Usually by pure substances we mean either pure elements (all one type
of atom), regular crystals (atoms arranged in a repeating pattern), or
things made of only one type of molecule (a tightly bound structure of
one or more types of atoms). So copper is a pure substance in any form
(only copper atoms). Liquid water is a pure substance (all H2O
molecules). Table salt is a pure substance (a regular crystal of Na and
Cl atoms, or, to be picky, ions).
Things that have different types of atoms or molecules, not
arranged in regular patterns, are typically called mixtures. Vodka, for
example, has some ethanol molecules and some water molecules, mixed
together in an irregular, changing pattern. It is at least a solution,
with the molecules mixed on the scale of single molecules. Other things
(like salami) are mixtures of different types of molecules (like fats
and proteins) all clumped up on a much larger scale. Mixtures with
large clumps of dissimilar materials are often called "suspensions".
Air is a complicated mixture of nitrogen, oxygen, argon, water, carbon
dioxide and other molecules, and usually has dust, pollen, soot, or
other particles suspended in it. A mixture of different kinds of metal
atoms is called an "alloy" if they are mixed on an atomic scale.
Mike W. (and Tom J.)
(republished on 07/13/06)