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Definition and types of chemical reactions


Definition of a chemical reaction

A chemical reaction is a process in which one or more substances, called reactants, are converted to one or more different substances, known as products. During the chemical reaction, atoms are rearranged. This requires that bonds are broken in the reactants and new bonds are formed in the products.

Representation of a chemical reaction : Reactant1 + Reactant2  —>  Product1 + Product2.

In our every day life we can find lot of chemical reactions around us such as combustion, digestion, photosynthesis of the plants and fermentation, in addition backing, washing hands and rusting metals are also considered as chemical reactions. There is so many other examples that you can find.


Difference between chemical reactions and physical changes

Let’s consider some situations and identify if the change that has occurred is physical or chemical:

  • A paper is torn
  • A paper is burnt
  • Water is converted to ice
  • Human respiration

To differentiate between chemical reaction and physical change we need to know their definition.

A physical change is a process in which the substance experiences change in its physical properties, like shape, size, appearance, and state. There is no change in the internal structure or molecular composition. On the other hand a chemical change results in the formation of entirely new substances. One or more molecules are rearranged or combined to form a new molecules.

Now let’s get back to the examples. First a paper is torn: Is it a physical change or a chemical change ? In fact there is no change of substance neither of the chemical properties, the paper is just torn into two pieces, paper is still paper and nothing else, hence it’s a physical change.

What about paper being burnt? Here paper is converted to ash and smoke new substances appear, this is a chemical change.

What about water being converted to ice? Here only the state of the matter is changing which is a physical property, ice can be converted back to water too with a little bit of heating hence it’s a physical change.

The last one is interesting. What happens when we respire? We use the oxygen we inhale and after the respiration process carbon dioxide is exhaled, clearly a new substance is formed, so it’s a chemical change. Whenever a chemical change occurs we say there is a chemical reaction.

Types of chemical reactions

  • Synthesis reactions
  • Decomposition reactions
  • Single displacement reactions
  • Double displacement reactions
  • Combustion reactions

Synthesis reaction

In this type of reaction there is two or more reactants and one product. The general representation of this reaction is : A + B –> AB


Decomposition reaction

In the decomposition reaction, there is one reactant and two or more products. The general representation of this reaction is : AB —> A + B


Single displacement

A single displacement reaction, also called a single replacement reaction, is a reaction in which one element from the reactants is replaced by another element from the second reactant. The general equation of this reaction is: AB + C –> AC + B.


Double displacement

A double displacement reaction is the type of reaction in which two reactants exchange ions to form two new compounds. Double displacement reactions has the form: AB + CD → AD + CB


Combustion reaction

In a combustion reaction a substance reacts with oxygen gas, and releases energy in the form of light and heat. Combustion reactions always involve Oxygen as one reactant. The products of this reaction are carbon dioxide (CO2) and water (H2O).