In chemistry, a chemical equation is a way of denoting a chemical reaction using chemical symbols for the participating particles (atoms, molecules, ions, etc.) and a reaction arrow to signify time (before 'state' on going to an 'after' state). [1] A chemical reaction equation, said another way, is a symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities are given on the right hand side. [2] The basic chemical reaction equation involves, typically, a one-way arrow, signifying an irreversible reaction in going from reactants to products:

In the above standard chemical equation, the symbols A, B, C, D, represent the molecules involved in the reaction, and the numbers x, y, z, and w are called stoichiometric coefficients, and represent the relative numbers of reacting molecules. A chemical equation thus expresses a chemical change quantitatively by means of chemical formulas, e.g. H2O, chemical symbols, e.g. H=hydrogen, and chemical process notation, e.g. reaction arrows (→), equilibrium reaction signifier (⇌), chemical bonds (-, =, ≡), heated reaction (Δ), among others. [3]

History
See main: History of the chemical equation
The first chemical reaction diagrams were made by Scottish physician and chemist William Cullen in 1757, such as shown below, who pioneered the development of affinity reaction diagrams during his lectures. Cullen called the reaction arrow a 'dart', which he said expressed the elective affinity preference of the reacting species. Modified types of Cullen reaction diagrams were later employed in the lecture notes of Joseph Black, Cullen's student. [4]

The upgrade from the equal sign " = " symbol to the two-way reaction arrow "$\leftrightarrows \,$" symbol (or ), signifying a reversible reaction, to note, did not come about until 1884 through the work of Dutch physical chemist Jacobus van't Hoff. [5]

In modern terms, the agreed upon notation, according to IUPAC, is as follows: the symbol '' for a stoichiometric relation; '' for a net forward reaction; '' for a reaction in both directions; and '' for equilibrium. [2]

5. (a) Nernst, Walther. (1895). Theoretical Chemistry: from the Standpoint of Avogadro’s Rule & Thermodynamics ($\leftrightarrows \,$, pg. 358). MacMillan and Co.