Chemistry (5070)
Topic 4 of 5Cambridge O Levels

Chemical Reactions & Equations

Balancing equations and types of reactions

A chemical reaction happens when substances change into new substances with different properties. The atoms rearrange but are never created or destroyed.


Signs a Reaction Has Occurred:

  • Colour change
  • Gas produced (bubbles)
  • Temperature change (exothermic = heats up, endothermic = cools down)
  • Precipitate formed (solid appears in a liquid)

    • Balancing Equations:

    Same number of each type of atom on both sides.

    Unbalanced: H₂ + O₂ → H₂O

    Balanced: 2H₂ + O₂ → 2H₂O (4H and 2O on each side)


    Types of Reactions:

  • Combination: A + B → AB (two things join)
  • Decomposition: AB → A + B (one thing breaks apart)
  • Displacement: More reactive element kicks out less reactive one
  • Neutralisation: Acid + Base → Salt + Water
  • Redox: Involves transfer of electrons
  • Oxidation: Loss of electrons (OIL)
  • Reduction: Gain of electrons (RIG)
  • Remember: OIL RIG

    • Exothermic vs Endothermic:

  • Exothermic: Releases heat (combustion, neutralisation). Surroundings get warmer.
  • Endothermic: Absorbs heat (thermal decomposition, photosynthesis). Surroundings get cooler.

    • Key Points to Remember

    • 1Atoms are conserved — same number on both sides of equation
    • 2OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
    • 3Exothermic = releases heat, Endothermic = absorbs heat
    • 4Reactivity series determines displacement reactions

    Pakistan Example

    Roti Making = Combustion!

    When you cook roti on a gas stove, you're watching a combustion reaction: CH₄ + 2O₂ → CO₂ + 2H₂O + heat. The methane (natural gas/sui gas) reacts with oxygen from air, producing carbon dioxide, water vapour, and loads of heat (exothermic). The blue flame means complete combustion. A yellow/orange flame means incomplete combustion — producing carbon monoxide (CO), which is poisonous. That's why proper kitchen ventilation is important!

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    Acids, Bases & SaltsMetals & Reactivity