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Difference: Combination Reaction and Decomposition Reaction

Aug 23, 2022


Chemical Reaction: 

When a chemical change occurs in a substance, we can say that a chemical reaction has taken place. Representing a chemical reaction to show the change that occurred in a compound separated by an arrow is called a chemical equation. 

An example of a chemical reaction is the combining of nitrogen and hydrogen to form the ammonia compound. 

The equation can be written as N2  +  3H2  →   2NH3 


Based on the properties of the reactants and the products, state of the reactants, and products the chemical reactions are classified into different types. They are: 

What are the different types of chemical reactions? 

  • Combination Reactions 
  • Decomposition Reactions 
  • Displacement Reactions 
  • Double Displacement Reactions 
  • Oxidation – Reduction Reaction 

Combination Reaction: 

When two or more substances combine to form products, such a reaction is called a combination reaction. If the reaction has one and only one product formed as a result of reaction between two or more reactants.  


  • Take a small amount of calcium oxide in water and observe the changes. 
  • Calcium oxide is a white solid that dissolves in water and forms a clear liquid. 
  • Beaker in which the solution is mixed becomes hot and releases energy. 


  • Formation of water:  
  • Formation of calcium carbonate: 
  • Formation of aluminum oxide:  

Uses of compounds: 

  • Calcium carbonate is used in the making of marble stones, chalk pieces. 
  • Water is used for drinking. 

Combination reactions help in the formation of a large number of compounds. 


A Combustion reaction is that in which a substance burns in the presence of air or oxygen and produces heat and light energy. Combustion is an exothermic reaction as heat energy is released during the process. 

The general equation for the combustion of a hydrocarbon can be written as : 

For example, the equation for the combustion reaction of propane, C3H8 is : 



  • Burning of petrol 
  • Burning of paper 
  • Burning of wood 

Exothermic Combination: 

The reaction in which two or more reactants combine and produce heat energy after the reaction is completed is called an exothermic reaction. 

Exothermic reactions in the human body: 

Sweating of the body releases heat energy of the body to outside and cools the body. 


A reaction in which one substance gets separated or divided into the smallest substances possible is called decomposition. The word decomposition means division.  

Types of decomposition: 

  • Thermal decomposition-using heat or thermal energy 

E.g.:  Decomposition of calcium carbonate 

  • Electrolytic decomposition – using electrical energy  

E.g.: Decomposition of water 

  • Photolytic decomposition-using light energy  

E.g.: Decomposition of silver chloride or silver bromide.  


  1. Manufacture of cement or calcium oxide. 
  1. Helps in relief from indigestion. 
  1. Extraction of metals from oxides. 


  • Chemical reactions can be of different types.
  • When two elements combine and form one product such reaction is a combination reaction.
  • When one reactants split into two or more products such reaction is a decomposition reaction.
  • During the formation of a compound if a heat energy is released then it is called a combustion reaction.
  • Decomposition reactions are of 3 types:
  1. Thermal decomposition
  2. Photolytic decomposition
  3. Electrolytic decomposition


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