Mixtures can be either homogeneous or heterogeneous [local]. A homogeneous mixture is one in which the components are uniformly distributed. This also defines a solution. A homogeneous mixture may use any phase of matter as components. An alloy is a homogeneous mixture or solution of metals. Solutions involving liquids as the major component should have particle sizes of 1 nm or less. Larger particle sizes produce a mixture that is not uniform in composition.

Heterogeneous mixtures have at least two visible phases. Most of the time this is obvious when you look at the material, you can see differences in color or texture. An example of this is granite, breakfast cereal with milk, and wood. Some mixtures are more subtle, milk scatters light and thus must contain solid particles as well as the liquid water. Blood contains both red and white cells along with the fluid. Since the both types of blood cells are relatively large (gt; 2000nm) they are not uniformly distributed. They can easily be seen under low power magnification.

Our perceptions of homogeneous and heterogeneous are a reflection of how closely a substance is examined. Notice that, in this scanning electron microscope image of toilet paper at 500X magnification, the differences are much more obvious than when viewed without assistance.