Observations in Daily Life:

  1. Milk spoils when left at room temperature in summers.
  2. Iron objects rust when exposed to humid air.
  3. Grapes undergo fermentation.
  4. Cooking food changes its nature.
  5. Digestion alters food composition.
  6. Respiration produces energy by breaking down food.
  • In all these examples, the original substance changes its nature and identity, indicating chemical changes.
  • A chemical reaction happens when a chemical change occurs.

Indicators of a Chemical Reaction:

  • Change in state (solid, liquid, gas).
  • Change in color (e.g., rust forming on iron).
  • Evolution of gas (e.g., bubbles in a reaction).
  • Change in temperature (e.g., heat produced or absorbed).

Word Equations:

  • Word Equation Example:
    • Burning magnesium:
      Magnesium + Oxygen → Magnesium oxide
  • Key Terms:
    • Reactants: Substances that react (e.g., magnesium and oxygen).
    • Products: Substances formed (e.g., magnesium oxide).
    • The arrow (→) indicates the direction of the reaction.

Chemical Equations:

  • Replacing words with chemical symbols makes reactions concise:
    • Example: Mg + O₂ → MgO
  • Balanced vs. Unbalanced Equations:
    • A balanced equation has the same number of atoms of each element on both sides.
    • An unbalanced equation violates the law of conservation of mass, which states:
      • “Mass cannot be created or destroyed in a chemical reaction.”

Steps to Balance a Chemical Equation:

  1. Identify reactants and products: Write their chemical formulas.
  2. Draw boxes around formulas to avoid altering them.
  3. Count atoms of each element on both sides.
  4. Balance one element at a time:
    • Use coefficients (numbers before formulas) to equalize atoms.
  5. Check the balance for all elements.
  6. Add physical states (if needed):
    • (s): solid, (l): liquid, (g): gas, (aq): aqueous (dissolved in water).

Examples:

  1. Magnesium Burning:
    • Skeletal equation: Mg + O₂ → MgO
    • Balanced equation: 2Mg + O₂ → 2MgO
  2. Reaction of Zinc with Sulfuric Acid:
    • Zn + H₂SO₄ → ZnSO₄ + H₂ (already balanced).
  3. Balancing Iron and Water Reaction:
    • Skeletal equation: Fe + H₂O → Fe₃O₄ + H₂
    • Step-by-step balancing:
      • Balance oxygen: 4H₂O → Fe₃O₄
      • Balance hydrogen: 4H₂
      • Balance iron: 3Fe
    • Final equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

Enhancing Equations:

  • Add reaction conditions (e.g., temperature, pressure, catalyst):
    • Example: CO(g) + 2H₂(g) → CH₃OH(l) (at 340 atm).
  • Include physical states for better clarity:
    • Example: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

Basics of Chemical Reactions

  • Atoms don’t change into other elements, nor do they disappear or reappear; reactions involve breaking and forming bonds.
  • Chemical reactions produce new substances through rearranging atoms.

1. Combination Reaction

  • Definition: Two or more reactants combine to form one product.
    • Example: CaO + H2​O → Ca(OH)2​ (Calcium oxide reacts with water to form calcium hydroxide).
  • Exothermic Reaction: Heat is released during combination reactions.
    • Examples:
      • Burning coal: C + O2​ → CO2​
      • Formation of water: 2H2​ + O2 ​→ 2H2​O
  • Real-life relevance:
    • Whitewashing: Slaked lime reacts with CO2​ in air to form shiny calcium carbonate (CaCO3).

2. Decomposition Reaction

  • Definition: A single compound breaks into two or more simpler substances.
    • Example: CaCO3 → CaO+CO2 (Limestone decomposes to quicklime and carbon dioxide).
  • Types:
    • Thermal Decomposition: Heat is used to break the compound.
      • Example: 2FeSO4 → Fe2O3+SO2+SO3
    • Photolytic Decomposition: Light causes the breakdown.
      • Example: 2AgCl → ​2Ag+Cl2​​ (Used in black-and-white photography).
    • Electrolytic Decomposition: Electricity breaks the compound.
      • Example: Electrolysis of water produces hydrogen and oxygen gases.
  • Endothermic Reaction: Energy is absorbed for decomposition to occur.

3. Displacement Reaction

  • Definition: A more reactive element displaces a less reactive one from its compound.
    • Example: Fe+CuSO4​ → FeSO4​+Cu
  • Other Examples:
    • Zn+CuSO4​ → ZnSO4​+Cu
    • Pb+CuCl2​ → PbCl2​+Cu
  • Competitive insight: The reactivity series of metals determines which element can displace another.

4. Double Displacement Reaction

  • Definition: Exchange of ions between two reactants to form new products.
    • Example: Na2​SO4​+BaCl2​ → BaSO4​+2NaCl
  • Precipitation Reaction: A solid (precipitate) is formed during the reaction.
    • Example: Formation of barium sulfate (BaSO4).

5. Oxidation and Reduction (Redox Reactions)

  • Oxidation: Gain of oxygen or loss of hydrogen.
    • Example: 2Cu+O2​ → 2CuO (Copper oxidized to copper oxide).
  • Reduction: Loss of oxygen or gain of hydrogen.
    • Example: CuO+H2 → Cu+H2O (Copper oxide reduced to copper).
  • Simultaneous process: Oxidation and reduction happen together in redox reactions.
    • Example: ZnO+C → Zn+CO

6. Effects of Oxidation in Everyday Life

  1. Corrosion:
    • Metals like iron rust (reddish-brown coating), copper tarnishes (green), and silver blackens.
    • Caused by reactions with moisture, oxygen, and other environmental factors.
    • Preventive measures: Paint, galvanization, or alloying.
  2. Rancidity:
    • Fats and oils oxidize over time, changing taste and smell.
    • Prevention:
      • Use antioxidants in food.
      • Store food in airtight containers or flush with inert gases like nitrogen.

Key Concepts and Practice for Competitive Exams

  • Combination vs. Decomposition: Opposite processes; one forms a single product, the other breaks into simpler substances.
  • Exothermic vs. Endothermic: Know examples and energy flow direction.
  • Redox Identification: Practice identifying which substances are oxidized and reduced in a reaction.
  • Practical Applications: Relate reactions to daily life processes (e.g., respiration, whitewashing).

Sample Questions

  1. Why is heat released during slaked lime formation?
  2. Write balanced equations for the decomposition of lead nitrate and the burning of methane.
  3. Identify oxidized and reduced substances in CuO+H2​ → Cu+H2​O.

These notes simplify the text while covering essential concepts for better understanding and competitive exam preparation.

These all are the notes of chapter 1 in science. After some time you get important questions HERE. *#THANKS FOR VISITING, VISIT AGAIN#*😊