State three rules for waiting the …

Rules followed in writing chemical equations are:
1. A chemical equation should be written with the reactants (if there are two or more should be separated by ‘+’ symbol, same applicable to products) on the left side of an arrow and the products of the chemical reaction on the right side of the equation.
2. The head of the arrow should always points toward the right or toward the product side of the equation (sometimes indicate equilibrium with the reaction proceeding in both directions simultaneously).
3. The condition required for the reaction to proceed (say temperature, pressure, catalyst etc.) should be written on the top of the arrow.
4. The physical state of reactants and product should be informed by writing the state inside the bracket next to the reactants and product. If gas you need to write e.g. O2(g), similarly for liquids (l), solids (s), aqueous solutions (aq) etc.
5. Chemical equations should be balanced on the basis of law of conservation of mass, i.e. number of atoms of the elements on the left side(reactants) should be equal to number of atoms of same elements on right side(products).
An example of a balanced chemical equation following above rules may be seen in the combustion of methane:

                            heat

CH₄ (g) + 2O₂(g) -------→ CO₂(g) + 2H₂O(l)

 Reactants(LHS)           Products (RHS)