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Chemical Reactions : Defination, Characteristics and Examples

Grade 8
Aug 20, 2022

Key Concepts:

  • Reactants
  • Products
  • Catalyst
  • Decomposition
  • Photochemical reactions


Think about the following changes of day-to-day life and think what happens when: 

  • Apple gets fermented. 
  • Food is cooked. 
  • Food is digested in our bodies. 
  • Plants prepare food by the process of photosynthesis. 
  • Dirt in the clothes getting removed on washing with soap and water. 

In all the changes, the initial substance’s nature and character have somewhat changed. 

The change in which a substance changes its physical properties is called a physical change. In this change, no new substance is formed. While the changes in which one or more new substance formed is known as a chemical change. In the chemical changes, the chemical properties of the substance change.   

If we recall the above changes, all are examples of chemical changes. We can say that chemical reactions occur whenever a chemical change occurs as all the examples are of chemical reactions. Chemical reactions are a vital part of daily life. 

Not every substance we get or use is readily available on earth. Some of them are to be synthesized, and some are processed. Some have to be stored in proper places. In order to solve the entire problem, chemical reactions allow us to analyze, predict and measure the product or compound we want. So let us learn more about chemical reactions in this session. 


Chemical Reactions 

A chemical reaction is a process in which a new product is formed. 

One or more substances combine to form a new substance in a chemical reaction. 

In a chemical reaction, breaking of chemical bonds of the reactant substances take place, to make new bonds which leads to the formation of new substances(products). 

It can be represented as below: 

(Reactants)       →     (Product) 


What are the reactants and products? 

Reactants:  The substances or compounds that undergo chemical change in the chemical reaction is known as reactants. 

Products: The new substance formed during the reaction is called the products

In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place. 
Only a rearrangement of atoms takes place in a chemical reaction. 

For example, in the reaction below when methane reacts with the oxygen, only rearrangement of atoms takes place, but no new atom or element is obtained during the reaction. 

A chemical bond: A chemical bond is an attractive force that holds the atoms of a molecule together in a compound. 

Example: Burning a magnesium ribbon in air and collecting magnesium oxide in a watch-glass. 

When a magnesium ribbon is burnt with oxygen, it gets converted to magnesium oxide. 

The above explanation of a chemical reaction in a sentence is fairly long. So, this can be written in a shorter form.  

The easiest way to write this is in the manner of a word equation. 

The word-equation for the above reaction can be-  

Magnesium + Oxygen → Magnesium oxide 

(Reactants)             (Product) 

2Mg(s) + O2(g) △→ 2MgO(s) 

  1. The substances undergoing the change in the above reaction are magnesium and oxygen; therefore, they are called reactants. 
  1. The new substance formed in this reaction is magnesium oxide, and it is called a product. 
  1. A word-equation displays the change of reactants to form the products through an arrow placed between them. 
  1. The reactants are always written on the left-hand side (LHS) with a plus sign (+) between them in the word equation.  
  1. While the products are written on the other side, i.e., the right-hand side (RHS) with a plus sign (+) between them. 
  1. The arrowhead points towards the products and shows the direction of the reaction. 

Some other examples are: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc. 

A Chemical Change or Reaction Occurs When Particles Collide. 

The collision occurs when reactants are in close contact or by supplying energy. Thus, one or more conditions are necessary for chemical change. 

A necessary condition for a Chemical Change/reaction: 

  1. Mixing 
  1. Solution 
  1. Heat 
  1. Light 
  1. Electricity 
  1. Pressure 
  1. Catalyst 

1.Mixing (close contact) 

Chemical reactions occur when two substances are mixed in their solid state. 

Example: Iodine and phosphorous reacts explosively when brought into close contact. 

P4(s) + 4I2(g) → 2P2I4(g) 

2.Lead nitrate(white) and potassium iodide(white) 

 reacts to make lead iodide(yellow) 

chemical reactions


In some cases, a chemical reaction occurs when substances are mixed in a molten or aqueous state. 

Example 1: Oxalic acid crystals and sodium carbonate react in water solution only. 

Example 2: Sodium chloride and silver nitrate react in a solution state to form the precipitates of silver chloride and sodium nitrate. 

Sodium nitrate


Some chemical reactions occur only on heating. 

Example 1: Copper carbonate decomposes on heating  

(∆ symbol for heating) into copper oxide and carbon dioxide. 

Example 2: Lead nitrate decomposes on heating, leaving yellow residue lead monoxide, brown gas nitrogen dioxide and colourless gas oxygen. 


Some of the chemical reactions take place by the action of light. These are called photochemical reactions or photolysis. 

Molecules of reactants absorb light energy to get activated and then reacts rapidly.  

Example 1: Plants form glucose from carbon 

 dioxide and water in the presence of light 

 dioxide and water in the presence of light 

Example 2:  Hydrogen and chlorine react in the presence of sunlight. 

