A heterogeneous material is one whose individual components can be discerned. The material can be separated into its components more easily than a homogenous one, usually through mechanical means.
Heterogeneous materials are non-uniform and composed of diverse parts that occupy the same volume. In chemistry, heterogeneous materials are often composed of different states of matter, but this isn’t always the case. A suspension of solid particles in water is heterogeneous, but so too is an immiscible mixture of water and oil. The former is a heterogeneous material consisting of a solid and a liquid, while the latter consists of two liquids.
Whether two components form a homogenous or heterogeneous mixture depends on the relative sizes of the atoms and the type and strength of their intramolecular interactions. If the atoms of the components interact strongly with one another, they may form a homogenous mixture. Component atoms that do not interact with one another are more likely to form a homogenous mixture. If the component atoms occupy the intramolecular vacancies between one another, they may form homogenous mixtures with little intermolecular interaction. Heterogeneity holds a different meaning in physics, where it implies that the components of a system must exist in different phases.