When two or more elements combine to form a compound, the ratio of their masses is constant, regardless of the source or method of preparation. This principle, concerning consistent elemental ratios within compounds, contrasts with the scenario where the same elements can combine in different proportions to create multiple distinct compounds. For instance, water (HO) always contains hydrogen and oxygen in a mass ratio of approximately 1:8. Conversely, carbon and oxygen can form both carbon monoxide (CO) and carbon dioxide (CO), each possessing a unique and fixed mass ratio of the constituent elements.
These concepts are fundamental to the development of modern chemistry. They provide a cornerstone for understanding chemical formulas, stoichiometry, and the nature of chemical reactions. Their historical significance lies in their contribution to the acceptance of the atomic theory, providing experimental evidence for the existence of atoms and their consistent combining ratios. The ability to predict and understand chemical behavior is predicated on these foundational principles, impacting fields ranging from pharmaceutical development to materials science.