              H2+Cl2⇇ Δ 2HCl

Example 3:  If chlorine water is exposed to sunlight, oxygen is evolved. 

Example 4:   Silver nitrate decomposes in the presence of sunlight. 

Note: Solutions of silver nitrate, hydrogen ‘per oxide are kept in brown bottles in the laboratory because they decompose in the presence of light. 


Like the decomposition of certain compounds, certain chemical reactions occur only when electricity passes through the substance. 

Example 1: Electrolysis of acidulated water occurs only in the presence of electricity to give oxygen and hydrogen. 

Electrolysis of Water 

Electrolysis of Water 

Example 2: On passing the current through molten sodium chloride, sodium and chloride are separately obtained.  


Some chemical reactions occur only when the involved substances are subjected to high pressure. 

Example: (a) Mercuric chloride and potassium iodide give scarlet colour substance called mercuric iodide when rubbed in mortar. 

(b) Nitrogen and Hydrogen, when subjected to high pressure, produce ammonia (in the presence of an iron catalyst). 


Some chemical reactions need a catalyst to accelerate or decelerate the rate they occur. The catalyst do not themselves take part in the reaction. 

 Example 1: Potassium chlorate decomposes only at 700°C, and even the release rate of oxygen is very slow. However, when potassium chlorate is heated in the presence of manganese dioxide, decomposition begins much lower temperature, 300°C, and manganese dioxide remains unaffected. 

Thus, manganese dioxide (MnO2) acts as a catalyst in the reaction. 

Example 2: Ammonia reacts with oxygen to produce nitric oxide and water vapour in the presence of a platinum catalyst. 

Some of the examples of catalytic chemical reactions are: 

Catalytic Process and their Catalyst 

Certain chemical reactions are characterized by changes that are quite easily observed. Some of these typical changes help us to determine whether a chemical reaction has taken place – 

Characteristics of Chemical Reactions 

  1. Evolution of gas 
  1. Change in state 
  1. Formation of precipitate 
  1. Change in temperature 
  1. Change in color 

1.Evolution of Gas  

Example 1: Heating of ferrous sulphate 

Ferrous sulphate breaks down into Ferric oxide(solid), sulphur dioxide (gas), sulphur trioxide (gas). 

Example 2: When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with effervesces. 

Zn [zinc] +H2SO4 [dil. sulphuric →ZnSO4 [zinc sulphate] +H2 [hydrogen] Zn [zinc] +H2SO4 [dil. sulphuric →ZnSO4 [zinc sulphate] +H2 [hydrogen] 


Effervescence: The formation of gas bubbles in a liquid during a reaction is called effervescence. In the reaction given above, one of the reactants is a liquid. In such cases, i.e. when one of the reactants is a liquid, the gas produced forms bubbles in the liquid, i.e. effervescence takes place. 

2.Change of Color: 

Example: When a few iron pieces are dropped in the blue colored copper sulphate solution, the blue color of solution fades and eventually turns to light green due to the formation of ferrous sulphate. 

Fe         +        CuSO4(aq)         →              FeSO4+                    CuFe         +        CuSO4aq         →              FeSO4+                    Cu

[iron]                 [blue solution]               [green solution]         [copper] 
                                                                                                            (red deposit) 

3.Formation of precipitates: 

Example 1: When sodium sulphate solution reacts with barium chloride, it forms barium sulphate precipitate and sodium chloride. 

Example 2:  When ferrous sulphate is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed. 

4.Change In saTemperature 

Quick lime reacts with water to produce calcium hydroxide and heat. 



5.Change of State: 

In many chemical reactions, a change of state is observed. The reaction might start with gaseous or liquid reactants and end with solid products and vice versa. 

Example 1: When ammonia and hydrochloric acid react, it forms the ammonium chloride in the solid-state. 

NH3(g)+HCl(g)⇇  NH4Cl(s)NH3(g)+HCl(g)⇇  NH4Cl(s)

Both hydrochloric acid and ammonia are gases which are fairly soluble in water and exist in the vapor phase above their solutions. The reaction is a neutralization reaction between a strong acid (hydrochloric acid) and a weak base (ammonia) to produce water and salt (ammonium chloride). 


A Chemical Reaction is a process in which a new substance is formed.

During the Chemical reaction, the bonds between the reactant molecules breaks and new bonds form to give new substance as products. In the chemical reaction, the substances that undergo chemical change in the reaction is called as Reactants and the substances formed during the reaction are called product. The necessary conditions to occur the Chemical reactions are:

a. Mixing the substance in solution state

b. By heat

c Light

d. Electricity

e. Pressure

f. in the presence of a catalyst

A chemical reaction can be confirmed by:

a. Evolution of gas

b. Change of colour

c. Formation of precipitates

d. Change of state

e. Change in the temperature



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