Final answer:
The law of definite proportions, or the law of constant composition, is a principle in chemistry that asserts a chemical compound contains elements in a fixed ratio by mass, which illustrates why compounds have a consistent composition.
Step-by-step explanation:
The law of definite proportions, also known as the law of constant composition, states that a chemical compound always contains its component elements in a fixed ratio by mass and that this proportional relationship applies regardless of the sample's source.
For instance, water (H₂O) always has two hydrogen atoms for every one oxygen atom, corresponding to a mass ratio of approximately 11.19% hydrogen to 88.81% oxygen. This law is foundational to chemistry as it helps explain why compounds have constant composition with respect to mass: the atoms that make up a compound are always present in whole-number ratios, making the substance's makeup consistent and predictable.