Chemical Reaction – Summary Notes

Chemical Reaction
│
├── Indicators (gas, heat, color, ppt)
├── Types
│ ├── Combination
│ ├── Decomposition (heat/light/electricity)
│ ├── Displacement
│ ├── Double Displacement
│ └── Redox
├── Law: Conservation of Mass
├── Effects: Corrosion, Rancidity
└── Real-life relevance (industry, environment)

 

🔹 1. Indicators of a Chemical Reaction

Signs that a chemical change has occurred:

• Gas formation (e.g. bubbles in acid–metal reactions)

• Heat/light emission (exothermic or endothermic reactions)

• Color change (e.g. iron rusts to reddish-brown)

• Precipitate formation (solid formed from liquid solutions)

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🔹 2. Types of Chemical Reactions

🔸 (a) Combination Reaction

• Two or more substances combine to form one product.

• Ex:
  CaO + H₂O → Ca(OH)₂

🔸 (b) Decomposition Reaction

• A single compound breaks into two or more products.

• Types:

  • Thermal decomposition (heat)

  • Photolytic decomposition (light)

  • Electrolytic decomposition (electricity)

• Ex:
  2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (by heat)

🔸 (c) Displacement Reaction

• More reactive element replaces a less reactive one from a compound.

• Ex:
  Zn + CuSO₄ → ZnSO₄ + Cu

🔸 (d) Double Displacement Reaction

• Exchange of ions between two compounds.

• Often leads to precipitate or gas formation.

• Ex:
  Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

🔸 (e) Redox Reaction

• Reaction involving both oxidation and reduction.

• Oxidation: Loss of electrons / gain of oxygen

• Reduction: Gain of electrons / loss of oxygen

• Ex:
  CuO + H₂ → Cu + H₂O

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🔹 3. Law of Conservation of Mass

“Mass can neither be created nor destroyed during a chemical reaction.”

• Total mass of reactants = Total mass of products

• Basis of balancing chemical equations

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🔹 4. Effects of Chemical Reactions

✅ Corrosion

• Oxidation of metals due to moisture and air.

• Example:
  Rusting of iron (Fe → Fe₂O₃·xH₂O)

✅ Rancidity

• Oxidation of fats/oils, causing foul smell/taste.

• Prevented by:

  • Refrigeration

  • Airtight containers

  • Antioxidants

  • Nitrogen flushing

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🔹 5. Real-life Relevance

🏭 Industry

• Redox used in metallurgy, battery operation, electroplating

• Decomposition used in cement and lime production

🌱 Environment

• Combustion reactions → Greenhouse gases

• Corrosion → Infrastructure degradation

• Photosynthesis: A combination reaction

• Pollution control relies on precipitation reactions