Compounds

📚 Key Concepts

🔹 What are Compounds?

Compounds are pure substances formed when two or more different elements combine chemically in a fixed ratio to form a new substance with entirely different properties.

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Key Characteristics:

  • Formed by chemical combination of elements
  • Fixed ratio of constituent elements
  • Properties different from constituent elements
  • Cannot be separated by physical methods
  • Can be broken down by chemical methods

🔹 Formation of Compounds

Water Formation:

  • 2 Hydrogen atoms + 1 Oxygen atom → 1 Water molecule
  • Ratio H:O = 2:1 (always fixed)
  • 2H + O → H₂O

Common Salt Formation:

  • 1 Sodium atom + 1 Chlorine atom → 1 Sodium chloride molecule
  • Ratio Na:Cl = 1:1 (always fixed)
  • Na + Cl → NaCl

🔹 Properties of Compounds vs Elements

Water (H₂O):

  • Hydrogen: Highly flammable gas
  • Oxygen: Supports combustion
  • Water: Extinguishes fire, liquid at room temperature

Sodium Chloride (NaCl):

  • Sodium: Soft, highly reactive metal
  • Chlorine: Poisonous, greenish gas
  • Salt: Harmless, essential for life, crystalline solid

🔹 Examples of Compounds

Common Compounds:

  • Sugar (C₁₂H₂₂O₁₁) – Carbon, Hydrogen, Oxygen
  • Carbon dioxide (CO₂) – Carbon, Oxygen
  • Methane (CH₄) – Carbon, Hydrogen
  • Ammonia (NH₃) – Nitrogen, Hydrogen
  • Iron sulfide (FeS) – Iron, Sulfur

🧪 Important Chemical Reactions

🔸 Decomposition of Water: 2H₂O → 2H₂ + O₂ (by electrolysis)

🔸 Decomposition of Sugar: C₁₂H₂₂O₁₁ → 12C + 11H₂O (by heating)

🔸 Formation of Iron Sulfide: Fe + S → FeS (by heating)