When we take a closer look at substances, we discover that they consist of either one single element or of mixtures of several elements. Chemists therefore divide the world of substances into pure and mixed chemical substances. A pure substance is of homogeneous composition. Substance mixtures, however, consist of two or more pure substances. The many mixtures are subdivided not only into homogeneous and heterogeneous mixtures but depending on the respective aggregate states of their components, are classified into various groups of mixtures.