Chemistry belongs to the category of exact sciences, and along with mathematics and physics it establishes the laws of existence and development of matter, consisting of atoms and molecules. All processes that occur both in living organisms and among objects of inanimate nature, are based on the phenomenon of the transformation of mass and energy. The law of constancy of the composition of the substance, the study of which this article will be devoted to, is the basis of the processes in the inorganic and organic world.
Atomic molecular theory
To understand the essence of the laws governing material reality, you need to have an idea of what it consists of. According to the great Russian scientist M. V. Lomonosov, “Physicists, and especially chemists, should not be in the dark, not knowing the internal particles of the structure.” It was he who in 1741, first theoretically, and then confirming by experiments, discovered the laws of chemistry, which serve as the basis for the study of living and non-living matter, namely: all substances are composed of atoms capable of forming molecules. All these particles are in continuous motion.
Discoveries and mistakes of J. Dalton
After 50 years, the ideas of Lomonosov began to develop the English scientist J. Dalton. The scientist performed the most important calculations to determine the atomic masses of chemical elements. This served as the main proof of such assumptions: the mass of a molecule and substance can be calculated by knowing the atomic weight of the particles that make up its composition. Both Lomonosov and Dalton believed that, regardless of the method of preparation, the molecule of the compound will always have the same quantitative and qualitative composition. Initially, it was in this form that the law of constancy of the composition of the substance was formulated. Recognizing Dalton's enormous contribution to the development of science, one cannot remain silent about the annoying mistakes: the denial of the molecular structure of simple substances such as oxygen, nitrogen, hydrogen. The scientist believed that only complex chemicals have molecules. Given the great authority of Dalton in the scientific community, his errors negatively affected the development of chemistry.
How atoms and molecules are weighed
The discovery of such a chemical postulate as the law of constancy of the composition of a substance was made possible thanks to the idea of preserving the mass of substances that entered into the reaction and formed after it. In addition to Dalton, the measurement of atomic masses was carried out by I. Berzelius, who compiled a table of atomic weights of chemical elements and proposed their modern designation in the form of Latin letters. Currently, the mass of atoms and molecules is determined using a carbon nanotube. The results obtained in these studies confirm the existing laws of chemistry. Previously, scientists used a device such as a mass spectrometer, but the sophisticated weighing technique was a serious drawback in spectrometry.
Why is the law of conservation of mass of substances so important
The above chemical postulate formulated by MV Lomonosov proves the fact that during the reaction, the atoms that make up the reagents and products do not disappear anywhere and do not appear from nothing. Their number remains unchanged before and after the chemical process. Since the mass of atoms is constant, this fact logically leads to the law of conservation of mass and energy. Moreover, the scientist declared this regularity as a universal principle of nature, confirming the interconversion of energy and the constancy of the composition of matter.
The ideas of J. Proust as a confirmation of the atomic-molecular theory
Let us turn to the discovery of such a postulate as the law of constancy of composition. Chemistry of the late 18th - early 19th century is a science in the framework of which scientific disputes were conducted between two French scientists, J. Proust and C. Bertollet. The first claimed that the composition of substances formed as a result of a chemical reaction mainly depends on the nature of the reagents. Bertollet was sure that the relative amount of interacting substances also affects the composition of the compounds - reaction products. Most chemists at the beginning of the research supported the ideas of Proust, who formulated them as follows: the composition of a complex compound is always constant and is not dependent on how it was obtained. However, further study of liquid and solid solutions (alloys) confirmed the thoughts of K. Bertollet. For these substances, the law of constancy of composition was not applicable. Moreover, it does not work for compounds with ionic crystal lattices. The composition of these substances depends on the methods by which they are extracted.
Each chemical substance, regardless of the method of its production, has a constant qualitative and quantitative composition. This formulation characterizes the law of constancy of the composition of a substance proposed by J. Proust in 1808. As evidence, he gives the following figurative examples: malachite from Siberia has the same composition as the mineral mined in Spain; there is only one cinnabar substance in the world, and it does not matter from which deposit it is obtained. Thus, Proust emphasized the constancy of the composition of the substance, regardless of the place and method of its extraction.
There are no rules without exceptions
From the law of constancy of composition it follows that when a complex compound is formed, chemical elements are connected to each other in certain weight ratios. Soon in chemical science, information appeared on the existence of substances having a variable composition, which depended on the method of preparation. Russian scientist M. Kurnakov proposed calling these compounds bertollides, for example titanium oxide, heavy water, zirconium nitride.
For these substances, 1 weight part of one element accounts for a different amount of another element. So, in the binary compound of bismuth with gallium, one weight part of gallium accounts for 1.24 to 1.82 parts of bismuth. Chemists later found that, in addition to combining metals with each other, substances that do not obey the law of constancy of composition are in such a class of inorganic compounds as oxides. Bertollides are also characteristic of sulfides, carbides, nitrides and hydrides.
The role of isotopes
Having at its disposal the law of the constancy of matter, chemistry, as an exact science, was able to link the weight characteristic of the compound with the isotopic content of the elements that form it. Recall that atoms of the same chemical element with identical proton but different nucleon numbers are considered isotopes. Given the presence of isotopes, it is clear that the weight composition of the compound can be variable, provided that the elements included in this substance are constant. If an element increases the content of any isotope, then the weight composition of the substance also changes. For example, ordinary water contains 11% hydrogen, and heavy water formed by its isotope (deuterium) - 20%.
Characterization of Bertollides
As we already found out earlier, conservation laws in chemistry confirm the basic principles of the atomic-molecular theory and are absolutely true for substances of constant composition - daltonides. But bertollides have boundaries in which a change in the weight parts of elements is possible. For example, in tetravalent titanium oxide, from 0.65 to 0.67 parts of oxygen per weight part of the metal. Substances of variable composition do not have a molecular structure, their crystal lattices are composed of atoms. Therefore, the chemical formulas of the compounds only reflect the boundaries of their composition. For different substances, they are different. Temperature can also affect the intervals of the weight composition of the elements. If two chemical elements form among themselves several substances - bertollides, then the law of multiple relations is also not applicable to them.
From all the above examples, we conclude: theoretically in chemistry there are two groups of substances: with constant and variable composition. The presence of these compounds in nature is an excellent confirmation of atomic-molecular studies. But the law of composition constancy is no longer dominant in chemical science. But he clearly illustrates the history of its development.