The law of constant composition was first formulated in 1808 by French scientist and chemist Joseph Louis Proust. After the principle of conservation of mass and energy, the law of constant composition of chemical compounds the second most important law between the main concepts of the modern chemistry.
This law is formulated as follows: the substance, regardless of the method of its preparation, always has a constant qualitative and quantitative composition. From the law of constant composition it follows that when a complex substance is formed, the elements of simple substances are connected to each other in strictly defined mass fractions. The law of constant composition was refined and became the cornerstone of the chemistry. Proust established that the constancy of the ratios of the components is also observed in a number of other compounds. He formulated a general rule that all compounds contain elements in strictly defined proportions (and not in any combination), regardless of the conditions for obtaining these compounds. This rule is called the law of constant composition, or sometimes the Proust’s law. In other words, if we recognize the atomic structure of the substance, then the law of constant composition follows from this position as a natural consequence. Moreover, since the validity of the law of constant composition is an indisputable fact, then, consequently, the atoms are indeed indivisible particles.
Along with compounds for which the law of constant composition is valid, there are compounds of variable composition – many solid oxides, sulfides, nitrides, carbides, etc. Different samples of the same substance, representing a chemical compound, contain the same elements in their composition and they are always in the same proportions. However, no matter how we get this connection, it always has the same composition. This is the meaning of the known law of constant composition. A systematic approach to the study of the substance offered, according to which all substances in nature do not contain molecular but multicomponent stochastic systems (MSS) with a random distribution of composition. The composition of compounds with a non-molecular structure (with an atomic, ionic, and metal lattice) is not constant and depends on the conditions of their production. The law of constant composition does not apply to multicomponent systems. With some accuracy, it is valid only for the main component that dominates in this system.
Another French chemist, Claude Louis Berthollet, argued with Proust that the elements can be connected to each other in any proportions, depending on the mass of the reactants. He tried to prove that the composition of certain compounds varies depends on the method of their production. Proust showed that Berthollet came to erroneous conclusions, due to inaccurate analyzes and insufficiently pure initial compounds. At the beginning of the 19th century, this discussion ended in the favor of Proust and the law of constant composition was recognized by most chemists. Proust defended the idea that the substance, irrespective of the methods of preparation, possesses the same composition. This statement was formulated in the law of constancy of the composition. Based on the data on the composition of the substance, its chemical formula was derived with a constant quantitative ratio of the elements. Therefore, compounds of constant composition were called stoichiometric compounds (the stoichiometry term comes from the Greek stoi heian – element and metreo – measure). The law of constant composition and the stoichiometry of the compounds have long been considered unshakable. However, at the beginning of the 20th century, the chemist Kurnakov discovered the existence of chemical substances of variable composition foreseen by Berthollet, named bertholids in his honor. This discovery resolved the contradiction between the seemingly incompatible views of Berthollet and Proust on the composition of substances. In this way, the development of chemistry has shown that along with substances that have a constant composition, there are substances with a variable composition, which depends on the method of production.
Finally, the use of the law of constant composition and the law of multiple ratios allowed Dalton to establish the values of the relative atomic masses of the elements, taking for the unit mass the mass of the hydrogen atom. Substances with a permanent composition are called daltonids in honor of the English chemist John Dalton, who widely applied the molecular-atomic theory to chemical phenomena. The composition of daltonids is described by chemical formulas with whole stoichiometric ratios, for example, H2O, HCl, CH4, CO2, C2H5OH.
For the most diverse chemical compounds, the proof of the constant composition was already a testimony in favor of the discreteness of the compound structure. The application of the law of constant composition for the analysis of any of indicated series shows that the existence of two (or several) compounds formed by the interaction of any pair of chemical elements is possible only if the composition of the compounds will differ from each other by the entire set of atoms. It should be noted that the law of constant composition is applied under the condition that the elements, forming the chemical compound, have a strictly constant isotopic composition. This is natural since the law of the constant composition provides for the constancy of the atomic mass of any chemical element, which is possible only under the condition of the strict determination of its isotopic composition.
In conclusion, we can note once again that Proust’s law of constant composition, in which elements are combined in the formation of a certain chemical compound, are not able to continuous changes. Is valid only for molecules, consisting of a small number of atoms, and interacting so little with each other that this can be neglected. Any crystalline substance that does not have a molecular structure must, to a greater or lesser extent, have a variable composition. The reason for this lies in the energy laws, the manifestation of the entropy factor.