Chapter 2: Acids, Bases, and Salts

Introduction to Acids

• Nature: Acids are substances that release H⁺ (hydrogen) ions when dissolved in water. They are proton donors.
• Acid Dissociation:
  • General formula: HA→H⁺+A−
  • For strong acids (like HCl): HCl→H⁺+Cl⁻
  • For weak acids (like acetic acid): CH₃COOH⇌CH₃COO⁻+H⁺
• Acid-Base Reactions:
  • With bases: HCl+NaOH→NaCl+H₂O
  • With carbonates: 2HCl+CaCO₃→CaCl₂+H₂O+CO₂
• Hydronium Ion (H₃O⁺): When an acid dissolves in water, a proton combines with a water molecule to form the hydronium ion.

Bases

• Nature: Bases are substances that release OH⁻ (hydroxyl) ions when dissolved in water. They are proton acceptors or electron pair donors.
• Base Dissociation:
  • General formula: BOH→B⁺+OH⁻
  • For strong bases like NaOH: NaOH→Na⁺+OH⁻
  • For weak bases like NH₃ in water: NH3+H2O⇌NH4⁺+OH⁻
• Base-Acid Reactions:
  • Neutralization with acids: NaOH+HCl→NaCl+H₂O

pH Scale

• Understanding pH: pH is a measure of hydrogen ion concentration in a solution. Mathematically, pH = -log[H⁺]. Lower pH means more acidic.
• pH Measurement: Can be done using pH meters, pH paper, or indicators.
• pH in Daily Life:
  • Human body pH: Blood pH is around 7.4, stomach acid pH is about 1-2.
  • Soil pH affects plant growth; different plants prefer different pH levels.

Indicators

• Litmus: Turns red in acid, blue in base.
• Phenolphthalein: Colorless in acid, pink in base.
• Methyl Orange: Red in acid, yellow in base.
• Universal Indicator: Provides a broader range of color changes for different pH levels.

Salts

• Formation: Salts result from the neutralization reaction between acids and bases.
• Types of Salts:
  • Normal salts: Completely neutralized (e.g., NaCl from NaOH and HCl).
  • Acid salts: Not fully neutralized, leaving some hydrogen attached (e.g., NaHSO₄).
  • Basic salts: Result from partial neutralization of a base (e.g., Zn(OH)Cl).
• Salt Properties:
  • Hydration: Some salts can absorb water from the air (hygroscopic) or can lose water to become anhydrous.
  • Acidic, Basic, or Neutral: Depends on the nature of the constituent ions.
    • NH₄Cl (ammonium chloride) is acidic because NH₄⁺ ion hydrolyzes to produce H⁺.
    • Na₂CO₃ (sodium carbonate) is basic due to CO₃²⁻ ion which hydrolyzes to produce OH⁻.
    • NaCl is neutral as both ions do not hydrolyze significantly.

Common Salts and Their Uses

• Sodium Chloride (NaCl): Common salt, seasoning, preservation.
• Sodium Bicarbonate (NaHCO₃): Baking soda, used in baking, neutralizing acids, fire extinguishers.
• Calcium Sulfate (CaSO₄.2H₂O): Plaster of Paris for making casts, sculptures.
• Copper Sulfate (CuSO₄): Fungicide, used in agriculture, electroplating.
• Alum (KAl(SO₄)₂.12H₂O): Used in water purification, dyeing, and as a mordant in textiles.

Chemical Reactions Involving Salts

• Double Displacement:
• Decomposition:
• Hydrolysis: Some salts react with water to change pH, e.g., 

AgNO₃+NaCl→AgCl+NaNO₃

CaCO₃→heatCaO+CO₂

NH₄Cl+H₂O→NH₄OH+HCl

Water of Crystallization

• Definition: The fixed number of water molecules present in one formula unit of a salt in its crystalline form.
• Example: CuSO₄.5H₂O (blue vitriol), where 5 water molecules are part of the crystal structure.