It is convenient to represent a chemical reaction as an equation of the following form: reactants -> products. These equations are most useful if they correctly reflect the relative amounts of each substance consumed and produced and if they are 'balanced' such that the amount of each element in the products is the same as the amount in the reactants. Conservation of mass implies that since atoms and other particles are not created or destroyed in the reaction, anything present on the reactants side must also be accounted for on the products side.
Mass is always conserved in a reaction, though volume may not be. Reactants may change phase as well. Subscripts are used to indicate such a process. For example, the dissolution of table salt (NaCl) in water can be concisely represented as
In such a process, it would be quite common to write over the reaction arrow to indicate the medium in which the dissolution takes place.
