The same chemical compound has a constant composition, regardless of the place and method of obtaining the sample. One of the first to reach this conclusion was the founder of modern chemistry, the French scientist A. de Lavoisier. He was able to determine the composition of water, air, organic substances, believed that particles are part of substances in certain proportions. Later, another chemist from France, J.L. Proust, formulated the law of constancy of composition. Both researchers worked hard to defend their ideas in a dispute with the scientific community.
Views on the structure of matter at the turn of the XVIII and XIX centuries
Chemical elements, connecting with each other in different ratios, form many substances. Each of them is characterized by a specific set of atoms and the ratio of their masses. But until the end of the 18th century, many scientists thought differently. The problem for them was the inaccuracy of quantitative methods. In addition, the positions of atomic-molecular views at that time were unstable, the theory of phlogiston - a mythical element dominated. A significant contribution to the establishment of the quantitative composition of the substance was made by:
- A. de Lavoisier;
- M.V. Lomonosov;
- J. L. Proust;
- D. Dalton.
The merit of J. L. Proust lies in the fact that he improved the quantitative methods of Lavoisier, strictly followed them. The scientist proposed the law of constancy of composition, preservation of the ratios of elements in samples of compounds. Proust began work in this direction at the end of the 18th century, but his works were recognized only in 1808. At about the same time, John Dalton introduced the idea of the atoms and masses of these particles, multiple ratios.
Biography of J. L. Proust
The scientist who proposed the law of constancy of composition was born on September 26, 1754 in the west of France. Joseph Louis's father was a pharmacist in Angers. He introduced his son to experiments with
chemicals. The young man continued his studies in Paris, where he met with Lavoisier and his scientific ideas. In 1776, Proust published his first serious works in the field of chemistry. From 1799 to 1806, the scientist led the laboratory in Madrid. Proust returned to France in 1806. After a brief stay in Paris, Joseph Louis went to his native land. In 1808-1816, his works in the field of studying the composition of substances, including glucose, were recognized. In 1817, he retired and lived alone until the end of his days (1826). Proust was one of the prominent scientists of his time, the Knight of the Legion of Honor, a member of the
Royal Academy of Sciences of Naples.
The law of constancy of composition. Examples
J. L. Proust discovered glucose, became famous for his brilliant victory in a scientific polemic with compatriot Bertollet, the cause of which was the law of constancy of composition. The wording proposed by Proust states: when several elements form a chemically pure sample, then it consists of the same atoms. The ratios of their masses and numbers are also permanent. Examples:
- Sodium chloride (NaCl) can be obtained by reacting hydrochloric acid with sodium hydroxide. The second method is the treatment with hydrochloric acid of sodium carbonate. In two different chemical reactions, we obtain a compound whose formula is NaCl. In both the first and second cases, the substance contains 39.33% sodium and 60.66% chlorine.
- Oxygen (chemical sign O) during the formation of a water molecule combines with the same amount of hydrogen (H). If 1.11 g of hydrogen interacts with 8.89 g of oxygen, then 10 g of water (H 2 O) is formed. An increase in the amount of one of the substances leads to the same result. The atoms of the element that was taken in excess will not react. The mass of water in this experiment remains the same - 10 g, the composition of its molecules is reflected by the formula H 2 O.
The value of discovering the constancy of composition
At the beginning of the XIX century in theoretical chemistry, laws were established that connect substances, describe their relationships. Qualitative and quantitative characteristics were studied by many scientists who proposed universal formulations. Fundamental were the ideas of MV Lomonosov on the conservation of mass in chemical transformations. The law of constancy of composition established by J. L. Proust is also of great importance for science and practice. Based on this regularity, the water formula is written only in the form of H 2 O, and the composition of sulfuric acid is H 2 SO 4 . But Proust’s law is not as comprehensive as the teachings of Lomonosov. Therefore, its wording was clarified after the discovery of isotopes. So called atoms of the same element, having different masses. The ratio of particles in the composition of the sample is a constant, but only subject to a constant isotopic composition. For example, the mass composition of hydrogen and oxygen in ordinary and heavy water is different. The second liquid contains a hydrogen isotope - deuterium. The mass of heavy water is greater than ordinary.
Modern views on the composition of the substance
In accordance with Proust's formulation, the ratio of the masses of the atoms that make up a particular substance is constant, it does not depend on the method of obtaining the sample. At the beginning of the XX century, when studying metal alloys, compounds with a variable composition were discovered. In this case, the unit mass of one chemical element may correspond to different masses of the other element. For example, in compounds of thallium with bismuth per unit mass of the first accounts for 1.2 to 1.8 units of weight of the second element. Such examples can be found among intermetallic substances, oxides, compounds of sulfur, nitrogen, carbon, hydrogen with metals. Consequently, the laws discovered by Proust and Dalton are fully valid only for those substances that have a molecular structure. These include many acids, oxides, hydrides. The qualitative and quantitative structure of such compounds is constant. For example, the composition of water in the atmosphere, oceans, glaciers and living organisms reflects the formula H 2 O.