What is a chemical equation? What are the essentials of a chemical equation?

Chemical equation is a symbolic representation of actual chemical change in terms of symbols and formulae of the reactants and products. The substances which react among themselves are called reactants. These are written on left hand side of the equation. The substances formed during the chemical change are called products. These are written on the right hand side of the equation. For example, reaction between zinc and hydrochloric acid to form zinc chloride and hydrogen can be represented by a chemical equation.     
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Essentials of a chemical equation.A chemical equation must satisfy the following conditions:
(i) It should represent a true chemical change i.e. the change represented by the chemical equation should be practically possible.
For example, the equation

is not a true chemical equation since copper does not react with sulphuric acid to produce copper sulphate and hydrogen.
(ii) It should be arithmetically balanced. 
Atoms of each element on both the sides of the arrow  must be equal. For example,
   
is an arithmetically balanced equation.
(iii) It should be molecular. If any elementary gas takes part in a chemical equation it must be expressed in the molecular form e.g.