Law of definite composition

In chemistry, the law of definite proportions, sometimes called Proust's law or the law of definite composition or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition by mass. For example, oxygen makes up about ⁸/₉ of the mass of any sample of pure water, while hydrogen makes up the remaining ¹/₉ of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.

This observation was first made by the French chemist Joseph Proust, based on several experiments conducted between 1798 and 1804. Based on such observations, Proust made statements like this one, in 1806:

I shall conclude by deducing from these experiments the principle I have established at the commencement of this memoir, viz. that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen. In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible.

The law of definite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound. At the end of the 18th century, however, when the concept of a chemical compound had not yet been fully developed, the law was novel. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust's fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion. The existence of this debate demonstrates that, at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed.

The law of definite proportions contributed to, and was placed on a firm theoretical basis by, the atomic theory that John Dalton promoted beginning in 1803, which explained matter as consisting of discrete atoms, that there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions.

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