Page 10 - NCERT Science Class 10 English Medium

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 O )
3 4
(ii) To balance 1×3 3

To equalise Fe, we take three atoms of Fe on the LHS.

3 Fe + 4 H O → Fe O + 4 H (1.8)
2 3 4 2
Step VI: Finally, to check the correctness of the balanced equation, we
count atoms of each element on both sides of the equation.
(1.9)
3Fe + 4H O → Fe O + 4H
2 3 4 2 (balanced equation)
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.
Writing
Writing Symbols of Physical States
Physical
States
of
Writing Symbols of Physical States
Symbols
Step VII: Writing Symbols of Physical StatesWriting 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 O(g) → Fe O (s) + 4H (g) (1.10)
2 3 4 2
Note that the symbol (g) is used with H O to indicate that in this
2
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 –
340atm
CO(g) + 2H (g)  CH OH(l) (1.11)
→
2 3
Sunlight
+
+
6CO (aq) 12H O(l)  C H O (aq) + 6O (aq) 6H O(l) (1.12)
→
2 2 Chlorophyll 6 12 6 2 2 2
(Glucose)
Using these steps, can you balance Eq. (1.2) given in the text earlier?
Chemical Reactions and Equations 5

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