Chemical Equation
Definition
A shorthand recipe for a chemical reaction, showing reactants turning into products with formulas and balanced numbers.
Technical definition
A chemical equation is a symbolic representation of a chemical reaction: reactant formulas appear left of an arrow, product formulas right of it, with stoichiometric coefficients chosen so atoms of each element balance on both sides, expressing conservation of mass.
Example
2H2 + O2 -> 2H2O shows two molecules of hydrogen reacting with one molecule of oxygen to make two molecules of water.
Common confusion
A balanced equation does not prove a reaction actually happens. It only shows what atom counts would look like if it did; real reactions also need favorable energy and conditions.