The law of constant proportion, also known as the law of definite proportions, was proposed by Joseph Proust. This law states that in a given compound, the elements are always present in definite and fixed proportions by mass.

Explanation:

Background:
Before the law of constant proportion was proposed, there were different theories regarding chemical compounds and their composition. Some scientists believed that elements combine in variable proportions to form compounds, while others thought that elements combine in fixed proportions. Joseph Proust's experiments and observations led him to propose the law of constant proportion.

Statement of the Law:
The law of constant proportion states that in a given compound, the elements are always present in definite and fixed proportions by mass. This means that regardless of the source or method of preparation, a compound will always contain the same elements in the same proportions. For example, water (H2O) will always consist of two hydrogen atoms and one oxygen atom in a fixed ratio of 2:1 by mass.

Experimental Evidence:
Joseph Proust conducted numerous experiments to support his law. One of his famous experiments involved the analysis of copper carbonate. He heated a sample of copper carbonate and observed that it decomposed into copper oxide (CuO) and carbon dioxide (CO2). By analyzing the masses of the reactants and products, he found that the ratio of copper to oxygen in copper carbonate was always 1:1 by mass, regardless of the source or method of preparation.

Significance of the Law:
The law of constant proportion is significant because it laid the foundation for the development of atomic theory. It implied that elements combine in fixed ratios because they are made up of discrete particles called atoms. This idea was further supported by the work of John Dalton, who proposed the atomic theory based on Proust's law and other experimental evidence.

Conclusion:
In conclusion, the law of constant proportion, proposed by Joseph Proust, states that in a given compound, the elements are always present in definite and fixed proportions by mass. This law provided crucial evidence for the development of atomic theory and has played a significant role in our understanding of chemical compounds.