Final answer:
A pure substance has a fixed composition, which means it is consistent throughout, unlike a mixture. Pure substances include elements and compounds with unique properties such as melting point and boiling point.
Step-by-step explanation:
The student asked what distinguishes a pure substance from a mixture. To answer this, unlike a mixture, a pure substance has a fixed composition. A pure substance is a form of matter with constant properties throughout the sample and includes elements, which consist of only one type of atom and compounds, which consist of two or more types of atoms. For example, any sample of water (H2O), which is a compound, will have the same proportion of hydrogen and oxygen atoms. In contrast, mixtures can have varying compositions and do not have uniform properties at the most basic level.