Difference Between Chemical and Physical Reaction

Key Difference – Chemical vs Physical Reaction
 

Chemical and physical reaction are two types of changes in matter and the key difference between chemical reaction and physical reaction is that when a substance undergoes a chemical reaction it is no longer the original compound that was there before the reaction whereas, a substance that undergoes a physical reaction remains the original substance while it’s state or the shape changes. However, in both chemical and physical reactions, the total energy remains constant.

What is a Chemical Reaction?

A chemical reaction takes place when two or more substances are combined to form an entirely new substance(s) or to change the original properties of the initial compound(s). During a chemical reaction, chemical properties of the initial compounds are changed. This involves breaking or making of chemical bonds.

The substances present at the beginning of the reaction are called “reactants” and the newly formed substances are called “products”. The number of elements present in the reactants is equal to the number of elements present in the products.

Example1: Combustion of fossil fuels

2C₂H₆  +  7O₂              →          4 CO₂  +  6 H₂O

(Reactants)                                (Products)

What is a Physical Reaction?

Physical reactions in substances are also known as “physical changes“. To understand the physical reaction, it is essential to have a clear idea about the physical properties of the matter. Physical properties are the properties which do not change the chemical nature of the matter. Those properties can be measured without changing the composition of the matter. Physical properties include appearance, texture, colour, odor, melting point, boiling point, density, solubility, etc.

Physical reactions involve a change in the form of matter or shape, but no changes in its composition.

Example1: Mixing of sugar in water

This is a physical reaction. Because nothing new is created by mixing sugar with water. The result is just sugar in water. If you evaporate the mixture, you can obtain the starting compounds.    

Example2: Freezing of water, melting of ice and evaporation of water.

All these three processes are physical changes of water. In any of these, changes do not involve the changes in the composition. It is water in different forms.

What is the difference between Chemical and Physical Reaction?

Definition of Chemical and Physical Reaction

Chemical reaction: Chemical reaction is any change that results in the formation of new chemical substances.

Physical reaction:Physical reaction is a change affecting the form of a chemical substance, but not its chemical composition.

Characteristics of Chemical and Physical Reaction

Changes in Original Compounds and Composition

Chemical reaction: Chemical reactions result in a change of  the original properties of the initial compounds or form an entirely new compound(s).

Physical reaction: Physical reactions do not change the composition of the elements or the compounds, but it results in a change in the state.

Reversibility

Chemical reaction: Most of the chemical reactions are irreversible.

Physical reaction:  Physical reactions are reversible.

Changes of Properties

Chemical reaction: At least one of the following changes takes place in a chemical reaction.

Changes in a chemical reaction:

• Colour change
• Formation of a solid (Precipitation reactions)
• Formation of gas or smell (effervescence reactions)
• Change in energy (endothermic or exothermic reaction)

 Physical reaction: A substance which undergoes a physical reaction; changes its shape or the phase, remaining the substance as it is.

Energy Requirement

Chemical reaction: There is a certain energy barrier that  needs to be  overcome in order to undergo a chemical reaction. It is called “activation energy”.

Physical reaction:There’s no such an energy requirement in physical reactions.