1. What is a Chemical Reaction?

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with new chemical properties.

Example:
Magnesium + Oxygen → Magnesium Oxide
2Mg + O₂ → 2MgO

2. Characteristics of Chemical Reactions

  • Change in state

  • Change in color

  • Evolution of gas

  • Change in temperature

  • Formation of precipitate

🔹 3. Chemical Equation

A chemical equation represents a chemical reaction using symbols and formulae of the substances involved.

Example:
Hydrogen + Oxygen → Water
2H₂ + O₂ → 2H₂O

4. Balancing Chemical Equations

In a balanced chemical equation, the number of atoms of each element is equal on both sides of the equation.
Law of Conservation of Mass is followed.

🔹 5. Types of Chemical Reactions

🔸 (a) Combination Reaction

Two or more substances combine to form a single product.
Example:
CaO + H₂O → Ca(OH)₂

🔸 (b) Decomposition Reaction

A single compound breaks down into two or more substances.
Example:
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

(c) Displacement Reaction

A more reactive element displaces a less reactive element from a compound.
Example:
Zn + CuSO₄ → ZnSO₄ + Cu

🔸 (d) Double Displacement Reaction

Exchange of ions between two compounds.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

🔹 6. Exothermic & Endothermic Reactions

  • Exothermic Reaction: Releases heat
    Example: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

  • Endothermic Reaction: Absorbs heat
    Example: N₂ + O₂ → 2NO (Heat absorbed)

🔹 7. Oxidation and Reduction

  • Oxidation: Gain of oxygen or loss of hydrogen

  • Reduction: Loss of oxygen or gain of hydrogen

Example:
CuO + H₂ → Cu + H₂O

8. Corrosion

  • Slow destruction of metals due to air, moisture, or chemicals.

  • Example: Rusting of iron:
    Fe + O₂ + H₂O → Fe₂O₃·xH₂O

🔹 9. Rancidity

  • Spoiling of food due to oxidation of fats and oils.

  • Prevention: Use of antioxidants, refrigeration, airtight containers.

📌 Short Definitions for Revision:

Reactant: Starting substances
Product: Final substances formed
Oxidation: Addition of oxygen
Reduction: Removal of oxygen