Which statement best describes pure substances?

Pure substances are defined as materials that have a uniform and definite composition. This characteristic allows them to be classified into two primary categories: elements and compounds.

Elements are pure substances that consist of only one type of atom and cannot be broken down into simpler substances by chemical means. Compounds, on the other hand, are substances formed when two or more elements are chemically bonded together in a fixed ratio. Both elements and compounds exhibit consistent properties, such as boiling point, melting point, and density, that distinguish them from mixtures.

The other statements do not accurately describe pure substances. For instance, the idea that they have a variable composition refers to mixtures rather than pure substances. Additionally, while some pure substances can exist as solids at room temperature, many can be gases or liquids, so stating that they always exist as solids is misleading. Lastly, pure substances are not mixtures; mixtures consist of two or more pure substances that are physically combined and can vary in composition. Thus, the classification of pure substances as either elements or compounds is the most accurate statement.