A chemical formula describes what inputs are necessary for a chemical reaction to occur and what products will result from the process. A complete formula indicates the state of matter -- solid, liquid or gas -- of each of these inputs and products in the reaction, ensuring that the chemist knows exactly what to expect.
The State of Affairs
For example, in the chemical formula for hydrolysis -- the splitting of water -- the liquid state of the reactant would be indicated by a lowercase "l" in parentheses next to the formula for water. Similarly, the gaseous state of the resulting hydrogen and oxygen would be indicated by a (g) next to the chemical formulas for these products. A solid reactant is marked by (s), while a solution of a reactant in water, or an aqueous solution, is indicated by (aq).