NCERT Textbook: Chemical Reactions & Equations Notes | Study Science Class 10 - Class 10

 Page 1


Chemical Reactions
and Equations
1 CHAPTER
C
onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Figure 1.1
Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn  the
magnesium ribbon keeping it
away as far as possible from
your eyes.
n What do you observe?
“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
2020-21
Page 2


Chemical Reactions
and Equations
1 CHAPTER
C
onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Figure 1.1
Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn  the
magnesium ribbon keeping it
away as far as possible from
your eyes.
n What do you observe?
“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
2020-21
Science
2
Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2
Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2
Formation of hydrogen
gas by the action of
dilute sulphuric acid on
zinc
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3
n Take a few zinc granules in a conical flask or a test tube.
n Add dilute hydrochloric acid or sulphuric acid to this
(Fig. 1.2).
CAUTION: Handle the acid with care.
n Do you observe anything happening around the zinc
granules?
n Touch the conical flask or test tube. Is there any change in
its temperature?
n Take lead nitrate
solution in a test
tube.
n Add potassium
iodide solution
to this.
n What do you
observe?
1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMICAL EQUA AL EQUA AL EQUA AL EQUA AL EQUATIONS TIONS TIONS TIONS TIONS
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide. This description of a
chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen ? Magnesium oxide (1.1)
            (Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
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.
You must have observed that 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.
2020-21
Page 3


Chemical Reactions
and Equations
1 CHAPTER
C
onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Figure 1.1
Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn  the
magnesium ribbon keeping it
away as far as possible from
your eyes.
n What do you observe?
“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
2020-21
Science
2
Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2
Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2
Formation of hydrogen
gas by the action of
dilute sulphuric acid on
zinc
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3
n Take a few zinc granules in a conical flask or a test tube.
n Add dilute hydrochloric acid or sulphuric acid to this
(Fig. 1.2).
CAUTION: Handle the acid with care.
n Do you observe anything happening around the zinc
granules?
n Touch the conical flask or test tube. Is there any change in
its temperature?
n Take lead nitrate
solution in a test
tube.
n Add potassium
iodide solution
to this.
n What do you
observe?
1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMICAL EQUA AL EQUA AL EQUA AL EQUA AL EQUATIONS TIONS TIONS TIONS TIONS
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide. This description of a
chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen ? Magnesium oxide (1.1)
            (Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
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.
You must have observed that 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.
2020-21
Chemical Reactions and Equations 3
1.1.1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words. A chemical equation represents a
chemical reaction. If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O
2
  ?  MgO (1.2)
Count and compare the number of atoms of each element on the
LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If yes, then the equation is balanced. If not,
then the equation is unbalanced because the mass is not the same on
both sides of the equation. Such a chemical equation is a skeletal
chemical equation for a reaction. Equation (1.2) is a skeletal chemical
equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction. That is, the
total mass of the elements present in the products of a chemical reaction
has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a
skeletal chemical equation. Is the chemical Eq. (1.2) balanced? Let us
learn about balancing a chemical equation step by step.
The word-equation for Activity 1.3 may be represented as –
Zinc + Sulphuric acid  ?  Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H
2
SO
4
 ? ZnSO
4
 + H
2
(1.3)
Let us examine the number of atoms of different elements on both
sides of the arrow.
Element Number of atoms in Number of atoms
reactants (LHS) in products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4
As the number of atoms of each element is the same on both sides of
the arrow, Eq. (1.3) is a balanced chemical equation.
Let us try to balance the following chemical equation –
Fe + H
2
O ? Fe
3
O
4
 + H
2
(1.4)
2020-21
Page 4


