Page 8 - NCERT Science Class 10 English Medium

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 → MgO (1.2)
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 SO → ZnSO + H (1.3)
2 4 4 2
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 O → Fe O + H (1.4)
2 3 4 2

Chemical Reactions and Equations 3

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