If molecules are too small to see, then how do scientists tell different chemicals apart?
(Lansing State Journal, August 4, 1993)

Some of these tests use direct measurements.  These methods do not alter the chemical composition of the substance, but often give only very limited information.  Some direct measurements include weigh samples on a sensitive scale and comparing its mass to its volume.  This is called a density measurement.  Alternatively, a substances mass can be compared to its ability to be dissolved in a liquid, such as water or acetone.  This property is called solubility.  Other direct measurements include visual inspection of color and texture.  Reddish colors often indicate the presence of iron, and shiny materials are usually metallic.

Indirect measurements alter the properties of a material to get information about its chemical nature.  To make an indirect measurement, one often mixes a known substance with the unknown under investigation.  If a reaction takes place that changes the properties of the unknown, some information about its structure becomes apparent.  For example, dyes are used to determine if a liquid is acid or alkali.  Acid samples react with the dyes to produce one color, while alkali samples react to produce a different color.

Spectroscopy uses light to determine a number of chemical properties, and can be used as a direct or in combination with indirect tests.  When someone shines light on a material, it may be absorbed, transmitted, reflected or scattered.  These properties help to define the composition of different chemicals.  Spectroscopy is a very powerful tool and can be used to identify the exact composition of a material.  More recently, very sophisticated microscopes have been constructed that can actually visualize materials at an atomic level.  However, these machines are very expensive to own and run, so most scientists rely on other methods, such as those described above, to tell different chemicals apart.

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