Chemical Reactions
and Equations
1 CHAPTER
C
onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Figure 1.1
Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn  the
magnesium ribbon keeping it
away as far as possible from
your eyes.
n What do you observe?
“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
2020-21
Science
2
Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2
Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2
Formation of hydrogen
gas by the action of
dilute sulphuric acid on
zinc
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3
n Take a few zinc granules in a conical flask or a test tube.
n Add dilute hydrochloric acid or sulphuric acid to this
(Fig. 1.2).
CAUTION: Handle the acid with care.
n Do you observe anything happening around the zinc
granules?
n Touch the conical flask or test tube. Is there any change in
its temperature?
n Take lead nitrate
solution in a test
tube.
n Add potassium
iodide solution
to this.
n What do you
observe?
1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMICAL EQUA AL EQUA AL EQUA AL EQUA AL EQUATIONS TIONS TIONS TIONS TIONS
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide. This description of a
chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen ? Magnesium oxide (1.1)
            (Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
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.
You must have observed that 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.
2020-21
Chemical Reactions and Equations 3
1.1.1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words. A chemical equation represents a
chemical reaction. If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O
2
  ?  MgO (1.2)
Count and compare the number of atoms of each element on the
LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If yes, then the equation is balanced. If not,
then the equation is unbalanced because the mass is not the same on
both sides of the equation. Such a chemical equation is a skeletal
chemical equation for a reaction. Equation (1.2) is a skeletal chemical
equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction. That is, the
total mass of the elements present in the products of a chemical reaction
has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a
skeletal chemical equation. Is the chemical Eq. (1.2) balanced? Let us
learn about balancing a chemical equation step by step.
The word-equation for Activity 1.3 may be represented as –
Zinc + Sulphuric acid  ?  Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H
2
SO
4
 ? ZnSO
4
 + H
2
(1.3)
Let us examine the number of atoms of different elements on both
sides of the arrow.
Element Number of atoms in Number of atoms
reactants (LHS) in products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4
As the number of atoms of each element is the same on both sides of
the arrow, Eq. (1.3) is a balanced chemical equation.
Let us try to balance the following chemical equation –
Fe + H
2
O ? Fe
3
O
4
 + H
2
(1.4)
2020-21
Science
4
Step I: To balance a chemical equation, first draw boxes around each
formula. Do not change anything inside the boxes while balancing the
equation.
Fe   +   H
2
O   ?   Fe
3
O
4
   +   H
2
(1.5)
Step II: List the number of atoms of different elements present in the
unbalanced equation (1.5).
Element Number of atoms Number of atoms
in reactants (LHS) in products (RHS)
Fe 1 3
H 2 2
O 1 4
To equalise the number of atoms, it must be remembered that we
cannot alter the formulae of the compounds or elements involved in the
reactions. For example, to balance oxygen atoms we can put coefficient
‘4’ as 4 H
2
O and not H
2
O
4
 or (H
2
O)
4
. Now the partly balanced equation
becomes –
Fe   +   4 H
2
O   ?   Fe
3
O
4
   +   H
2
Step IV: Fe and H atoms are still not balanced. Pick any of these elements
to proceed further. Let us balance hydrogen atoms in the partly balanced
equation.
To equalise the number of H atoms, make the number of molecules
of hydrogen as four on the RHS.
Step III: It is often convenient to start balancing with the compound
that contains the maximum number of atoms. It may be a reactant or a
product. In that compound, select the element which has the maximum
number of atoms. Using these criteria, we select Fe
3
O
4
 and the element
oxygen in it. There are four oxygen atoms on the RHS and only one on
the LHS.
To balance the oxygen atoms –
The equation would be –
  Fe   +   4   H
2
O   ?   Fe
3
O
4
   +   4 H
2
Atoms of In reactants In products
oxygen
(i) Initial 1 (in H
2
O) 4 (in Fe
3
O
4
)
(ii) To balance 1×4 4
Atoms of In reactants In products
hydrogen
(i) Initial 8 (in 4 H
2
O) 2 (in H
2
)
(ii) To balance 8 2 × 4
(1.6)
(partly balanced equation)
(1.7)
(partly balanced equation)
2020-21
Page 5


