Substances and mixtures

In physical chemistry, A chemical substance is a kind of matter with a definite composition and set of properties.  So it could be a chemical compound or an amount of molecules.

A mixture is two or more chemical substances which have distinct properties that can be separated, usually through physical means due to the different properties that each substance has. 

Separatory_funnel_with_oil_and_colored_water

Set up for a mixture of oil and water. image: wikimedia

A container filled with sand and iron dust can be separated by a magnet for example. 

For example air is a mixture of various gases. Metal alloys are also mixtures.

There are  various kinds of mixtures that often depend on the phases of the two substances. 

A colloid is a substance microscopically dispersed throughout another substance 

An emulsion is a mixture of two or more liquids that are normally immiscible (nonmixable or unblendable). Emulsions are part of a more general class of two-phase systems of matter called colloids.

Mixtures of oil and water are also examples of substances that can be separated.