In chemistry, the Law of Constant Proportions either Law of definite proportionsholds that a chemical compound is always formed by the same proportion by mass of its constituent elements regardless of the method of preparation or source of obtaining.
This law was enunciated at the end of the 18th century by the French chemist Joseph Louis Proust, which is why it is also known as Proust's Law, and continued to develop it during the early 19th century. Next to the Law of multiple proportionsfrom the British chemist John Daltonconstitute the two basic laws of stoichiometry.
Example of constant proportions in a compound
Fundamentally, the Law of Constant Proportions is due to the fact that a certain element is always made up of the same type of atoms. For example, an oxygen atom is always an oxygen atom with the same atomic weight.
Take for example water (HtwoO), a simple compound made up of two types of elements, hydrogen and oxygen.
Oxygen represents 8/9 of the total mass of any water sample, while hydrogen represents the remaining 1/9. The hydrogen:oxygen mass ratio in water is always 1:8.be it water from the sea, a river, a lake or a fountain.
Similarly, if you analyze carbon dioxide (COtwo), a mass ratio of 3:8 will always be obtained.
Another example. Common salt is an ionic compound made up of chlorine and sodium (NaCl). The atomic weight of sodium is 23 and that of chlorine is 35. From this it can be deduced that 58 grams of salt always contain 23 grams of sodium and 35 grams of chlorine.
History of the Law of Constant Proportions
The Law of Constant Proportions may seem obvious today to any chemist, to the point that it is an implicit law in the very definition of a compound, but when Proust began to spread it, it was heavily criticized.
It would not be until Dalton's postulates and his atomic theory when the Law of Constant Proportions began to be accepted.
Proust carried out numerous experiments with binary inorganic compounds, especially sulfates, sulfides and metallic oxides, and from these experiments he deduced the Law of Constant Proportions. The first time it was published was in an article on iron oxides from 1774two.
Previously, Joseph Priestley Y Antoine Lavoisierproposed the constant ratio for the oxidation phenomena of metals with oxygen and extended it to all known combustion phenomena, but Proust postulated it for any type of compound or combination of elements.
One of the main opponents of Proust's Law was Claude-Louis Bertholletwho still maintained that the elements could be combined in any proportion, but it should be noted that at that time the distinction between compound and mixture was not yet very well developed.
The Law of Constant Proportions dovetailed perfectly with Dalton's atomic theory of the early 19th century, considered the first scientifically based atomic theory. In fact, the Law of Constant Proportions greatly contributed to Dalton being able to develop his theory.
Non-stoichiometric compounds
Non-stoichiometric compounds are defined as chemical compounds whose elemental atomic composition cannot be represented by integers and whose bulk composition may vary from sample to samplewhich means that the Law of Constant Proportions, although very useful in practice, is not universally true.
A well-known example in wustite, a mineral composed of iron (II) oxide or ferrous oxide but in which there may be between 23 and 25% oxygen by mass and whose elemental formula may vary from Faith0.83Or even Faith0.95EITHER.
The ideal formula of iron (II) oxide is FeO, but due to the crystal structure it adopts at the molecular level in wustite, there may be variations with absent atoms or with atoms of other elements substituting for iron or oxygen atoms in certain positions.
That is, non-stoichiometric compounds that do not follow constant proportions would have their explanation in crystallographic defects.
On the other hand, the law of constant proportions is also not completely fulfilled in stoichiometric compounds that should follow the law of constant proportions. The existence of isotopes can cause different samples of the same compound to have a different mass ratio between its elements.
The differences due to isotopes are so small that they could not be detected by Proust with the technology available at the time, when the existence of atoms, let alone isotopes, was not even known.