The Law of Constant Composition, also known as the Law of Definite Proportions, states that a pure compound is always composed of the same elements in the same proportion by mass. This means that the ratio of the masses of each element in a compound is always constant, regardless of the source or method of preparation of the compound.
The Law of Constant Composition was first proposed by French chemist Joseph Louis Proust in the late 18th century. Proust conducted experiments on the composition of copper carbonate and iron carbonate, and found that they always contained the same proportions of copper, carbon, and oxygen, regardless of their source.
The Law of Constant Composition is a fundamental principle in chemistry and is used to identify and classify substances. It allows chemists to determine the chemical formula of a compound based on the masses of its elements, and to verify the purity of a substance by comparing its composition to the expected proportions.
There are a few rare exceptions to the Law of Constant Composition, such as isotopic variations in elements or compounds. For example, water can exist as either H2O or D2O (deuterium oxide), which have slightly different proportions of hydrogen and oxygen due to the presence of the heavier isotope deuterium.
Chemical reactions involve the rearrangement of atoms and the formation of new compounds. The Law of Constant Composition applies to the reactants and products of a chemical reaction, ensuring that the total mass of elements in a closed system remains constant. This allows chemists to accurately calculate the amounts of reactants needed to produce a desired amount of product.