How does the slow movement of a Classical sonata contrast with its first movement?

In the Classical sonata, the first movement is typically a fast, serious allegro structured in sonata form, which serves as the weightiest part of the work.12

The second movement provides a deliberate contrast by utilizing a slower tempo, often characterized as andante or adagio, and a more lyrical quality. While the first movement follows a highly uniform structural pattern, the slow movement is less predictable in its organization. It frequently employs:12

• Sectional forms: Such as ternary (A B A) or other recurring patterns.12
• Variation form: A theme followed by a series of variations.12
• Rondo form: Where a principal theme alternates with intervening themes.12

Though less common, the slow movement may also be written in sonata form; however, when this occurs, the development section is often omitted to maintain appropriate proportions. Ultimately, the slow movement functions to complement the first movement by offering a new, related range of musical expression.1