What is a pure substance?

A pure substance is defined as a material that consists of only one type of particle or component throughout. This singularity means that all its physical and chemical properties are consistent and uniform. For example, elements like gold (Au) or compounds like water (H₂O) are considered pure substances because they are made up entirely of one type of atom or one type of molecule, respectively.

In contrast, a mixture contains two or more different substances that are physically combined, which is characterized by the presence of multiple components varying in proportion. This variability leads to different properties in different samples of a mixture. Additionally, a pure substance does not vary in composition; it remains constant regardless of the sample size or form.

Therefore, recognizing that a pure substance is composed exclusively of a single component clarifies why the choice that identifies a pure substance in this manner is the correct one.