Chapter 1st Science Chemical Reactions and Equations

Chemical Reactions and Equations
Chemical Reactions and Equations

Such changes, which only change the physical state, shape, and physical properties of matter, are called physical changes. Let us see Chapter 1 Science Chemical Reactions and Equations.


1) Melting of ice
2) Freezing of water
3) Evaporation of water.

Such changes, which also change the chemical composition and properties of substances, are called chemical changes or reactions.

1) Milk bursting
2) Combustion
3) Rusting

Chemical reactions are written with the help of chemical equations.

Chemical Reaction

When two or more substances combine to form a new substance, And if the physical and chemical properties of this new substance are different from the previous substance, then this chemical reaction is called a chemical reaction. 


1) Hydrogen and oxygen reactions produce water (water), whose physical and chemical properties are different from hydrogen and oxygen.
                  H+O2 —————–> H2O 
       Hydrogen. oxygen water

also read:- Metals and Non-Metals Notes

2) Carbon is burnt in the presence of oxygen to form carbon dioxide. The physical and chemical properties of CO2 are different from that of carbon.
C+O2 —> CO2

Identification of chemical

• Change in state • Change
in temperature • Change
in color • Emission
of gas • Emission
of an odor
Formation of a precipitate.

chemical reactions in daily life 

Digestion of food
Rusting of iron
Formation of curd

Chemical equation: 

A chemical reaction is represented by a chemical equation. The chemical formulas of the substances participating in a chemical equation are written.


Magnesium (Mg) is burnt in the air (O2) to form magnesium oxide (MgO). The chemical equation for this reaction is;

 Magnesium Oxygen. magnesium               

Substances that take part in a chemical reaction are called ” reactants”.

Eg: Magnesium and Oxygen

Substances that are formed in a chemical reaction are called “products”.
eg: magnesium oxide

Methods to write the chemical Equation

1) Mark the arrow between the reactants and the products, the
reactants are written on the left side of the arrow (LHS) and the products are written on the right side (RHS).

2) Put a positive (+) sign between the molecules of the reactants. Similarly, there is a positive (+) mark between the products as well.

3) The head of the arrow points towards the products, indicating the direction of the reaction.

Such a chemical equation of a chemical reaction is called a Skeletal Chemical Equation.

balanced chemical equation; (Balanced Chemical Equation)
Equations that have the same number of atoms in both reactants and products.
Example; 2Mg+O2——>2MgO

unbalanced chemical equations; unbalanced chemical equation)
Equations in which the number of atoms is not equal in both the reactant and the product.
Example; Mg+O2——–>2MgO

Why is it necessary to balance a chemical equation? (why is it important to balance a chemical equation?)

According to the law of indestructibility of matter, in the reactants and products of any chemical reaction, mass is neither created nor destroyed.

Therefore, the number of atoms of the reactant and the product of a chemical reaction should be the same.

also read:- Acid, Base and Salt

Balancing of chemical Equation

The chemical equation is balanced by the following method.

Hit and Trial Method

1) Write the unbalanced equation with a bracket around the formula.
(Fe) + (H2O)———> (Fe2O3)+ (H2)
2) Note the number of atoms of different elements present in the equation.

Atoms in the element reactants.
            The number of atoms in the product. Number of
  Fe 1 3
  H 2 2
  O 1 4

3) First, we will balance the substance with the highest number of atoms. On this basis let us take Fe3O4.
There are 4 atoms of oxygen on the right, while only one on the left. To balance the oxygen atom;
[Fe]+ 4× [H2O]——> [Fe3O4] + [H2]

4) To make the hydrogen atom equal, the number of right and hydrogen molecules is increased to ‘4’.
[Fe]+4[H2O]———–> [Fe3O4] + 4× [H2]

5) Now Fe (Iron) has to be balanced.
3× [Fe]+4[H2O]——–>[ Fe3O4 ]+ 4 [ H2 ]

6) The balanced equation is followed by the Sarsian formulas of the reactants and products along with the signs of their physical states.