Chemical Reactions
and Equations
1 CHAPTER
C
onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Figure 1.1
Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1 Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn  the
magnesium ribbon keeping it
away as far as possible from
your eyes.
n What do you observe?
“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
2020-21
Science
2
Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2 Activity 1.2
Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2 Figure 1.2
Formation of hydrogen
gas by the action of
dilute sulphuric acid on
zinc
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3 Activity 1.3
n Take a few zinc granules in a conical flask or a test tube.
n Add dilute hydrochloric acid or sulphuric acid to this
(Fig. 1.2).
CAUTION: Handle the acid with care.
n Do you observe anything happening around the zinc
granules?
n Touch the conical flask or test tube. Is there any change in
its temperature?
n Take lead nitrate
solution in a test
tube.
n Add potassium
iodide solution
to this.
n What do you
observe?
1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMIC 1.1 CHEMICAL EQUA AL EQUA AL EQUA AL EQUA AL EQUATIONS TIONS TIONS TIONS TIONS
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide. This description of a
chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen ? Magnesium oxide (1.1)
            (Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
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.
You must have observed that 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.
2020-21
Chemical Reactions and Equations 3
1.1.1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words. A chemical equation represents a
chemical reaction. If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O
2
  ?  MgO (1.2)
Count and compare the number of atoms of each element on the
LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If yes, then the equation is balanced. If not,
then the equation is unbalanced because the mass is not the same on
both sides of the equation. Such a chemical equation is a skeletal
chemical equation for a reaction. Equation (1.2) is a skeletal chemical
equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction. That is, the
total mass of the elements present in the products of a chemical reaction
has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a
skeletal chemical equation. Is the chemical Eq. (1.2) balanced? Let us
learn about balancing a chemical equation step by step.
The word-equation for Activity 1.3 may be represented as –
Zinc + Sulphuric acid  ?  Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H
2
SO
4
 ? ZnSO
4
 + H
2
(1.3)
Let us examine the number of atoms of different elements on both
sides of the arrow.
Element Number of atoms in Number of atoms
reactants (LHS) in products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4
As the number of atoms of each element is the same on both sides of
the arrow, Eq. (1.3) is a balanced chemical equation.
Let us try to balance the following chemical equation –
Fe + H
2
O ? Fe
3
O
4
 + H
2
(1.4)
2020-21
Science
4
Step I: To balance a chemical equation, first draw boxes around each
formula. Do not change anything inside the boxes while balancing the
equation.
Fe   +   H
2
O   ?   Fe
3
O
4
   +   H
2
(1.5)
Step II: List the number of atoms of different elements present in the
unbalanced equation (1.5).
Element Number of atoms Number of atoms
in reactants (LHS) in products (RHS)
Fe 1 3
H 2 2
O 1 4
To equalise the number of atoms, it must be remembered that we
cannot alter the formulae of the compounds or elements involved in the
reactions. For example, to balance oxygen atoms we can put coefficient
‘4’ as 4 H
2
O and not H
2
O
4
 or (H
2
O)
4
. Now the partly balanced equation
becomes –
Fe   +   4 H
2
O   ?   Fe
3
O
4
   +   H
2
Step IV: Fe and H atoms are still not balanced. Pick any of these elements
to proceed further. Let us balance hydrogen atoms in the partly balanced
equation.
To equalise the number of H atoms, make the number of molecules
of hydrogen as four on the RHS.
Step III: It is often convenient to start balancing with the compound
that contains the maximum number of atoms. It may be a reactant or a
product. In that compound, select the element which has the maximum
number of atoms. Using these criteria, we select Fe
3
O
4
 and the element
oxygen in it. There are four oxygen atoms on the RHS and only one on
the LHS.
To balance the oxygen atoms –
The equation would be –
  Fe   +   4   H
2
O   ?   Fe
3
O
4
   +   4 H
2
Atoms of In reactants In products
oxygen
(i) Initial 1 (in H
2
O) 4 (in Fe
3
O
4
)
(ii) To balance 1×4 4
Atoms of In reactants In products
hydrogen
(i) Initial 8 (in 4 H
2
O) 2 (in H
2
)
(ii) To balance 8 2 × 4
(1.6)
(partly balanced equation)
(1.7)
(partly balanced equation)
2020-21
Chemical Reactions and Equations 5
To equalise Fe, we take three atoms of Fe on the LHS.
3   Fe   +   4   H
2
O   ?   Fe
3
O
4
   +   4   H
2
(1.8)
Step VI: Finally, to check the correctness of the balanced equation, we
count atoms of each element on both sides of the equation.
3Fe + 4H
2
O ? Fe
3
O
4
 + 4H
2
The numbers of atoms of elements on both sides of Eq. (1.9) are
equal. This equation is now balanced. This method of balancing chemical
equations is called hit-and-trial method as we make trials to balance
the equation by using the smallest whole number coefficient.
Step VII: Writing Symbols of Physical States Writing Symbols of Physical States Writing Symbols of Physical States Writing Symbols of Physical States Writing Symbols of Physical States  Carefully examine
the above balanced Eq. (1.9). Does this equation tell us anything about
the physical state of each reactant and product? No information has
been given in this equation about their physical states.
To make a chemical equation more informative, the physical states
of the reactants and products are mentioned along with their chemical
formulae. The gaseous, liquid, aqueous and solid states of reactants
and products are represented by the notations (g), (l), (aq) and (s),
respectively. The word aqueous (aq) is written if the reactant or product
is present as a solution in water.
The balanced Eq. (1.9) becomes
3Fe(s) + 4H
2
O(g)  ? Fe
3
O
4
(s) + 4H
2
(g) (1.10)
Note that the symbol (g) is used with H
2
O to indicate that in this
reaction water is used in the form of steam.
Usually physical states are not included in a chemical equation unless
it is necessary to specify them.
Sometimes the reaction conditions, such as temperature, pressure,
catalyst, etc., for the reaction are indicated above and/or below the arrow
in the equation. For example –
CO(g) + 2H (g)
2
340atm
CH OH(l)
3
? ? ????
(1.11)
6CO (aq) 12H O(l) C H O (aq) 6O
2 2 6 12 6
+ +
Sunlight
Chlorophyll
? ? ????? ?
2 2 2
(aq) 6H O(l) +
(1.12)
   (Glucose)
Using these steps, can you balance Eq. (1.2) given in the text earlier?
Step V: Examine the above equation and pick up the third element
which is not balanced. You find that only one element is left to be
balanced, that is, iron.
Atoms of In reactants In products
iron
(i) Initial 1 (in Fe) 3 (in Fe
3
O
4
)
(ii) To balance 1×3 3
(1.9)
(balanced equation)
2020-21