What do coefficients in a balanced chemical equation show?

In a balanced chemical equation, coefficients are the numbers placed to the left of a chemical formula to indicate the relative number of molecules or formula units involved in the reaction.12

These coefficients serve as stoichiometric factors, defining the ratio in which reactants combine and products are formed. For example, in the equation ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}$, the coefficient 2 indicates that two molecules of oxygen are required to react with one molecule of methane to produce two molecules of water.2

It is important to distinguish coefficients from subscripts:
* Coefficients represent the number of molecules and are the only values adjusted to balance an equation.1
* Subscripts appear in smaller print to the right of an element and represent the number of atoms of that element within a single molecule; these must remain unchanged to preserve the chemical identity of the substance.1

By adjusting coefficients, chemists ensure that the equation adheres to the law of conservation of mass, confirming that the total number of atoms for each element remains equal on both sides of the reaction.12