Formulas and equations in chemistry don’t always mean the same as in mathematics. Formulas in chemistry are representations of a chemical compound using symbols for the elements and subscripts for the number of atoms present. For example, the chemical formula for water is H₂O, in which there are two atoms of hydrogen (H) bonded to an atom of oxygen (O). The subscript 2 indicates that there are two atoms of hydrogen in the molecule; if there is no subscript number, as with the oxygen (O), a subscript of 1 is implied. (Remember, not all compounds are molecular; for example, NaCl, or sodium chloride [regular table salt], is called an ionic compound. In these cases, the formula shows the proportion of the atoms of each element making up the compound.) There are other types of formulas in chemistry, but this is the most familiar.

Equations in chemistry also differ from mathematics. Chemical equations represent the reaction relationship between two or more chemical compounds—along with the products of the chemical reaction. For example, the chemical equation 2H₂ + O₂ → 2H₂O is the reaction of hydrogen with oxygen to form water. The arrow indicates the direction of the reaction toward the product; the reactants (or the substances that react) are hydrogen and oxygen. There is also a methodology in writing chemical equations. Simply put, first there is the process to determine the reactants and outcome; next, determine the formula for each substance; and finally, balance the equation.