DEDICATED
TO
MY FATHER
LATE SHRI. M. S. MALLAYYA
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - D -
, CHAPTER – 1
CHEMICAL REACTIONS AND EQUATIONS
CHEMICAL REACTIONS
Any change can be classified as physical change and chemical change. Physical changes
can be easily reversed but, it is not easy to reverse a chemical change.
In chemical changes, new substances are formed and it is difficult to regenerate the original
substances. Chemical changes are more permanent than physical changes.
Chemical reaction involves chemical changes.
Chemical reactions are the processes in which new substances with new properties are formed.
During a chemical reaction, atoms of one element do not change into those of another element.
Only a rearrangement of atoms takes place in a chemical reaction.
Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This
powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen
present in the air.
Magnesium Oxygen Magnesium Oxide
Heat
(As ribbon) (From air) (White powder)
The burning of magnesium in air to form magnesium oxide is an example of chemical reaction.
REACTANTS AND PRODUCTS
The substances which take part in a chemical reaction are called reactants.
The new substances produced as a result of chemical reaction are called products.
In the above chemical reaction, there are two reactants : Magnesium and Oxygen but only one
product : Magnesium oxide.
CHARACTERISTICS OF CHEMICAL REACTIONS
In a chemical reaction, reactants are transformed into products.
The important characteristics of chemical reaction are:
Evolution of a gas
Formation of a precipitate
Change in colour
Change in temperature and
Change in state.
Any one of these characteristics can tell us whether a chemical reaction has taken place or not.
CHEMICAL EQUATIONS
The method of representing a chemical reaction with the help of symbols and formulas of the
substances involved in it is known as chemical equation.
A word-equation shows change of reactants to products through an arrow placed
between them. The reactants are written on the left-hand side (LHS) with a plus sign (+)
between them. Similarly, products are written on the right-hand side (RHS) with a plus
sign (+) between them. The arrowhead points towards the products, and shows the
direction of the reaction.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow
shows the direction of chemical reaction. Condition, if any, is written generally above the
arrow.
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 1 -
,When hydrogen reacts with oxygen, it gives water. This reaction can be represented by
following chemical equation:
Hydrogen + Oxygen ⇨ Water
H2 + O2 → H2O
In first equation words are used and in second symbols of substances are used to write the
chemical equation. For convenience, symbol of substance is used to represent chemical
equations. Chemical Equation is a way to represent the chemical reaction in concise and
informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and
Unbalanced Chemical Equation.
Balanced Chemical Equation: A balanced chemical equation has number of atoms of each
element equal on both sides.
Example: Zn + H2SO4 → ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a
balanced chemical equation.
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not
equal to the number of atoms of each element present in product, then the chemical equation is
called unbalanced chemical equation.
Example: Fe + H2O → Fe3O4 + H2
In this example number atoms of elements are not equal on two sides of the reaction. For
example, on the left hand side only one iron atom is present, while three iron atoms are present
on the right hand side. Therefore, it is an unbalanced chemical equation.
BALANCING A CHEMICAL EQUATION:
To balance the given or any chemical equation, follow these steps:
Fe + H2O → Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table; as shown
here.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 1 3
Hydrogen 2 2
Oxygen 1 4
Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number
of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O → Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the
RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O → Fe3O4 + 4 x H2
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of
iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the
LHS by 3.
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 2 -
, 3 x Fe + 4 x H2O → Fe3O4 + 4 x H2
Now the number of atoms of each element becomes equal on both sides. Thus, this equation
becomes a balanced equation.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 3 3
Hydrogen 8 8
Oxygen 4 4
After balancing, the above equation can be written as follows.
3Fe + 4H2O → Fe3O4 + 4H2
Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
Gaseous state is represented by symbol ‘g’
Liquid state is represented by symbol ‘l’
Solid state is written by symbol ‘s’
Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above
and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which
reaction takes place, a chemical equation can be made more informative.
INTEXT QUESTIONS PAGE NO. 6
Q1: Why should a magnesium ribbon be cleaned before it is burnt in air?
Answer : Magnesium is an extremely reactive metal. When stored, it reacts with oxygen to
form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable
and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by
sand paper for removing this layer so that the underlying metal can be exposed to air.
Question 2: Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer :
Question 3: Write a balanced chemical equation with state symbols for the following
reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble
barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in
water) to produce sodium chloride solution and water.
