A heterogeneous mixture is a type of combined substance classified in chemistry. In this type of mixture, the different components are distinctly visible and possess clearly identifiable separate properties with no blending of substances or traits. Heterogeneous mixtures can involve any combination of gasses, liquids, or solids, so long as no chemical transformation, alteration, or catalyst is present. The mixture requires no specific composition or ratio of ingredients for the various ingredients to retain their properties. Everyday examples of heterogeneous mixtures include trail mix, air, or oil floating on the surface of water.

In chemistry, mixtures — especially a heterogeneous mixture — differ from compounds because the mixture possesses no unique chemical properties caused by the combination of ingredients. Compounds create new chemical bonds when the ingredients are blended; mixtures may combine without altering the chemical properties of the mixture even if some characteristics may seem to differ. For instance, adding olives to a salad will create a heterogeneous mixture that may alter the flavor of the salad, but doesn't alter the physical composition of the lettuce or dressing. The lettuce is still lettuce, and not some other substance transformed by the olives.

A heterogeneous mixture differs from a homogenous mixture by the distribution of its materials. Heterogeneous mixtures possess components distributed unevenly. This is opposed to an equal homogenous blend, with proportionate ratios of the ingredients present throughout.

A particular type of heterogeneous mixture is the suspension, which involves solid particles of relatively large size suspended and distributed in a liquid or gas. When solids in a suspension begin to settle, the process is called sedimentation. Although for a time, the mixture may appear evenly distributed and wholly blended, over time the suspension will always settle, with the heavier solid sinking below the suspension medium or to the bottom of a container.

Heterogeneous mixtures can be separated by simple processes of separation or filtration, such as separating wheat from chaff or straining noodles from water. Components with different densities can be separated by the process of flotation, in which lighter components rise while heavier ones sink. Filtration is used to separate mixtures containing at least one solid ingredient. Conversely, compounds often require heating or some other chemical catalyst to trigger the reaction that would separate the ingredients.

The word "heterogeneous" itself originates from two Greek words. The first, heteros, means "different," and the second, genos, refers to a type or kind of thing. Together, they become "different thing," or a mixture of different things.