Page 107 - ICSE Chemistry 8

Based on the above-men oned steps, let’s write down the chemical equa ons for some chemical
reac ons.
• Hydrogen combines with oxygen to form water.
H 2 + O H O
2
2
Hydrogen Oxygen Water
• Methane burns in air to form carbon dioxide and water.
CH + O CO 2 + H O
2
2
4
Methane Oxygen Carbon dioxide Water
Note that in the fi rst reac on, the number of atoms of hydrogen and oxygen on the le -hand side and the
right-hand side are not equal.
In the second reac on also, the number of atoms of hydrogen, oxygen and carbon on the le -hand side
and right-hand side are not equal.

The chemical equa on that has an unequal number of atoms of one or more reactants and products is
called a skeletal or unbalanced equa on.

The equa ons given above violate the law of conserva on of mass. According to the law of conserva on of
mass, ma er can neither be created nor destroyed in a chemical reac on. This means that the total mass
of all the reactants must be equal to the total mass of all the products. In other words, we can say that the
number of atoms of the reactants must be equal to the number of the same type of atoms of the products.

Need for Balancing a Chemical Equation

To make the number of diff erent types of atoms equal in reactants and products we balance an equa on.
The chemical equa on that has an equal number of atoms of one or more elements in the reactants and
products is called a balanced equa on. Such an equa on confi rms to the atomic theory and the law of
conserva on of mass.

BALANCING A CHEMICAL EQUATION

Let’s learn to balance chemical equa ons by applying the atomic theory and the law of conserva on of
mass.

Applying Atomic Theory

The atomic theory states that atoms of the reac ng molecules
involved in a chemical reac on do not change. These molecules only Thinking Fountain
share or transfer electrons to form new compounds. Therefore, we Why is there a need to balance a
can say that the number of atoms of each element must be equal on chemical equa on? Balance the
both the sides of the chemical equa on. Using this fact, we balance a following chemical equa ons.
chemical equa on. • CaCl + Na PO
4
2
3
Ca (PO ) + NaCl
4 2
3
Skeletal or unbalanced equa ons are balanced by hit and trial method.
Let’s consider the following example to understand balancing of • HClO + P O
10
4
4
H PO + Cl O
chemical equa ons. 3 4 2 7
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