Can the composition of a compound ever change?

The composition of a compound is defined by the specific ratio of its constituent elements, and this ratio remains constant regardless of the physical state or sample size. A compound is formed through chemical bonds between elements, and their properties, including composition, are determined by these relationships.

For example, water is always composed of two hydrogen atoms and one oxygen atom (H2O). This ratio does not change whether you're looking at a glass of water, an ice cube, or steam. Any variation in composition would imply a change in the chemical identity of the substance, meaning it would no longer be classified as that particular compound.

In contrast, mixtures can have varying compositions, which is a main distinguishing factor between mixtures and compounds. This principle is fundamental in chemistry, as it underpins the law of definite proportions, stating that a chemical compound always contains the same proportion of elements by mass. Therefore, the assertion that a compound's composition stays constant is indeed correct.