Matter can be divided into two main types, pure substances and mixtures. Pure substances are elements and compounds. Elements exist as atoms (or molecules of the same type of atom) and compounds exist as molecules made of more than one type of atom. Pure substances are different from mixtures in that all of their particles are identical. For example, a sample of the compound water contains only H2O particles. Compounds always contain the same ratio of elements.
Mixtures, which contain more than one type of particle, fall into two groups: Homogeneous mixtures, which are the same throughout, and heterogeneous mixtures, which have particles that are unevenly distributed. Salt water is an example of a homogeneous mixture and soil is an example of a heterogeneous mixture. Mixtures can vary in composition, for example rubbing alcohol is available as 70% alcohol and as 90% alcohol.
Mixtures can be separated by physical means such as distillation or filtration. The elements the make up a compound can only be separated if a chemical reaction takes place.
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