Answer :
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 3 -
TO
MY FATHER
LATE SHRI. M. S. MALLAYYA
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - D -
, CHAPTER – 1
CHEMICAL REACTIONS AND EQUATIONS
CHEMICAL REACTIONS
Any change can be classified as physical change and chemical change. Physical changes
can be easily reversed but, it is not easy to reverse a chemical change.
In chemical changes, new substances are formed and it is difficult to regenerate the original
substances. Chemical changes are more permanent than physical changes.
Chemical reaction involves chemical changes.
Chemical reactions are the processes in which new substances with new properties are formed.
During a chemical reaction, atoms of one element do not change into those of another element.
Only a rearrangement of atoms takes place in a chemical reaction.
Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This
powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen
present in the air.
Magnesium Oxygen Magnesium Oxide
Heat
(As ribbon) (From air) (White powder)
The burning of magnesium in air to form magnesium oxide is an example of chemical reaction.
REACTANTS AND PRODUCTS
The substances which take part in a chemical reaction are called reactants.
The new substances produced as a result of chemical reaction are called products.
In the above chemical reaction, there are two reactants : Magnesium and Oxygen but only one
product : Magnesium oxide.
CHARACTERISTICS OF CHEMICAL REACTIONS
In a chemical reaction, reactants are transformed into products.
The important characteristics of chemical reaction are:
Evolution of a gas
Formation of a precipitate
Change in colour
Change in temperature and
Change in state.
Any one of these characteristics can tell us whether a chemical reaction has taken place or not.
CHEMICAL EQUATIONS
The method of representing a chemical reaction with the help of symbols and formulas of the
substances involved in it is known as chemical equation.
A word-equation shows change of reactants to products through an arrow placed
between them. The reactants are written on the left-hand side (LHS) with a plus sign (+)
between them. Similarly, products are written on the right-hand side (RHS) with a plus
sign (+) between them. The arrowhead points towards the products, and shows the
direction of the reaction.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow
shows the direction of chemical reaction. Condition, if any, is written generally above the
arrow.
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 1 -
,When hydrogen reacts with oxygen, it gives water. This reaction can be represented by
following chemical equation:
Hydrogen + Oxygen ⇨ Water
H2 + O2 → H2O
In first equation words are used and in second symbols of substances are used to write the
chemical equation. For convenience, symbol of substance is used to represent chemical
equations. Chemical Equation is a way to represent the chemical reaction in concise and
informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and
Unbalanced Chemical Equation.
Balanced Chemical Equation: A balanced chemical equation has number of atoms of each
element equal on both sides.
Example: Zn + H2SO4 → ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a
balanced chemical equation.
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not
equal to the number of atoms of each element present in product, then the chemical equation is
called unbalanced chemical equation.
Example: Fe + H2O → Fe3O4 + H2
In this example number atoms of elements are not equal on two sides of the reaction. For
example, on the left hand side only one iron atom is present, while three iron atoms are present
on the right hand side. Therefore, it is an unbalanced chemical equation.
BALANCING A CHEMICAL EQUATION:
To balance the given or any chemical equation, follow these steps:
Fe + H2O → Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table; as shown
here.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 1 3
Hydrogen 2 2
Oxygen 1 4
Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number
of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O → Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the
RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O → Fe3O4 + 4 x H2
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of
iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the
LHS by 3.
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 2 -
, 3 x Fe + 4 x H2O → Fe3O4 + 4 x H2
Now the number of atoms of each element becomes equal on both sides. Thus, this equation
becomes a balanced equation.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 3 3
Hydrogen 8 8
Oxygen 4 4
After balancing, the above equation can be written as follows.
3Fe + 4H2O → Fe3O4 + 4H2
Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
Gaseous state is represented by symbol ‘g’
Liquid state is represented by symbol ‘l’
Solid state is written by symbol ‘s’
Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above
and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which
reaction takes place, a chemical equation can be made more informative.
INTEXT QUESTIONS PAGE NO. 6
Q1: Why should a magnesium ribbon be cleaned before it is burnt in air?
Answer : Magnesium is an extremely reactive metal. When stored, it reacts with oxygen to
form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable
and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by
sand paper for removing this layer so that the underlying metal can be exposed to air.
Question 2: Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer :
Question 3: Write a balanced chemical equation with state symbols for the following
reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble
barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in
water) to produce sodium chloride solution and water.
Answer :
Prepared by: M. S. KumarSwamy, TGT(Maths) Page - 3 -
