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A [[pure substance]] (Latin ''purus'', from [[Greek]] '''πυρ''' or ''pyr'' fire and '''πυροω''' ''pyroo'' I refine) is any substance having unique and defining properties. The concept of a pure substance requires one to separate the substance from all other substances ''without'' any change to that substance. One can, for example, separate the constituent [[metal]]s of an alloy without changing any one of them--but if one separates the [[hydrogen]] and [[oxygen]] in water, one obtains two new substances having properties radically different from those of the compound of which they were once part.

A [[pure substance]] (Latin ''purus'', from [[Greek language|Greek]] '''πυρ''' or ''pyr'' fire and '''πυροω''' ''pyroo'' I refine) is any substance having unique and defining properties. The concept of a pure substance requires one to separate the substance from all other substances ''without'' any change to that substance. One can, for example, separate the constituent [[metal]]s of an alloy without changing any one of them--but if one separates the [[hydrogen]] and [[oxygen]] in water, one obtains two new substances having properties radically different from those of the compound of which they were once part.

Pure substances are either [[chemical element]]s or [[chemical compound]]s. An element or [[molecular compound]] consists of only one type of atom or molecule.<ref>Wile, Dr. Jay L. ''Exploring Creation With Chemistry''. Apologia Educational Ministries, Inc. 1998</ref> An [[ionic compound]] can also be a pure substance, but must contain not more than three different species of ion in a ratio expressible in single digits. Any one of these ions may be monatomic or polyatomic, however.

Pure substances are either [[chemical element]]s or [[chemical compound]]s. An element or [[molecular compound]] consists of only one type of atom or molecule.<ref>Wile, Dr. Jay L. ''Exploring Creation With Chemistry''. Apologia Educational Ministries, Inc. 1998</ref> An [[ionic compound]] can also be a pure substance, but must contain not more than three different species of ion in a ratio expressible in single digits. Any one of these ions may be monatomic or polyatomic, however.