PS-3.4    Classify matter as a pure substance (either an element or a compound) or as a mixture
               (either homogeneous or heterogeneous) on the basis of its structure and/or composition.
              
Key Concepts
Pure substance:  element, compound
Mixture: homogenous mixture, heterogeneous mixture
    All matter in the universe can be classified (sorted)  into two major categories, Pure substances and Mixtures. Pure substances can be either elements or compounds. Because elements consist of a single kind of atom, the represent a "pure sort" and can be sorted out no more. On the other hand, compounds consist of two or more elements chemically combined. The elements in a compound can only be separated ( sorted) by using chemical means. The statement "chemical means" usually refers to a chemical reaction.  One example of a chemical process that separates a compound into its constituent parts is shown here. 2H2O-->2H2 + O2 . In this chemical change the the compound water is separated the elements Hydrogen and Oxygen. Anytime a compound is separated into its parts, a chemical change must be carried out.
Mixtures can be subdivided into homogeneous mixtures and heterogeneous mixtures. The substances that are in a mixture are NOT chemically combined. As a result, the parts of a mixture can be separated from each other by physical methods such as physically picking out the different parts, filtering, or centrifuging.  The chart below shows how matter is classified.
ELEMENTS
COMPOUNDS
HOMOGENEOUS MIXTURES
HETEROGENEOUS MIXTURES
Pure substance
Made of only one kind of atom.
Cannot be separated into anything simpler than single atoms.
Pure substance
Made of more than one kind of atom chemically bonded to each other. This also means that they are made up of more than one kind of element.
Chemical methods must be used to separate compounds into constituent elements.
Physical combinations of two or more substances.
Particles are atomic or molecular in size.
Can be separated by physical methods such as distillation or chromatography.
Physical combinations of two or more substances.
Often the particles are visible.
Can be separated by physical means such as mechanical separation or  filtering.