A chemical compound always contains the same elements in the same proportion by mass, regardless of the source of the compound or how it was prepared. For example, water (HO) consistently exhibits a mass ratio of approximately 1:8 between hydrogen and oxygen. This constant ratio remains unchanged whether the water originates from a river, a laboratory synthesis, or any other source.
This principle is a cornerstone of stoichiometry, providing a predictable foundation for quantitative chemical analysis. Its recognition enabled the development of consistent chemical formulas and accurate calculations of reactant and product quantities in chemical reactions. Historically, its formulation was crucial in solidifying the atomic theory of matter, supporting the idea that elements combine in fixed, whole-number ratios.