Law of Definite Proportions (1799)

The Law of Definite Proportions, formulated by Joseph Proust in 1799, states that a chemical compound always contains exactly the same proportion of elements by mass, regardless of the sample's source or how the compound was prepared.

Proust's careful analysis of various chemical compounds, particularly those containing elements like oxygen and hydrogen, led to the observation that the masses of elements in compounds remained constant in fixed ratios.

This law solidified the concept that elements combine in specific, fixed proportions, which contributed to the development of stoichiometry and the understanding of chemical composition and reactions.

The Law of Definite Proportions laid the groundwork for the atomic theory and played a pivotal role in the advancement of modern chemistry, helping to establish the principles of chemical combination and the composition of compounds.