Types of chemical reactions

Combination reactions

  • It is a reaction in which single product is formed from two or more reactants.
  • Mostly combination reactions are exothermic in nature.
  • Combination reactions are also known as synthesis reactions because in this reaction new substance is synthesized. 

Example of combination reaction

  • Formation of water- H2  +  O2  →  H2O
  • Burning of coal-  CO  +  O2  →  CO2
  • When calcium oxide (quick lime) reacts with water, then it combines with water and forms calcium hydroxide (slaked lime) and large amount of heat is released. This reaction is also an exothermic reaction.


  • A solution of calcium oxide is used for white washing walls.
  • Calcium oxide with water form calcium oxide which reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. 
  • Calcium carbonate is formed after two to three days of whitewashing and gives a shiny finish to the walls.
  • Limestone, marble and chalk are different forms of calcium carbonate.

Decomposition reactions

  • The reaction in which a single reactant breaks down to give simpler products is called a decomposition reaction.
  • Decomposition reactions require energy either in the form of heat, light, or electricity to break down the reactant. Therefore, all decomposition reactions are endothermic reactions.
  • Decomposition reaction is the reverse of combination reaction.

  • Lead(II) nitrate undergo decomposition reaction to give yellow colour lead (II) oxide, colourless gas of oxygen and brown colour nitrogen dioxide.

Types of decomposition reaction

  1. Thermal decomposition
  2. Electrical decomposition
  3. Photochemical decomposition

Electrolytic decomposition reaction:

  • Electrolytic decomposition occurs when an electric current is passed through an aqueous solution of a compound. 
  • Electrolysis of water - Electrolysis of water is the decomposition of water into hydrogen and oxygen due to the passage of electric current through it.
  • Decomposition of sodium chloride - On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine. 
  • Mostly decomposition reactions are endothermic in nature.

Thermal decomposition reaction (Thermolysis):

  • Thermal decomposition is a chemical reaction where a single substance breaks into two or more simple substances when heated. 
  • Decomposition of calcium carbonate on heating -
  • Ferrous sulphate crystals loses water when heated - Green colour of FeSO4 disappears and reddish brown solid(Fe2O3) is formed and Smell of burning Sulphur is observed

Photochemical decomposition:

  • Decomposition reactions which require energy in the form of light for breaking down the reactants are called photolytic decomposition.
  • The reverse of the photochemical reaction is called chemiluminescence.



Decomposition of silver chloride in presence of sunlight –

  • Decomposition of silver chloride - Place a small quantity of silver chloride (AgCl) taken in a watch glass under sunlight for some time. The crystals slowly acquire a grey colour. On analysis, it is found that the sunlight has caused decomposition of silver chloride into silver and chlorine.


  • Decomposition of silver bromide -
  • The above reactions are used in black and white photography.

Displacement reactions

  • Those reactions in which more reactive metal displaces less reactive metal from its salt solution are called displacement reactions.


 Fe(s) + CuS04(aq) ——–> FeS04(aq) + Cu(s)

Note-  A good way to remember the order of a reactivity series of metals is to use the first letter of each one to make up a silly sentence. For example: People Say Little Children Make A Zebra Ill Constantly Sniffing Giraffes.

Reactivity series

Double displacement reactions

  • Reaction in which there is exchange of Ions between the reactants is called double displacement reaction.
  • This reaction is also known as precipitation reaction.

Precipitation Reaction: 

  • The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called Precipitation Reaction.
  • Precipitation reaction takes place when the concentration of dissolved ions in the solution exceeds solubility product.
  • Examples-
     Sodium hydroxide reacts with copper sulphate to form a pale blue precipitate of copper hydroxide.

  • Lead acetate reacts with potassium iodide to form a yellow precipitate of lead iodide and potassium acetate.