Compare the musical elements of classical music and impressionistic music.

When
contrasting classical music and impressionist music, let's assume "classical" refers
to the music of Haydn and Mozart's era. With that being the case, these are two very different
types of music. 

The most obvious difference is tonality. Classical music
draws on the tonality codified by J.S. Bach, that of major and minor scales. In contrast,
impressionist music uses expanded tonality, where the chords are not so clearly major or minor.
These composers stack notes upon notes such that there is much less of a sense of a key
center. 

A second difference would be instrumentation. Orchestras of Haydn's
and Mozart's time had fewer instruments, and some of the instruments were not in their final
form. For example, the French horn of the classical period was a natural horn (no valves). In
contrast, impressionist music makes use of a wider range of instruments and the instruments are
advanced enough that they are much less acoustically limited. Impressionist orchestrations
reflect this fact. 

Related to the differences in orchestras between the two
periods, there is a contrast between composers' approaches to tonal color. During the classical
era, composers did not "paint" with sound in quite the same way as the composers of
the impressionist era did. You can hear this in the opening bars of Debussy's "Afternoon of
a Faun," where the flute and the harp establish a rich sonority that signals the beginning
of a story, the sonic equivalent of a play taking place in one of Monet's garden
paintings.