Class X - CBSE - Science - Chemistry - Lesson 1: Chemical Reactions and Equations

Vidhya T.
02/12/2020 0 0

Chemical Reactions & Equations

Chemical Reactions:

  • change in colour, state, the evolution of gas, change in temperature

Chemical Equation:

  • Shorter way, concise & useful instead of using words
  • Representation: Reactions, Products, Arrow Head [include pressure, temperature, catalyst, etc.]
  • Skeletal chemical equations: Mg + O2 -> MgO
  • Balanced chemical equations: 2Mg + O2 -> 2MgO

Types of Chemical Reactions:

  1. Combination
  2. Decomposition
  3. Displacement
  4. Double Displacement
  5. Redox
  6. Exothermic
  7. Endothermic


CaO + H2O -> Ca(OH)2

Ca(OH)2 + CO2 -> CaCO3 + H2O


  1. Thermal: CaCO3 --heat--> CaO + CO2
  2. Photochemical: AgCl --light--> Ag + Cl2
  3. Electrolysis: H2O --electricity--> H2 + O2


Fe + CuSO4 --> Fe(SO)4 + Cu

Elements of higher reactivity displace elements of lesser reactivity. Fe > Cu

Double Displacement:

Na2(SO)4 + BaCl2 --> BaSO4 + NaCl (this is a precipitation reaction as well)

Redox: Reduction Oxidation

Reduction means the removal of Oxygen (or) addition of hydrogen

Oxidation means the addition of Oxygen (or) removal of hydrogen

2Cu + O2 --heat--> 2CuO [oxidation]

CuO + H2--> Cu + H2O [reduction CuO to Cu, oxidation H2 to H2O]


Burning of natural gas

CH4 + O2 --> CO2 + H2O


C6H12O6 + O2 --> CO2 + H2O + energy


ZnCO3 + Heat absorbed --> ZnO + CO2

Effect of Chemical Reactions:


  • undesirable change that occurs in metals in the presence of moisture, acids, bases & air
  • Fe + H2O --> Fe2O3.nH2O


  • undesirable change that takes place in oil-containing food
  • Chips: flushing out with air prevents oxidation of fatty acids.


  • Silver reacts with sulphides in the atmosphere (e.g. mercuric sulphide) to form a black layer of silver sulphide.
  • Toothpaste to it to remove the tarnish regains its colour.
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