Chemical equation

A chemical equation uses symbols and chemical formulas to represent a chemical reaction. Reactants appear on the left side, products on the right, with a plus sign between multiple substances on each side and an arrow pointing toward the products to show the reaction's direction. Chemical formulas can be symbolic, structural (pictorial diagrams), or a mix of both. Numbers called stoichiometric coefficients placed before formulas indicate how many entities (like molecules) of that substance are involved; if no number is written, the coefficient is 1. The first chemical equation was diagrammed by Jean Beguin in 1615.

The structure of a chemical equation includes a list of reactants (starting substances) on the left, an arrow, and a list of products (substances formed) on the right. For example, the reaction of hydrochloric acid with sodium is written as 2HCl + 2Na → 2NaCl + H₂. Simple equations might be read aloud by spelling out the symbols, while complex ones use IUPAC nomenclature.

Different arrow variants denote reaction types. To indicate a substance's physical state, symbols in parentheses follow the formula: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution. For instance, aqueous hydrochloric acid reacting with solid sodium to form aqueous sodium chloride and hydrogen gas is written 2HCl(aq) + 2Na(s) → 2NaCl(aq) + H₂(g). If gaseous hydrogen chloride replaces the acid, the equation becomes 2HCl(g) + 2Na(s) → 2NaCl(s) + H₂(g). Alternatively, an upward arrow (↑) can indicate gas formation, or a downward arrow (↓) a precipitate, especially when only one such species forms.

If a reaction requires energy, it is shown above the arrow: a capital Greek delta (Δ) or triangle (△) for heat, and hν for light. Other symbols denote specific types of energy or radiation. If a specific medium (like an acid or base) is needed, its name, H⁺, OH⁻, or simply "acid" or "base" may be placed above the arrow. Conditions like temperature, pressure, or the presence of catalysts can also be indicated there.

Standard notation places all reactants on one side, all products on the other, with positive coefficients. For example, dehydration of methanol to dimethylether is 2 CH₃OH → CH₃OCH₃ + H₂O. An extension moves some substances above or below the arrow, preceded by a plus sign (or nothing) for a reactant and a minus sign for a product, hiding less important substances to clarify the reaction type or facilitate chaining in multi-step mechanisms. These substances are not catalysts, as they are consumed or produced. Another extension places some substances on branches of the arrow.