Class 10 Science Chapter 1 – Chemical Reactions and Equations

# Chemical Reactions and Equations ## What is a Chemical Reaction? A chemical reaction is a process in which substances (reactants) are transformed into different substances (products) with different chemical compositions. **Signs of a Chemical Reaction:** - Change in colour - Evolution of a gas - Change in temperature - Formation of a precipitate - Change in state --- ## Types of Chemical Reactions ### 1. Combination Reaction Two or more substances combine to form a single product. **General form:** A + B → AB **Examples:** - 2Mg + O₂ → 2MgO (Burning of magnesium) - H₂ + Cl₂ → 2HCl - CaO + H₂O → Ca(OH)₂ (Slaking of lime) ### 2. Decomposition Reaction A single substance breaks down into two or more simpler substances. **General form:** AB → A + B **Examples:** - 2H₂O → 2H₂ + O₂ (Electrolysis of water) - CaCO₃ → CaO + CO₂ (Thermal decomposition) - 2AgBr → 2Ag + Br₂ (Photodecomposition) **Types of Decomposition:** - Thermal decomposition – by heat - Electrolytic decomposition – by electricity - Photolytic decomposition – by light ### 3. Displacement Reaction A more reactive element displaces a less reactive element from its compound. **General form:** A + BC → AC + B **Examples:** - Fe + CuSO₄ → FeSO₄ + Cu (Iron displaces copper) - Zn + H₂SO₄ → ZnSO₄ + H₂ - 2Na + 2H₂O → 2NaOH + H₂ ### 4. Double Displacement Reaction Two compounds exchange their ions to form two new compounds. **General form:** AB + CD → AD + CB **Example:** - Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl (Precipitation reaction) --- ## Oxidation and Reduction **Oxidation:** Gain of oxygen OR loss of hydrogen OR loss of electrons **Reduction:** Loss of oxygen OR gain of hydrogen OR gain of electrons > When oxidation and reduction occur simultaneously, the reaction is called a **Redox Reaction**. **Example:** CuO + H₂ → Cu + H₂O - CuO is reduced (loses oxygen) → Cu - H₂ is oxidized (gains oxygen) → H₂O --- ## Exothermic and Endothermic Reactions | Type | Energy Change | Examples | |------|--------------|---------| | Exothermic | Releases heat | Burning, Respiration, Neutralization | | Endothermic | Absorbs heat | Photosynthesis, Dissolution of NH₄Cl | --- ## Corrosion and Rancidity **Corrosion:** Slow destruction of metals due to reaction with air/moisture. - Example: Rusting of iron → Fe₂O₃·xH₂O - Prevention: Painting, Galvanization, Alloying **Rancidity:** Oxidation of fats and oils, making food taste/smell bad. - Prevention: Vacuum packaging, Addition of antioxidants, Refrigeration --- ## Important Formulas for Exams | Reaction | Equation | |----------|---------| | Burning of Magnesium | 2Mg + O₂ → 2MgO | | Electrolysis of Water | 2H₂O → 2H₂ + O₂ | | Thermite Reaction | 2Al + Fe₂O₃ → Al₂O₃ + 2Fe | | Photosynthesis | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ | --- ## Quick Revision – Previous Year Questions **Q1.** What is a combination reaction? Give two examples. **Q2.** Distinguish between exothermic and endothermic reactions. **Q3.** Define rancidity. How can it be prevented? **Q4.** What is a redox reaction? Give one example. **Q5.** Why does silver chloride turn grey when exposed to sunlight?