Everything about Law Of Definite Proportions totally explained
In
chemistry, the
law of definite proportions and also the elements states that a
chemical compound always contains exactly the same proportion of
elements by mass. An equivalent statement is the
law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example,
oxygen makes up
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.
History
This observation was first made by the
French chemist
Joseph Proust based on several experiments conducted between
1797 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've 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's to say, that it unites with two constant proportions of oxygen. In this respect it doesn't 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 hadn't 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 very existence of this debate underscores that at the time, the distinction between pure
chemical compounds and
mixtures hadn't 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.
Non-stoichiometric compounds
It may be noted that although very useful in the foundation of modern chemistry, the laws of definite proportions isn't universally true. There exist
non-stoichiometric compounds whose elemental composition can vary from sample to sample. An example is the
iron oxide wüstite, which can contain between 0.83 and 0.95 iron atoms for every oxygen atom, and thus contain anywhere between 23% and 25% oxygen. In general, Proust's measurements were not accurate enough to detect such variations.
In addition, the
isotopic composition of an element can vary depending on its source, hence its weight in a pure stoichiometric compound may vary. This fact is used in
geochemical dating since
astronomical,
atmospheric,
oceanic,
crustal and deep
Earth processes may concentrate lighter or heavier isotopes preferentially. With the exception of
hydrogen and its isotopes, the effect is usually small but measurable with modern instrumentation. An additional note: many natural
polymers vary in composition (for instance
RNA,
proteins,
carbohydrates) even when "pure". Polymers are generally not considered "pure chemical compounds" except when their molecular weight is uniform (monodisperse) and their stoichiometry is constant. In this unusual case, they still may violate the "Law" due to isotopic variations.
Further Information
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