Chemistry Lesson: Amphoteric Behavior of Alumina (Unit 5)

Introduction

• Lesson Focus: Amphoteric behavior of alumina
• BTEC Applied Science Unit 5
• Learning Aim: A1 (Looking at amphoteric behavior)
• Future lesson will cover metal oxides and hydroxides

Key Definitions

Acid

• Definition: Substance that dissociates in water to form H⁺ ions
• Examples:
  • HCl (Hydrochloric acid): Dissociates to H⁺ and Cl⁻
  • H₂SO₄ (Sulfuric acid): Releases two H⁺ and sulfate ion (SO₄²⁻)
• Types:
  • Monoprotic Acid: Releases 1 H⁺ (e.g., HCl)
  • Diprotic Acid: Releases 2 H⁺ (e.g., H₂SO₄)

Base

• Definition: Reacts with acids to form salt and water
• Distinction:
  • Alkali: A base that dissolves in water to form OH⁻ ions
  • Note: All alkalis are bases, but not all bases are alkalis

Amphoteric

• Definition: Substance that can act as both an acid and a base
• Example: Alumina (Aluminium oxide)

Alumina (Aluminium Oxide)

Chemical Properties

• Formula: Al₂O₃
• Composition:
  • Aluminium (Al): Group 3, forms 3⁺ ions
  • Oxygen (O): Group 6, forms 2⁻ ions
• Inertness: Chemically inert (unreactive except with hot acids or bases)

Amphoteric Behavior

• Reactions:
  • With Acids:
    • Example reaction with HCl (Hydrochloric Acid) or H⁺
    • Simplified to show reaction with H⁺
  • With Bases (Hydroxides):
    • Different reactions based on conditions (e.g., hot vs. not hot hydroxides)
    • Know one sample reaction for exams

Uses of Alumina

• Applications:
  • Fillers in paints, sunscreens, glasses
  • Chromatography (e.g., thin layer chromatography)
  • Effective desiccant (drying agent)
• Notes: Desiccants used in products like shoe boxes to remove moisture

Conclusion

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• Next Topic: Metal oxides and hydroxides