Why do chemical equations need to be balanced?

Answer- A chemical equation is to be balanced to equate the number of atoms of the reactants to the number of atoms of the products. Balancing a chemical equation is important in order to follow the law of conservation of mass. 

The law of conservation of mass overlooks the balancing of a chemical equation. According to the law, matter cannot be created or destroyed in a chemical reaction. The total quantity of matter of the elements present on the reactant side should be equal to the total quantity of matter of the elements present on the product side following the law of conservation of mass. 

For example- 

Combustion of methane: 

CH4 + O2 → CO2 + H2O 

In the above equation, there are 4 atoms of hydrogen, 1 atom of carbon and 2 atoms of oxygen on the reactant side. 

On the product side, there are 3 atoms of oxygen, 2 atoms of hydrogen and 1 atom of carbon. The number of atoms on the reactant side is not equal to the number of atoms on the product side, therefore the equation does not abide by the law of conservation of mass. 

The number of reactants and products should be changed by adding coefficients to balance the equation. 

The balanced equation for combustion of methane would be- 

CH4 + 2O2 → CO2 + 2H2O 

To balance the equation, the chemical formulas of the reactants and products remain unchanged. In the balanced equation for combustion of methane, there are 4 hydrogen atoms, 4 oxygen atoms and 1 carbon atom on both sides of the equation. 

An unbalanced equation is an inaccurate representation of a chemical equation. Chemical reactions need to be balanced to abide by the law of conservation of mass which states that matter cannot be created or destroyed. A balanced chemical equation gives a rough idea of the number of reactants that are required for a reaction.