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📑 In This Chapter Guide (Table of Contents)
1. Chemical Properties of Acids and Bases
Acids are sour in taste, turn blue litmus red, and dissociate in water to produce Hydronium ions (H₃O⁺ or H⁺(aq)). Bases are bitter, soapy to touch, turn red litmus blue, and yield Hydroxide ions (OH⁻(aq)) in solution.
Fundamental Chemical Reactions:
- Acid + Metal → Salt + Hydrogen Gas:
2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)↑(Pop-sound test confirms H₂ gas). - Base + Metal → Salt + Hydrogen Gas:
2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) [Sodium Zincate] + H₂(g)↑. - Acid + Metal Carbonate → Salt + Carbon Dioxide + Water:
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑(Turns lime water milky due toCaCO₃precipitate; excess CO₂ clears the solution forming solubleCa(HCO₃)₂). - Neutralization Reaction:
Acid + Base → Salt + Water.HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). - Metallic Oxide + Acid → Salt + Water: Metallic oxides are basic in nature (e.g.,
CuO + 2HCl → CuCl₂ [Blue-green] + H₂O). Non-metallic oxides are acidic in nature (e.g.,CO₂ + Ca(OH)₂ → CaCO₃ + H₂O).
2. The pH Scale & Importance in Everyday Life
The pH scale measures Hydrogen ion concentration in an aqueous medium. Calculated as pH = -log₁₀[H⁺]. The scale ranges from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral at 25°C.
Crucial Real-World Applications:
- Human Body pH: Operates within a narrow pH range of 7.0 to 7.8.
- Acid Rain: When rainwater pH falls below 5.6, it harms aquatic fauna and dissolves historical monuments (stone leprosy, as seen in the Taj Mahal).
- Digestive System: Stomach produces hydrochloric acid (pH ~1.2) for pepsin activity. Excess acidity causes heartburn, cured using antacids like Milk of Magnesia (
Mg(OH)₂) or Sodium Hydrogen Carbonate. - Tooth Decay: Begins when mouth pH drops below 5.5. Bacteria degrade food sugars into acids that corrode tooth enamel (calcium hydroxyapatite). Toothpaste is mildly basic to neutralize this acid.
- Self-defense by Plants & Animals: Honeybee stings inject formic/methanoic acid (cured with mild base like baking soda). Nettle plant leaves inject methanoic acid, relieved by rubbing leaves of the dock plant.
3. Commercially Important Salts: Formulas & Preparation
Sodium Chloride (Common Salt, NaCl) serves as a vital raw material for synthesizing several crucial industrial compounds:
| Chemical Name | Formula | Preparation Reaction | Key Industrial Uses |
|---|---|---|---|
| Bleaching Powder | CaOCl₂ |
Dry slaked lime + Chlorine gas:Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O |
Textile bleaching, disinfectant for drinking water, oxidizer in chemical plants. |
| Baking Soda | NaHCO₃ |
Solvay process:NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ |
Baking powder (mixed with tartaric acid), antacid medicine, soda-acid fire extinguishers. |
| Washing Soda | Na₂CO₃·10H₂O |
Recrystallization of sodium carbonate:Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O |
Glass, soap and paper industries, removing permanent hardness of water. |
| Plaster of Paris (POP) | CaSO₄·½H₂O |
Heating Gypsum at 373 K (100°C):CaSO₄·2H₂O --[373K]--> CaSO₄·½H₂O + 1½H₂O |
Fractured bone casts, decorative false ceilings, architectural statues and molds. |
💡 Frequently Asked Questions (FAQ)
❓ Why should Plaster of Paris be stored in a moisture-proof container?
Plaster of Paris (Calcium Sulphate Hemihydrate) is highly hygroscopic. In the presence of moisture or water, it absorbs water and recrystallizes back into Gypsum, forming a solid hard mass and losing its setting property: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
❓ Why does dry HCl gas not change the color of dry litmus paper?
Acids exhibit acidic properties and turn blue litmus red only in the presence of water, where they ionize to produce free Hydronium (H₃O⁺) ions. In dry gaseous form without moisture, HCl molecules do not dissociate, so no color change occurs.
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