Pure substances are defined as?

Pure substances are defined as materials that have a uniform and definite composition. They can be categorized into two main types: elements and compounds. Elements are basic substances that cannot be broken down into simpler substances by chemical means. Compounds, on the other hand, consist of two or more elements that are chemically bonded together in a fixed proportion.

This definition emphasizes that pure substances maintain consistent properties throughout and cannot be separated into other substances by physical processes. The other choices include terms that either indicate mixtures or lack specificity in defining pure substances. Homogeneous and heterogeneous mixtures involve combinations of different substances, which can vary in composition and do not meet the criteria for being classified as pure substances.