A chemical reaction is a process where a chemical substance may decompose into two or more substances; two or more substance may combine to yield new substance(s).
A chemical equation describes a chemical reaction using chemical symbols and formulae.
- The chemical formula on the left hand side shows the starting substances (also called as reactants).
- The chemical formula on the right hand side shows the final substances (called products).
- The reactants and products are separated by the arrow symbol (). The arrow sign stands for “yields” or “react to form”.
Reactants 
Products
For example, zinc and sulphuric acid reacts to form zinc sulphate and hydrogen. We can write this statement in terms of chemical equation as
Zn + H2SO4 ZnSO4 + H2
A chemical equation must satisfy following conditions:
- The chemical formula of reactants and products must be correct.
- It must be balanced. The number of atoms of each element appearing in the equation should be same on both side of arrow. Coefficients are used for balancing atoms on both sides.
- It must be molecular. The reactant(s) and product(s) must be in molecular state. Some elements like hydrogen, oxygen, nitrogen and halogens (fluorine, iodine, bromine, chlorine) exist in diatomic state. These elements should always be written as H2, O2, N2, F2, Cl2, I2, and Br2.
Coefficient
The number written in front of a molecule is called coefficient. While balancing an equation, we can change the coefficients. We usually drop the coefficient of 1, while writing equation. The coefficients are never written as fractions. While balancing the equation, remove fraction by multiplying both sides of equation by denominator of fractional coefficient.
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