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CBSE NOTES ⇒ Class 10th ⇒ Science ⇒ 1. Chemical Reactions and Equations

1. Chemical Reactions and Equations

Types of Chemical Reaction

Types of Chemical Reaction


Chemical reactions involve the breaking and making of bonds between atoms to produce new substances. Ex. The breaking of water molecule produces oxygen and hydrogen, while making of bond between carbon and oxygen produce carbon dioxide.


The types of reaction are as follow;

(1) Combination reaction

(2) Decomposition reaction

(3) Displacement reaction

(4) Double displacement reaction

(5) Oxidation and reduction reaction


1. Combination Reaction


A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

General form : A + B → AB

CaO(s)  +  H2O(l) → Ca(OH)2 (aq)

(Quick lime)               (Slaked lime) 

According to definition to comparison with reaction equation we see calcium oxide and water are two reactants which form a single product calcium hydroxide.


** Calcium hydroxide is used for white washing on walls. This is a precipitate reaction, when calcium hydroxide is painted on walls it reacts with CO2 present in air to form a thin layer of calcium carbonate and also produce water which get evapourated.    

Some Other Examples of Combination reactions.

Burning of coal:

(i) C(s) + O2(g) → CO2(g)

Formation of water:

(ii) 2H2(g) + O2(g) → 2H2O(l)

Formation of sulphur dioxide:

(iii) S(s) + O2(g) → SO2(g)

Rusting (Formation of Ferrous oxide):

(iv) 2Fe + O2(g) → 2FeO 

2. Decomposition Reaction

The reaction in which a single reactant breaks down into two or more than two simpler products is known as decomposition reaction. 

General form: A → B + C 

Example of Decomposition Reaction

(Ferrous sulphate)     (Ferric oxide)   (sulphur dioxide)  (sulphur trioxide)

Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3),
sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid,
while SO2 and SO2 are gases.

Another Example of Decomposition Reaction

(Limestone)                     (Quick lime)

On heating calcium carbonate decomposes to calcium oxide and carbon dioxide. 

Calcium Oxide: It's common name is quick lime or burnt lime. It is white in colour, caustic, alkaline and crystalline. It obtained from thermal decomposition of calcium carbonate (limestone).

Uses of Calcium oxide:

(I) Manufacturing of cement

(II) Manufacturing of various types of glass. 

(III) In agriculture It is used for treating acidic soils.

(IV) White washing: it is used with water

Thermal Decomposition: When a decomposition reaction is carried out by heating, it is called thermal decomposition.


Decomposition of Silver chloride: 

white silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

*  This Reaction is used in black and white photography. 

*  All decomposition reactions require energy either in the form of heat, light or electricity for breaking down reactants. 


On the basis of heat there are two kinds of reaction. 

A. Exothermic Reactions: Reactions in which heat is released along with the formation of products are called exothermic.

  • During the reaction large amount of heat (energy) is evolved
  • This heat makes the reaction mixture warm. 

Examples of exothermic reactions: 

(i) Burning of natural gas

CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)

(ii) Respiration is also an exothermic reaction. 

When we take food to get energy to stay alive. During digestion, food is broken down into simpler substances likes carbohydrates and other nutrients. These carbohydrates further broken down to form glucose. This glucose combines with oxygen in cells of our body in the process of cellular respiration and provide energy. Therefore respiration is also an exothermic reaction. 

(iii) The decomposition of vegetable matter into compost is also an
example of an exothermic reaction.

B. Endothermic Reactions: Reactions in which energy is absorbed are known as endothermic reactions.

3. Displacement Reaction: 

The reaction in which the highly reactive element displaces the less reactive element from the reaction, such reaction is known as displacement reaction

General formulla for displacement reaction:

Ab + C → Cb + A 

Here C is highly reactive element which displace A from the reaction and joins b. 


When a iron nail is placed in the equous solution of copper sulphate Iron displaces copper from the reaction or its compound and blue colour of copper sulphate turns grey brown because Iron is highly reactive metal than copper. 

The reaction is as follows; 

(i)  CuSO4(aq) + Fe(s) → FeSO4(aq) + Cu(s)

When solid zinc is placed in the equous solution of copper sulphate zinc displaces copper from the reaction or its compound because zinc is highly reactive metal than copper. 

Equation of this reaction;

(ii) CuSO4(aq) + Zn(s)  → ZnSO4(aq) + Cu(s)

Other example: 

(iii) CuCl2(aq) +  Pb(s)  → PbCl2(aq) + Cu(s)


4. Double Displacement Reaction: 

Such a reaction in which there is an exchange of ions between the reactants are called double displacement reactions. 

