Ace Your AP Chemistry Journey 2026 – Unleash the Elements and Score Big!

The law of definite proportions states that a chemical compound always contains its constituent elements in a fixed ratio by mass, regardless of the amount of the compound or how it was prepared. This principle is fundamental in chemistry and emphasizes that for any given compound, the relative masses of the elements present remain constant, no matter the source or method of synthesis.

This concept ensures that if you analyze two samples of the same compound, you would find they contain the exact same elements in the same proportions, differentiating compounds from mixtures. For example, water (H₂O) will always consist of hydrogen and oxygen in a 2:16 weight ratio, leading to the conclusion that 2 grams of hydrogen will combine with 16 grams of oxygen to form 18 grams of water.

The other choices do not accurately describe the law of definite proportions. The first choice speaks to the physical states of a compound rather than its ratios. The third option refers to mixtures, where the proportions can vary, which is different from the fixed ratios observed in compounds. The last choice pertains to mixtures as well, where composition can be uniform or variable, but does not align with the concept of fixed mass ratios in chemical compounds.