Acids, Bases, and Salts

Acids

Sour in taste, turn blue litmus red, and produce H⁺ (or H₃O⁺) ions in aqueous solution. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and acetic acid (CH₃COOH).

Bases

Bitter in taste, turn red litmus blue, and produce OH⁻ ions in water. Soluble bases are called alkalis (e.g., NaOH, KOH). Bases feel soapy and can be corrosive.

Indicators

Substances that change color in the presence of acids or bases:

• Litmus: Red in acid, blue in base
• Phenolphthalein: Colorless in acid, pink in base
• Methyl orange: Red in acid, yellow in base
• Natural indicators:
  • Turmeric: Yellow in acid, reddish-brown in base
  • Beetroot extract
• Olfactory indicators: Substances like onion, vanilla, and clove change odor in acidic or basic conditions

Types of Reactions

• Reaction with Metals:
  Acids react with metals to produce hydrogen gas and a salt (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂)

  Some bases, like NaOH, also produce hydrogen with metals like zinc (2NaOH + Zn → Na₂ZnO₂ + H₂).

• Reaction with Metal Carbonates/Hydrogencarbonates:
  Acids produce carbon dioxide, water, and a salt (e.g., Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂)
• Neutralization Reaction:
  Acid + Base → Salt + Water (e.g., HCl + NaOH → NaCl + H₂O)

  This is an exothermic reaction where acids and bases nullify each other's effects.

• Reaction of Metallic Oxides with Acids:
  Metallic oxides (basic) react with acids to form salt and water (e.g., CuO + 2HCl → CuCl₂ + H₂O)
• Reaction of Non-Metallic Oxides with Bases:
  Non-metallic oxides (acidic) react with bases to form salt and water (e.g., CO₂ + Ca(OH)₂ → CaCO₃ + H₂O)

Behavior in Water

• Acids in Water:

  Produce H⁺ ions, which combine with water to form hydronium ions (H₃O⁺). Dry acids (e.g., HCl gas) don't show acidic behavior without water.

• Bases in Water:

  Produce OH⁻ ions. Soluble bases are alkalis (e.g., NaOH → Na⁺ + OH⁻).

• Conductivity:

  Acids and bases conduct electricity in aqueous solutions due to the presence of ions (H⁺/H₃O⁺ or OH⁻).

• Dilution:

  Mixing acids or bases with water is exothermic. Acid must be added to water slowly to avoid splashing due to heat.

pH Scale Overview

Measures the concentration of H⁺ ions in a solution (0-14):

• pH 7 is neutral
• pH < 7 is acidic
• pH > 7 is basic

Universal Indicator

Shows a range of colors depending on pH, used to quantify acidity or basicity.

Applications

• Biological: Organisms function within a narrow pH range (e.g., human body: 7.0-7.8)
• Soil: Plants require specific pH for growth (e.g., farmers use lime to neutralize acidic soil)
• Digestive System: Stomach acid (HCl) aids digestion; antacids neutralize excess acid
• Tooth Decay: pH < 5.5 corrodes enamel; basic toothpaste neutralizes acids
• Acid Rain: pH < 5.6 harms aquatic life

Acids

• Strong Acids: Fully ionize in water (e.g., HCl, H₂SO₄), producing more H⁺ ions
• Weak Acids: Partially ionize (e.g., CH₃COOH), producing fewer H⁺ ions

Bases

• Strong Bases: Fully dissociate (e.g., NaOH, KOH)
• Weak Bases: Partially dissociate (e.g., NH₄OH)

Formation and Classification

• Formation: Salts are formed by neutralization or other reactions (e.g., NaCl from HCl + NaOH)
• Family of Salts: Salts with the same cation or anion belong to a family (e.g., NaCl, Na₂SO₄ are sodium salts)

pH of Salts

• Strong acid + strong base → neutral salt (pH 7, e.g., NaCl)
• Strong acid + weak base → acidic salt (pH < 7, e.g., NH₄Cl)
• Strong base + weak acid → basic salt (pH > 7, e.g., Na₂CO₃)

Common Salts and Their Uses

• Common Salt (NaCl): Source of chemicals like NaOH, NaHCO₃, Na₂CO₃
• Chlor-Alkali Process: Electrolysis of brine (NaCl solution) produces NaOH, Cl₂, and H₂
• Bleaching Powder (CaOCl₂): Made from chlorine and slaked lime; used for bleaching and disinfection
• Baking Soda (NaHCO₃): Used in cooking, antacids, fire extinguishers
• Washing Soda (Na₂CO₃·10H₂O): Used in glass, soap industries, and water softening

Fixed number of water molecules in a salt's crystalline structure (e.g., CuSO₄·5H₂O).

Plaster of Paris (CaSO₄·½H₂O): Formed by heating gypsum (CaSO₄·2H₂O); used in casts, decorations.