CBSE Class 10th Science

Ch-1 Chemical Reactions and Equations

Chemical reaction

Chemical reactions are chemical changes in which reactants transform into products by making or breaking of bonds(or both) between different atoms.

Types of chemical reactions

Taking into consideration different factors, chemical reactions are grouped into multiple categories.
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation


Combination reaction

In a combination reaction, two elements or one element and one compound or two compounds combine to give one single product.
Chemical Reactions and Equations-4

Decomposition reaction

A single reactant decomposes on the application of heat or light or electricity to give two or more products.
Types of decomposition reactions:
a. Decomposition reactions which require heat – thermolytic decomposition or thermolysis.

Displacement reaction

More reactive element displaces a less reactive element from its compound or solution.

 Chemical Reactions and Equations-7

Double displacement reaction

An exchange of ions between the reactants takes place to give new products.
For example, Chemical Reactions and Equations-8

Precipitation reaction

An insoluble compound called precipitate forms when two solutions containing soluble salts are combined.
Chemical Reactions and Equations- 9

Redox reaction

Oxidation and reduction take place simultaneously.
Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.
Reduction: Substance gains electrons or loses oxygen or gains hydrogen.
Oxidising agent – a substance that oxidises another substance and self-gets reduced.
Reducing agent – a substance that reduces another substance and self-gets oxidised.

Chemical Reactions and Equations-11

Endothermic and exothermic reaction

Exothermic reaction – heat is evolved during a reaction. Most of the combination reactions are exothermic.


Endothermic – Heat is required to carry out the reaction.


Most of the decomposition reactions are endothermic.


Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.


 4Fe(s)+3O2(from air)+xH2O(moisture)→2Fe2O3.xH2O(rust)
Corrosion of copper:
Cu(s)+H2O(moisture)+CO2(from air)→CuCO3.Cu(OH)2(green)
Corrosion of silver:
Ag(s)+H2S(from air)→Ag2S(black)+H2(g)


It refers to the oxidation of fats and oils in food that is kept for a long time. It gives foul smell and bad taste to food. Rancid food causes stomach infection on consumption.
(i) Use of air-tight containers
(ii) Packaging with nitrogen
(iii) Refrigeration
(iv) Addition of antioxidants or preservatives

Balancing of a Chemical Reaction

Conservation of mass

According to the law of conservation of mass, no atoms can be created or destroyed in a chemical reaction, so the number of atoms for each element in the reactants side has to balance the number of atoms that are present in the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the total mass of the reactants participated in a chemical reaction.

Balanced chemical equation

The chemical equation in which the number of atoms of each element in the reactants side is equal to that of the products side is called a balanced chemical equation.

Steps for balancing chemical equations

Hit and trial method: While balancing the equation, change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of each element is same on each side of the chemical equation.

Short-cut technique for balancing a chemical equation



Set up a series of simultaneous equations, one for each element.

Ca: a=3c

C: a=d

O: 3a+4b=8c+3d

H: 3b=2d

P: b=2c

Let’s set c=1

Then a=3 and



So a=3; b=2; c=1; d=3

The balanced equation is


ncert solutions
(Next Lesson) Ch-2 Acids, Bases and Salts
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