Iodine crystals are not a homogeneous mixture, as they cannot be physically broken down into pure compounds or elements. Because the only element present in this case is iodine, which crystallizes in the solid phase, iodine crystals can be classified as a pure substance.
Matter can be classified into two categories: mixtures and pure substances. Mixtures are described as either homogeneous or heterogeneous. Homogeneous mixtures have a uniform composition, and heterogeneous mixtures do not. Pure substances can either be elements or compounds. Elements can exist as isolated atoms or in molecule chains, and compounds exist as molecules of at least two different elements.