This document classifies matter as either pure substances or mixtures. Pure substances are uniform throughout and can be elements, which contain only one type of atom, or compounds, which contain two or more elements joined together. Mixtures are not uniform and can be either heterogeneous, with noticeably different parts, or homogeneous, with evenly distributed parts. Mixtures are further classified as solutions, suspensions, or colloids based on the size of particles in the mixture.