Chemical Classification

Classifying matter by chemical composition gives a different, and more chemically meaningful, split: mixtures versus pure substances.

• A mixture consists of two or more chemical entities (elements and/or compounds) present together without any chemical reaction between them -- their individual identities and properties are preserved, and they can, in principle, be separated by physical means. Mixtures are further split by appearance into:

  • Homogeneous mixtures, which look uniform throughout (e.g. green tea, where the dissolved substances are evenly distributed and you cannot see separate components).
  • Heterogeneous mixtures, which visibly contain distinguishable regions or phases (e.g. oil and water, which separate into visible layers).

• A pure substance is composed of only one kind of simple atom or molecule, and is further split into:

  • Elements, which contain only one type of atom. An element can exist as monatomic units (a single atom acting as the basic unit, e.g. gold Au, copper Cu) or as polyatomic units (several atoms of the same element bonded together, e.g. hydrogen H₂, phosphorus P₄, sulphur S₈). Recall that an atom itself is the smallest electrically neutral particle of matter, built from electrons, protons and neutrons.
  • Compounds, which contain molecules made of two or more atoms of different elements chemically bonded together (e.g. carbon dioxide CO₂, glucose C₆H₁₂O₆, hydrogen sulphide H₂S, sodium chloride NaCl). …