basic salt, releases CO2 when heated, used in baking and as antacid/fire extinguisher.
  • Washing soda (Na2CO3·10H2O): used for cleaning, softening water, made by heating baking soda then recrystallization (water of crystallization).
  • Plaster of Paris (CaSO4·½H2O): made by heating gypsum (CaSO4·2H2O); used in construction and medical casts; must be kept airtight to avoid turning back to gypsum.

Key Terms & Definitions

  • Acid — Substance donating H+ ions in solution.
  • Base — Substance releasing OH– ions or accepting H+ ions.
  • Indicator — Substance changing color/smell with acids/bases.
  • Neutralization — Reaction: acid + base → salt + water.
  • pH — Scale (0-14) measuring acidity/alkalinity.
  • Salt — Ionic compound from acid-base reaction.
  • Chlor-alkali process — Electrolysis of brine to make NaOH, Cl2, H2.
  • Water of crystallization — Fixed water molecules in a salt’s structure.

Action Items / Next Steps

  • Complete homework: Write reactions for quick/slaked lime with acid, and explain formation principle of washing soda.
  • Review and memorize important compound formulas and processes (NaOH, CaOCl2, NaHCO3, Na2CO3·10H2O, CaSO4·½H2O).
  • Practice exam-style questions discussed.
  • Ensure understanding of pH applications and indicator color changes.

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