Mozart’s Sonata No. 7 in C major, K. 309, begins with an Allegro con spirito that sets broad, forceful gestures against much lighter melodic writing. Octaves and chords give the opening its weight, while broken-chord accompaniment and quick upper figures keep the quieter phrases moving. The music repeatedly exchanges these two kinds of texture as it passes through a contrasting middle section and a changed return.

Opening idea: A quick ornamental note leads into two long upper notes over bass octaves, followed by a dotted figure shared by the hands. A softer detached melody answers over a regular left-hand broken-chord pattern. Stronger octave statements and running sixteenths interrupt this lighter texture, and repeated accompaniment chords later support a rising upper line.

Secondary idea and first close: The contrasting theme first appears in the left hand, with the right hand joining afterward. Upper ornaments and sustained notes give it a different profile from the opening. A long trill sounds above the accompaniment as the passage moves toward its cadence and the first section’s repeat.

Middle section and return: The opening material reappears in a darker harmonic setting. Trills and running figures continue over broken chords, and the left hand sometimes takes the quicker line beneath longer upper notes. The returning opening includes altered harmonies and double-note figures. The secondary idea now starts in the right hand, reversing its earlier order of entry. A longer closing passage brings back the trill and finishes by recalling the opening gesture.

Interesting fact: The secondary theme changes hands between its first appearance and its return. Listening for the lower melody first, then the upper melody later, reveals a small but clear change in the movement’s design.