Chemists often write “equations” to describe chemical reactions. It is conventional to list the initial species, or reactants, on the left side of the equation, followed by an arrow, and the final species, or products, on the right side of the equation. The equation below shows the reaction of methane and oxygen to produce water and carbon dioxide.

The arrow to the right indicates that the reactant species are converted to the product species during the course of the reaction. In some cases, reactions are reversible, which will be represented by two arrows, one pointing in either direction (). Be careful, because in chemistry is not the same as !