Explain brefiely the chemical equations?

Chemical Equations :  A chemical equation is a shorthand way of describing a chemical reaction. For example, in the formation of water by the burning of hydrogen, we may write an equation as follows:

2H_2(g) +O_2(g) →2H₂O_(i) +? H

Substances entering the reaction, on the left side of the arrow, are called reactants. The arrow signifies that the reaction yields or produces the products, shown on the right side. The subscripts in parentheses refer to the gas or liquid states of the reactants or products. The D refers to a change, and the H represents heat given off during the reaction. The D H is sometimes referred to as enthalpy, or heat of reaction.
Actually, any reaction can proceed in both forward and reverse directions. If neither the reactants nor the products are removed from the system, the reaction will proceed toward an equilibrium, at which point the forward and reverse reactions will occur at the same rate. Equations must be balanced; that is, they must contain the same number and type of atoms on each side.