A mixture is an association of two or more substances that are not chemically combined.  There are two types of mixtures, heterogeneous and homogeneous.  A heterogeneous mixture is a substance making up the mixture which is not spread uniformly throughout.  Substances from different parts of the mixture have different compositions.  The other type of mixture is a homogeneous mixture.  In this mixture components are uniformly combined.  This happens because the particles are small enough and are easily spread throughout the substance.  Solutions are homogeneous mixtures made up of very small particles.  These particles are molecules, atoms or ions.  A solution is very important in experiments because solids with not react with other solids (Dorin 332).