What does the Law of Constant Composition refer to?

The Law of Constant Composition, also known as the Law of Definite Proportions, states that a given compound always contains the same proportion of elements by mass, regardless of the sample size or source of the compound. This means that the relative number of atoms of each element in the compound remains constant. For example, water (H2O) always consists of two hydrogen atoms and one oxygen atom; this ratio does not change whether you have a single drop of water or an entire river.

This principle is foundational in chemistry because it underscores the idea that compounds have specific formulas. It implies that when a chemical reaction occurs, the masses of the reactants and products are also consistent with this composition, thereby allowing scientists to predict how substances will behave in various reactions.

The other options do not accurately reflect this law. Transformations in atomic structure are covered by different concepts in chemistry, and the notion that all compounds have identical properties is not true as different compounds can have vastly different characteristics despite having the same elemental composition.