How can you tell the difference between a substance and a mixture?

A substance is a general word that can be used for any
kind of material including elements, compounds and mixtures. Perhaps the the word
substance is used by mistake instead of compound. A third type of substance often
referred in chemistry is an element.

An element is a
substance that has only one type of atoms in its molecules. A compound is a substance
formed by chemical combination of more than one elements. The molecules of compounds
contain more than one type of atoms. However, the molecules of any one compound are all
alike in composition.

A mixture is a combination of two or
more substances that have not combined chemically. The constituent substances can be
elements, compound, or a combination of the two. A mixture consists of combination of
molecules of different types.