A chemical reaction which have an equal number of atoms of different elements in the reactants and product sides is called a balanced chemical equation. For example;

In the above reaction; zinc reacts with sulphuric acid to form zinc sulphate and hydrogen. Let us examine the number of atoms of different elements on both sides of the arrow.

As the number of atoms of each element is the same on both sides of the arrow, hence, it is a balanced chemical equation.

The chemical equations are balanced to satisfy the law of conservation of mass which says that 'matter (or atoms) can neither be created nor destroyed in a chemical reaction'. So, the total mass of the elements present in the products of a chemical reaction should 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.