By definition a chemical equation is a written representation of a chemical reaction, showing both the reactants and products, their physical states, and the direction in which the reaction proceeds. In addition, many chemical equations designate the conditions necessary (such as high temperature) for the reaction to occur. A chemical equation provides quantitative information about a chemical reaction, but only if it is balanced.
For a chemical equation to be balanced, the same number of each kind of atom must be present on both sides of the chemical equation. The French chemist Antoine Lavoisier described the law of conservation of matter -- in a chemical reaction matter can neither be created nor destroyed. From Dalton's atomic theory we know that all substances are composed of atoms. During a chemical reaction atoms may be combined, separated, or rearranged, but not created or destroyed.
