Final answer:
A pure substance is characterized by a constant composition and properties throughout the sample, with sharply defined melting and boiling points and consistent chemical behavior. Examples of pure substances include elements and compounds. Chromatography is used for testing purity, yielding a single substance for pure substances.
Step-by-step explanation:
The characteristic that best identifies a pure substance is its constant composition, which means it is the same everywhere and shows a single set of properties such as melting point, color, and boiling point. This is contrasted with mixtures, which can vary in their components and properties. For example, substances such as elements, which have a single type of atom, and compounds, which are made of two or more types of atoms in a fixed ratio, are considered pure substances because every sample has the same makeup and properties.
By definition, a pure substance can be either an element or a compound. Elements consist of only one type of atom and cannot be chemically broken down into simpler substances. Compounds consist of two or more types of atoms chemically bonded together. Since pure substances have a constant composition, any sample you take will exhibit the same physical and chemical properties, such as a sharp melting or boiling point. Materials like sucrose (table sugar), consisting of 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen by mass, always show the same properties.
Chromatography serves as a practical technique to test for purity. A pure substance would result in a single component at the end of the chromatographic process, whereas an impure substance would show several components, indicating the presence of a mixture rather than a pure substance.