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    Everyone likes to group things. Language students group words as verbs, nouns, and so on; collections of words are classified as phrases, or clauses, or sentences, and these again are reclassified according to their function. In the same way, botanists classify plants as algae, or fungi, or gymnosperms, etc. Zoologists classify animals as vertebrates and invertebrates. The vertebrates can be further classified as mammals, reptiles, birds, fish, etc. classification enables us to keep hold of more information and, if it is based on the right data, enables us to understand better the ideas we are studying.

    Chemists are no exception. The chemical classification of materials, if it is based on a good system, should enable us to understand better the many substances which exist in our world. What is to be the basis of our classification? Perhaps the most obvious one is appearance. Materials could be classified as solid, liquid or gas with some mixed type as, for example, mud being solid/liquid material and steam a liquid/gas material. Appearance could enable us to subdivide our main classifications groups a little further; the solid may be green, or black, powdery or crystalline; the liquid may be colored, oily, thick, or free flowing; the gas may be colored. However, we soon realize that many probably quite different materials have the same appearance. Both air and the deadly-monoxide gas are colorless, odorless gases, but we would like not to group them as the same thing. Many different liquids are colorless, water-like materials.

The paragraph following the passage most likely deals with the classification of ....