Method of writing the essential features of a chemical reaction using chemical symbols (or other agreed-upon abbreviations).
By convention, reactants (present at the start) are on the left, products (present at the end) on the right. A single arrow between them denotes an irreversible reaction, a double arrow a reversible reaction. The law of conservation of matter (see conservation law ) requires that every atom on the left appear on the right (the equation must balance); only their arrangements and combinations change. For example, one oxygen molecule combining with two hydrogen molecules to form two water molecules is written 2H 2 + O 2 2192; 2H 2 O. The dissociation of salt into sodium and chloride ions is written NaCl 2192; Na + + Cl - . See also stoichiometry .