What is a Mixture?

A mixture contains two or more substances combined in such a way that each substance retains its chemical identity.  When you shake copper sulfate with sand, the two substances do not react with one another.  In contrast, when sodium is exposed to chlorine gas, a new compound, sodium chloride, is formed.
There are two forms of mixtures:
1.  Homogeneous or uniform mixtures are ones in which the composition is the same throughout.  Another name for a homogeneous mixture is a solution, which is made up of a solvent, usually taken to be the substance present in largest amount, and one or more solutes.  Most commonly, the solvent is a liquid, whereas solutes may be solids, liquids, or gases.  Soda water is a solution of carbon dioxide (solute) in water (solvent).  Seawater is a more complex solution in which there are several solid solutes, including sodium chloride; the solvent is water.  It is also possible to have solutions in the solid state.  Brass is a solid solution containing the two metals copper (67%-90%) and zinc (10%-33%).
2.  Heterogeneous or nonuniform mixtures are those in which the composition varies throughout.  Most rocks fall into this category.  In a piece of granite, several components can be distinguished, differing from one another in color.