Law of constancy of composition: formulation, examples, meaning

The same chemical compound has a constant composition, regardless of the place and method of obtaining the sample. To such a conclusion one of the first came 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 the particles are part of the substances in certain proportions. Later another scientist from France, J. L. Proust, formulated the law of constancy of the composition. Both researchers put a lot of effort into defending 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 a multitude of substances. Each of them is characterized by a certain set of atoms and the ratio of their masses. But until the end of the XVIII century, many scientists believed otherwise. The problem for them was the inaccuracy of quantitative methods. In addition, the positions of atomic-molecular views at that time were unsteady, the theory of the phlogiston-the mythical element prevailed. In determining the quantitative composition of the substance, a significant contribution was made by:

• A. de Lavoisier;
• MV Lomonosov;
• J.L. Proust;
• D. Dalton.

The merit of JL Proust lies in the fact that he perfected the quantitative methods of Lavoisier, followed them strictly. The scientist proposed a law of constancy of composition, preservation of the relationships of elements in compound samples. Proust began to work in this direction at the end of the 18th century, but his works were received only in 1808. At about the same time, John Dalton introduced the idea of atoms and masses of these particles, multiple ratios.

Biography of JL Proust

The scientist who proposed the law of constancy of composition, was born on September 26, 1754 in the west of France. The father of Joseph Louis was an apothecary in Angers. He also introduced his son to experiments with chemicals. The young man continued his studies in Paris, where he met Lavoisier and his scientific ideas. In 1776 Proust published his first serious works in the field of chemistry. From 1799 to 1806, the scientist headed the laboratory in Madrid. In France Proust returned in 1806. After a brief stay in Paris, Joseph Louis went to his native place. 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 outstanding scientists of his time, Knight of the Legion of Honor, a member of the Royal Academy of Sciences of Naples.

Law of constancy of composition. Examples

J.L. Proust discovered glucose, became famous for his brilliant victory in scientific controversy with compatriot Berthollet, whose reason was the law of constancy of the composition. The formulation proposed by Proust says: when several elements form a chemically pure sample, it consists of the same atoms. Relations between their masses and numbers are also permanent. Examples:

1. Sodium chloride (NaCl) can be produced by the reaction of hydrochloric acid with sodium hydroxide. The second method is the treatment of sodium carbonate with hydrochloric acid . In two different chemical reactions, we obtain a compound whose formula is NaCl. In both cases, the substance contains 39.33% sodium and 60.66% chlorine.
2. Oxygen (chemical sign O) when a water molecule is formed combines with the same amount of hydrogen (H). If 1.11 g of hydrogen reacts with 8.89 g of oxygen, 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 reflects the formula H ₂ O.

The importance of the discovery of constancy of composition

At the beginning of the nineteenth century, theoretical laws established the laws that connect substances, describe their relationships. We studied the qualitative and quantitative characteristics of 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 JL Proust is also of great importance for science and practice. On the basis of this pattern, the water formula is written only in the form of H ₂ O, and the sulfuric acid composition is H ₂ SO ₄ . But the Proust law does not have such a comprehensive character as Lomonosov's teaching. Therefore, his formulation was clarified after the discovery of isotopes. This is the name for atoms of the same element, which have different masses. The ratio of particles in the composition of the sample is a constant, but only under the condition of an unchanged isotopic composition. For example, the mass composition of hydrogen and oxygen in ordinary and heavy water is different. The second liquid contains the isotope of hydrogen-deuterium. The mass of heavy water is more than usual.

Modern views on the composition of matter

In accordance with Proust's formulation, the mass ratio of the atoms composing a certain substance is of a constant nature, does not depend on the method of obtaining the sample. At the beginning of the 20th century, when studying metal alloys, compounds with a variable composition were discovered. In this case, different weights of another element can correspond to a unit of weight of one chemical element. For example, in compounds of thallium with bismuth per unit mass of the first one, there are 1.2 to 1.8 units of weight of the second element. Such examples can be found among intermetallic substances, oxides, sulfur compounds, 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, the oceans, glaciers and living organisms reflects the formula H ₂ O.