Final answer:
A pure substance is matter with a constant composition and properties, which can be an element or a compound, and cannot be physically separated. Elements consist of one type of atom, while compounds consist of two or more types. Mixtures, which differ from pure substances, can be separated into their components physically.
Step-by-step explanation:
When considering pure substances, it's important to note that they possess a constant composition and set of properties throughout the given sample. Pure substances fall into two main categories: elements and compounds. An element is a substance that cannot be chemically broken down into simpler substances and consists of a single type of atom. Examples include aluminum and oxygen. On the other hand, a compound is composed of two or more types of atoms and can be broken down into simpler chemical substances. Water, for instance, is a compound made of hydrogen and oxygen atoms.
Contrary to pure substances, mixtures are combinations of two or more substances that can be separated by physical means. Mixtures retain the individual properties of their components, such as in the combination of table salt and ground pepper, which can be separated with patience. Mixtures come in two forms: homogeneous, where the composition is the same throughout, and heterogeneous, where the composition may vary.
Thus, the key characteristics of a pure substance include being a single kind of matter with a specific makeup that cannot be separated physically, as these would require chemical changes. Pure substances are identifiable because they have distinct and consistent physical properties such as melting point, boiling point, and color. The distinction between a pure substance and a mixture is essential in chemistry for understanding the nature of materials and how they can be utilized or transformed.