General formulla for double displacement reaction:

Ab + Cd → Ad + Cb


(i) Na2SO4 + BaCl2 → BaSO4  + 2NaCl 

(ii) NaOH + H2SO4 → Na2SO4 + H2O

(iii) NaCl + AgNO3 → AgCl + NaNO3

(iv) BaCl2 + H2SO4 → BaSO4 + HCl

(v) BaCl2 + KSO4 → BaSO4 + KCl2


5. Oxidation And Reduction: 

Oxidation Reaction: Oxidation is the gain of oxygen or loss of hydrogen.

In other words:

The addition of oxygen to a substance or the removal of hydrogen from a substance or both is called oxidation

Examples of Oxidation: 

Gain of Oxygen; 


[Carbon gains oxygen and gets oxidised in carbon dioxide]



[Phosporous gains oxygen and gets oxidised in Phosporous pentaoxide]


[Copper gains oxygen and gets oxidised in copper oxide]


Removal of Hydrogen: 

Examples of Oxidation:


[Sulpher gives/looses Hydrogen and gets oxidised]


[Sulpher gives/looses Hydrogen and gets oxidised]


[Carbon gives/looses Hydrogen and gets oxidised]

Here carbon gains oxygen and gets oxidised.  

Reduction Reaction: Reduction is the loss of oxygen or gain of hydrogen.

In other words: 

The addition of hydrogen to a substance or removal of oxygen from a substance or both is called reduction.

Example of reduction:




However both oxidation and reduction occur together. 

Redox Reaction: Such reaction in which one reactant gets oxidised while the other gets reduced during a reaction is called redox reaction.

In other words;

When oxidation and reduction both take place in same reaction is known as redox reaction. 


Here oxidation and reduction both reaction take place in same reaction. 


Oxidising Agent/Oxidants/Oxidisers: 

The substance which gives oxygen for oxidation or removes hydrogen for reduction is called oxidising agent. 

Reducing agent:

The substances which is responsible for removing oxygen or gives Hydrogen for reduction called reducing agent. 


Here in above example CuO is reduced to Cu so reduced substance is CuO. Hence CuO (copper oxide) gives oxygen for oxidation to oxidised H2 therefore It is a Oxidising Agent.   

And H2 is oxidised to H2O so H2 Is Oxidised and it is responsible for removing oxygen from CuO (copper oxide) Therefore H2 (Hydrogen) is Reducing Agent. 


(a) Oxidised substance : H2                   // Which gains in Oxygen

(b) Reduced substance: CuO            // Which looses Oxygen 

(c) Oxidising Agent :CuO                   // which provides Oxygen for oxidation

(d) Reducing Agent : H2         // Which is responsible for removal of Oxygen 


Effects oxidation reaction: 

There are many reactions are going on around us in our everyday life in which corrosion of metals and rancidity of foods are common example due to oxidation reaction. 

1. Corrosion

The process in which metals looses their surface gradually by the action of air, water and moisture is called corrosion

In other words: 

The process in which metals surface get corroded by reaction with air, water and moisture is called corrosion

***** Corrosion is not a rusting while rusting causes corrosion. 

Preventing Corrosion: 

There are following method to prevent corrosion: 

(i) By galvonisation 

(ii) By painting metal surfaces

(iii) By oiling or greesing metal surfaces

Galvonisation: Coating of thin layer of zinc (Zn) on metal surface by electro-plating is called galvonisation. 


2. Rancidity

The condition produced by aerail (airborne) oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity

Rancidity is an event, when fat/oil containing foods get oxidised after a long time their taste and smell change.

  • Rancidity spoils the food materials prepared in fat/oil. like, chips, Bhuiya and Sabji etc. 
  • Oxidised food materials' taste and smell are changed. 
  • Rancid food is unfit for eating. 

Preventing fat/oil containing foods from rancidity: 

Fat/oil containing food can be preveneted or the rate of rancidity can be retarded by using following methods:

(i) Rancidity can be prevented by adding anti-oxidants to fat/oil containing foods.

(ii) Rancidity can be prevented to remove oxygen gas from container and filling nitrogen gas during packaging.

(iii) Rancidity can be retarded by keeping food in air tight containers that helps to slow down oxidation. 

(iv) Rancidity can be retarded by storing food items away from heat and light. 

(v) Rancidity can be retarded by keeping food in a refrigerator that cause to slow down oxidation. 


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Study Materials List:

CBSE NOTES ⇒ Class 10th ⇒ Science
1. Chemical Reactions and Equations
2. Acids, Bases and Salts
3. Metals and Non-metals
4. Carbon and its Compounds
5. Periodic Classification of Elements
6. Life Processes
7. Control and Coordination
8. How do Organisms Reproduce
9. Heredity and Evolution
10. Light-Reflection and Refraction
11. Human Eye and Colourful World
12. Electricity
13. Magnetic Effects of Electric Current
14. Sources of Energy
15. Our Environment
16. Management of Natural Resources

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