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Combination Reaction

Last Updated : 04 Jan, 2024
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Combination Reaction is a type of chemical reaction that occurs when two or more substances combine to form a new substance without any separate byproducts, due to that, it is also called an Addition Reaction or Synthesis Reaction. Combination Reaction involves the synthesis of a single compound from its constituent elements or other compounds.

This article covers the concepts related to combination reaction, like the formula of combination reaction, types of combination reaction, synthesis, cause, examples and real-life examples of combination reaction.

What is a Combination Reaction?

A process in which two or more chemicals combine to create a single new substance is known as a combination reaction. Synthesis reactions are another name for combination reactions.

Generally, in any combination reaction reactants A and B, combine to form the product, which is AB. There is no restriction on the quantity of reactants that can participate in combination reactions. Moreover, a combination reaction forms a new bond between the reactant molecules, which releases heat and makes the reaction generally exothermic.

Definition of Combination Reaction

A process in which two or more chemicals combine to create a single new substance is known as a combination reaction.

Chemical Equation for Combination Reaction

General chemical equation for combination or addition reaction is give by:

A + B → AB

Read More about Types of Chemical Reaction.

Cause of Combination Reaction

In a chemical reaction, components or compounds come together to generate a result. In order to achieve stability, every element or compound participates in a chemical combination. According to Lewis theory, an atom becomes stable when it assumes the configuration of a noble gas.

As a result, atoms share electrons during a chemical reaction and establish a stable electronic configuration. Chemical combination is therefore caused by an atom’s tendency to acquire stability through the formation of a noble gas configuration.

Example of Combination Reactions

Some of common examples of Combination Reactions are:

  • Formation of Sodium Cholride
  • Formation of Magnesium Oxide
  • Formation of Barium Fluoride
  • Formation of Ammonia
  • Formation of Other Substances

Formation of Sodium Chloride

Solid sodium metal reacts with chlorine gas to form sodium chloride.


Formation of Magnesium Oxide

When magnesium is lit, it reacts rapidly, combining with oxygen in the air to form a fine powder known as magnesium oxide.


Formation of Barium Fluoride

Formed by the combination of barium metal and fluorine gas is a highly exothermic reaction.

Ba(s) + F2(g) → BaF2(s)

Formation of Ammonia

Hydrogen and nitrogen gas combines to form ammonia.

3H2+ N2 → 2NH3

Formation of Some Other Substances

Some other subtance that are formed with the help of combination reaction are listed as follows:

  • When sulfur dioxide gas reacts with oxygen gas, sulfur trioxide is produced.

2SO₂(g) + O₂(g) → 2SO₃(g)

  • Iron reacts with oxygen to form iron(III) oxide, commonly known as rust.

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

  • The reaction between calcium oxide and water produces calcium hydroxide.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Properties of Combination Reaction

Some properties of combination or addition reaction are:

  • Reactants can participate in a chemical reaction in combination reactions in an infinite number.
  • An ionic solid is always the final result of a combination reaction involving a metal and a non-metal.
  • Elements or compounds combine together to attain stability.

Types of Combination Reactions

There are 3 ways in which a combination reaction can take place.

  • Combination of Two or More Elements
  • Combination of Two or More Compounds
  • Combination of a Compound with an Element

Combination of Two or More Elements

This type of combination reaction takes place when two or more elements combine together and form a single compound.


  • Hydrogen and oxygen combine to form water.

H2(g) + O2(g) → 2H2O(l)

  • Carbon and oxygen combine to form Carbon Dioxide gas.

C(g) + O2(g) → CO2(g)

Combination of Two or More Compounds

This type of combination reaction takes place when two or more compounds combine together and form a single compound.

Example: The formation of calcium hydroxide (slaked lime) by the reaction of calcium oxide (quick lime) and water.

CaO(s) + H2O(l) → Ca(OH)2(s)

Combination of a Compound with an Element

This type of combination reaction takes place when compound react with an element to form a single compound.

Example: Formation of Carbon Dioxide gas from the compound carbon monoxide and the element oxygen

2CO(g) + O2(g) → 2CO2(g)

Read More,

Real-Life Examples of Combination Reaction

Some real life examples of combination reaction are listed as follows:

  • When it rains, pollutants in our environment, such as sulfur oxide, interact with the rainwater to generate sulfuric acid. As a result, some acidic materials are present in the rainwater.
  • During dental treatment, a white powder known as calcium hydroxide can be used to fill a root canal. A combined reaction between calcium oxide and water produces calcium hydroxide. Applications for calcium hydroxide include food processing, paper manufacturing, and sewage treatment.
  • Burning coal is a combustion reaction, and carbon dioxide is the byproduct of all combustion reactions.

Steps to Determine a Combination Reaction

To determine any reaction is a combination reaction or not, we can use the following steps:

Step 1: Identify if there is a single product in the reaction.

Step 2: Look for multiple reactants involved in the reaction.

Step 3: Double-check to ensure nothing is overlooked by carefully reviewing each potential answer twice.

Also Read,

Sample Question on Combination Reaction

Question 1: Because potassium is an extremely reactive alkali metal, it needs to be kept submerged in oil to keep from interacting with the air. Write the balanced chemical equation for the potassium-oxygen combination process.


The chemical equation according to the question:


Balanced chemical equation will be:


Question 2: Write combination reaction for the formation of aluminum oxide (Al2O3).


Chemical balance equation for formation of aluminum oxide (Al2O3) is given as follows:

4Al + 3O2 → 2Al2O3

Question 3: When Ag is exposed to air it gets a black coating of?


A black layer of silver sulfide is created when silver is exposed to air and reacts with the hydrogen sulfide gas in the atmosphere. In every solvent, silver sulfide remains insoluble.

Combination Reaction: Practice Questions

Q1: What white powder forms on burning magnesium in air?

Q2: What does the combination of nitrogen and hydrogen gas formed?

Q3: Write balanced combination reaction for photosynthesis.

Q4: Explain the formation of ferrous sulphide with balanced chemical reaction.

Q5: Write chemical reaction for formation of slaked lime.

Combination Reaction: FAQs

1. What is Combination or Addition Reaction?

Combination or addition reaction involves two or more substances combining to form a new compound.

2. What are the 4 Types of Chemical Reactions?

The four types of chemical reactions are synthesis, decomposition, single replacement, and double replacement.

3. What are some Other name for Combination Reaction?

Combination reaction is also called synthesis or composition reaction.

4. Is Combination Reaction Exothermic or Endothermic?

The exothermic or endothermic nature of a combination reaction depends on the specific reaction.

5. What is the General Formula for Combination Reaction?

There is no single general formula for combination reactions as they vary based on the specific substances involved, but for two subtance combining together we can write the general formula as:

A + B → AB

6. What are Some Examples of Combination Reaction?

Examples of combination reactions include the formation of water (2H₂ + O₂ → 2H₂O) and the synthesis of ammonia (N₂ + 3H₂ → 2NH₃).

7. Is Photosynthesis is Combination Reaction?

No, Photosynthesis is neither combination nor decomposition reaction as there are two product and two reactant as well.

6CO2 + 6H2O → C6H12O6 + 6O2

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