What does the law of definite proportions state?

The law of definite proportions states that a chemical compound always contains the same proportion of elements by mass, regardless of the source or how it was prepared. This principle emphasizes that the ratio by which elements combine to form a compound is fixed and unchanging. For example, water (H₂O) will always be composed of 2 hydrogen atoms and 1 oxygen atom, which translates to a specific mass ratio, regardless of whether the water comes from the ocean, a glacier, or a laboratory.

This law is fundamental to the understanding of chemical formulas and reactions, as it supports the idea that compounds have consistent and predictable compositions, allowing chemists to calculate amounts and predict behaviors in chemical reactions accurately.