Chapter Overview

Chapter 1 — Chemical Reactions and Equations introduces how substances change into new products with different properties. It explains representation of reactions using chemical equations and the need to balance them according to the Law of Conservation of Mass. The chapter covers major reaction types — combination, decomposition, displacement, double displacement, oxidation and reduction — and discusses practical phenomena such as corrosion and rancidity. Clear understanding of these fundamentals strengthens problem-solving skills and is essential for performing well in the CBSE Class 10 board examination.

Key Concepts and Definitions

• Chemical reaction: Process where reactants form new products with different properties.
• Chemical equation: Symbolic representation of a reaction using chemical formulas and symbols. Example: Zn + H₂SO₄ → ZnSO₄ + H₂
• Reactants: Substances that undergo change.
• Products: New substances formed after the reaction.
• Balanced chemical equation: Equal number of each atom on both sides (obeys conservation of mass).
• Oxidation: Addition of oxygen or removal of hydrogen.
• Reduction: Addition of hydrogen or removal of oxygen.
• Redox reaction: Reaction where oxidation and reduction occur simultaneously.

Types of Chemical Reactions (with examples and balanced equations)

1. Combination reaction
   Two or more reactants combine to form a single product.
   Example: 2Mg + O₂ → 2MgO
2. Decomposition reaction
   A compound breaks down into simpler substances (often by heat/electricity).
   Example: 2H₂O → 2H₂ + O₂
3. Displacement reaction
   A more reactive element displaces a less reactive element from its compound.
   Example: Zn + CuSO₄ → ZnSO₄ + Cu
4. Double displacement reaction
   Ions are exchanged between two compounds forming new products (one may be a precipitate).
   Example: Na₂SO₄ + BaCl₂ → 2NaCl + BaSO₄↓
5. Oxidation–reduction (redox) reaction
   Oxidation and reduction occur simultaneously.
   Example: CuO + H₂ → Cu + H₂O

Important Terms and Symbols

NCERT In-Text Concept Highlights

• Law of Conservation of Mass: Mass of reactants equals mass of products.
• Balancing chemical equations: Example — unbalanced: Fe + H₂O → Fe₃O₄ + H₂; balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
• Exothermic reaction: Releases heat. Example: C + O₂ → CO₂ + Heat
• Endothermic reaction: Absorbs heat. Example: CaCO₃ → CaO + CO₂
• Corrosion: Oxidation of metals (e.g., rusting of iron).
• Rancidity: Oxidation of fats and oils causing spoilage.

Revision Notes (Exam-oriented)

• Always balance chemical equations to obey the Law of Conservation of Mass.
• Classify reaction type by observing reactants and products.
• Redox reactions involve simultaneous oxidation and reduction.
• Prevent corrosion by painting, galvanization or oiling.
• Prevent rancidity using antioxidants or nitrogen flushing.
• Exothermic: heat released; Endothermic: heat absorbed.
• Practice balancing equations involving polyatomic ions and gases.

Important Questions for Practice

Real-life Applications of Chemical Reactions

• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
• Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
• Electrolysis of water: 2H₂O → 2H₂ + O₂
• Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O + Energy