Combination Reaction

Our Objective

Our objective is to study combination reactions.

The Theory

Before performing the experiment, let’s find out what combination reactions are!

A combination reaction is a general category of chemical reactions. It may be defined as a chemical reaction in which two or more substances combine to form a single substance under suitable conditions.

Combination reactions are also known as synthesis, because in these reactions new substances are synthesized.

The general equation that describes a combination reaction is:

What are the types of combination reaction?

Combination reactions are of three types.


Similarly, non-metals may react with highly active metals to form covalent compounds. Example, sulphur reacts with oxygen gas to form gaseous sulphur dioxide



  • Reaction between two or more elements:An example of this type of combination reaction is the reaction between a metal and a non- metal. Most metals react with non-metals to form ionic compounds.  A good example of this would be:




  • Reaction between elements and compounds: An element and a compound react to form another compound. Example, carbon monoxide reacts with oxygen gas to form carbon dioxide.
  • Reaction between two compounds:  Two compounds react with each other to form a new compound. Example, calcium oxide (quick lime) reacts with carbon dioxide gas to form calcium carbonate (lime stone).


Most combination reactions are exothermic in nature. Why?

Combination reactions involve the formation of new bonds and this process releases a large amount of energy in the form of heat.
Let us discuss some more examples of combination reactions.    

  • Formation of Calcium Hydroxide:Reaction between quick lime (Calcium oxide, CaO) and water is a combination reaction.

In this reaction, quick lime reacts with water to form slaked lime (calcium hydroxide, Ca(OH)2).
The reaction between quick lime and water is highly vigorous as well as exothermic.

  • Combustion Reactions: Combustion of coal and combustion of hydrogen are examples of combination reactions.

Coal burns in air to form carbon dioxide gas.

Hydrogen burns in the presence of oxygen to form water in the form of steam. Upon cooling, it becomes liquid.


  • Formation of Ammonium chloride:Ammonium chloride is formed by combining vapours of ammonia with hydrogen chloride gas. It is a white-coloured solid.



  • Formation of Sulphuric acid: Formation of sulphuric acid from sulphur trioxide is also a combination reaction. Sulphur trioxide on hydration forms sulphuric acid. This reaction is highly exothermic in nature.

  • Formation of ferrous sulphide:It is formed by heating fine pieces of iron with sulphur powder.

  • Manufacture of Ammonia:The manufacture of ammonia gas from nitrogen and oxygen is also a combination reaction.


  • Combination of sodium oxide and water:  Sodium oxide combines with water to form sodium hydroxide.


 Learning Outcomes

  1. Students acquire knowledge about combination reactions.
  2. Students acquire skills to perform a combination reaction using quick lime and water.
  3. Students will be able to distinguish a combination reaction from a given set of chemical reactions.

So, let’s do a combination reaction between water and quick lime…