This document defines the classification of matter. There are two main categories: pure substances and mixtures. Pure substances include elements, which are made of only one type of atom, and compounds, which are two or more elements chemically bonded together. Mixtures contain two or more pure substances mixed together without chemical bonding. Mixtures can be either heterogeneous, where the parts can be seen, or homogeneous, where the parts cannot be seen. Heterogeneous mixtures are less pure than homogeneous mixtures.