Signals of the physical states of the reactants and products
Solid (s) -solid
liquid (l) – liquid
gas (g) – gas
Aqueous solution (aq) -aqueous

7) Write the reaction conditions (temperature, pressure) and catalyst above or below the arrow.
8) The heating of the reactants is indicated by writing or shams above the arrow.

read also:- Difference Between Acid and Base

Types of chemical reactions

1. Combination Reaction
In this reaction, two or more reactants combine to form a single substance, then it is called combination reaction.
A+B=C ( structure of CR)


C(s) + O2 (g)—-> CO2 (g)
2H2 (g) + O2(g) —–> 2H2O (l)

2. Decomposition Reaction
In this reaction, one reactant breaks down to form two or more products.
A—– > B+ C 

Eg – heat

There are three types of decomposition reactions;

1. Thermal dissociation; (thermal decomposition) Decomposition  by heat.
example ;
                 On heating /
CaCo3 (s) —————–> CaO (s) + CO2 (g)
(limestone) (slaked lime)

~ Calcium oxide (lime / non-slaked lime) is used in various industries and in the manufacture of cement. (imp)

2. Electrolytic Decomposition
Example – When an electric shock is passed through acidic water, water decomposes into hydrogen and oxygen.
2H2O (l) ——————> 2H2(g) + O2 (g)

3. Optical dissociation: (photolysis) dissociation in the presence of sunlight.
2AgCl(s)—————-> 2Ag(s) +Br2(g)
(Silver chloride turns from white to grey)
★ Use of this reaction in black-white photography 

displacement/substitution reaction

In this reaction, the more reactive element displaces the more reactive element from its compound.
A+ BC—-> BA+C

1) Single substitution reactions
Fe(s)+ CuSO4 (aq)———> FeSO4 (aq)+Cu(s)
        copper sulphate. Iron sulphate

2. Double displacement or precipitation reaction
A reaction in which ions are exchanged between the reactants. and
 the reaction in which the precipitate is formed.
sodium barium barium sodium
sulfate chloride sulfate chloride

Exothermic and Endothermic Reactions

1) Exothermic Reaction
The reactions in which heat is also released along with the formation of the product. It is denoted by the sign of positive.

CH4(g)+ O2(g)—–>CO2(g)+2H2O(g)+heat

2) Endothermic
Reaction The reactions in which heat is absorbed. They are called endothermic reactions. They are denoted by the minus (-) sign.

: increase of oxygen or loss of hydrogen in a reaction 
C+O2 —–>CO2

of Oxygen in a reaction Addition of Hydrogen 
ZnO+C——–> Zn+CO

Oxidizing Agents
The substance which is reduced in a chemical reaction is called an oxidizing agent.
That is, the substance that gains electrons in the reaction. 
Example: Oxygen, Ozone, Hydrogen Peroxide 

Reducing Agents
The substance which gets oxidized in a chemical reaction is called a reducing agent.
That is the substance that loses electrons in the reaction.
Example- hydrogen carbon, sulfur dioxide

Redox Reaction, when both reduction and oxidation reactions occur together in a reaction, then it is called a redox reaction.

                | ,
Sulfuric. Stans. Mercurous Scenic
Chloride, Chloride, Chloride, Chloride
like here Hg2+ is being reduced and Sn2+ is being oxidized, so it is a redox reaction.

Effect of Oxidation Reactions in Everyday Life

When a metal comes in contact with moisture, air, water, acid, etc., the metal becomes corrosive (weak).
Like rusting of iron, black coating on silver, and green coating on copper.
Corrosion can be prevented by painting metals, forming alloys or coating other metals.

 Fatty and oily food ingredients, when exposed to air, become analyzed causing changes in their taste and smell.
 Like 1. spoilage of fatty foods.
        2. On keeping the paneer cooked, it starts giving a foul smell.

Measures to prevent rancidity
• By keeping food items in airtight containers.
• Refrigeration of 
potato chips by filling oxygen with nitrogen gas
• Protecting food and cooked food from exposure to direct sunlight.

Click here to learn more about Chemical Reactions and Equations

Leave a Comment