Which of the following statements best explains the Law of Constant Proportions?

Correct Option: A chemical compound always contains the same elements in the same proportion by mass

Explanation:

The Law of Constant Proportions (also known as the Law of Definite Proportions or Proust's Law) was stated by Joseph Proust in 1799.

This law states that a pure chemical compound, regardless of its source or method of preparation, always consists of the exact same elements combined together in a fixed ratio by mass.

For example, pure water ($H_{2}O$) always contains hydrogen and oxygen in a mass ratio of 1:8, whether it is obtained from a river, rain, or synthesized in a laboratory.

Let us evaluate the given options:

1. "Elements are always found in random ratios in a compound." - Incorrect, because compounds have definite, fixed ratios, not random ones.

2. "Atoms can be divided into smaller particles during a reaction." - Incorrect, this relates to atomic structure, not the Law of Constant Proportions.

3. "Mass can neither be created nor destroyed in a chemical reaction." - Incorrect, this describes the Law of Conservation of Mass, not Constant Proportions.

4. "A chemical compound always contains the same elements in the same proportion by mass" - Correct, as it directly states the Law of Constant Proportions.