What is the law of definite proportions?

The law of definite proportions states that a chemical compound is made up of elements that combine in fixed, definite ratios by mass, regardless of the amount of the compound formed. This principle indicates that if you take any sample of a pure compound, the ratio of the masses of the elements that compose it will always be the same. For example, in water (H2O), the ratio of hydrogen to oxygen by mass is always approximately 11:89, no matter how much water you have. This consistency supports the idea that compounds are uniform in their composition, confirming the fixed proportions of elements in compounds.

The other options describe different concepts in chemistry that do not align with the law of definite proportions. Elements mixing in any ratio relates more to mixtures rather than compounds, while a variable composition suggests a lack of fixed ratios, which is contrary to the definition of compounds. Lastly, stating that all elements are found in all compounds is incorrect because compounds are specific combinations of certain elements, not just any combination of elements.