Air, syrup and rain are examples of homogenous substances, while pizza, blood and gravel are examples of heterogeneous ones. The components of homogenous substances are uniformly distributed in a single phase, while those of heterogeneous substances are non-uniformly distributed and may be comprised of several phases.
The distinction between homogeneous and heterogeneous substances is important in the classification of chemical mixtures. It is usually possible to separate a heterogeneous substance into its individual components through physical means, such as sieving, filtration or centrifugal separation.
The separation of heterogeneous substances is usually much more difficult, needing distillation, recrystallization or another complex physiochemical processes. Some heterogeneous mixtures can be made homogeneous through a process called homogenization. This process is commonly used to mix milk with cream, a heterogeneous mixture, to yield homogenized milk.
The concepts of homogeneity and heterogeneity can be applied in other branches of science besides chemistry. Homogeneity and heterogeneity can be applied to the structure of galaxies in astronomy, for example. Galaxies with uniform distributions of stars are homogeneous, while those with non-uniform distributions are heterogeneous. In physics, systems with multiple phases of the same type of matter, such as ice, liquid water and water vapor, are called heterogeneous systems.