The part of a sound wave where the molecules are spread apart is called a rarefaction.

Here's why:

* Sound waves are longitudinal waves: This means the vibrations of the molecules travel in the same direction as the wave itself.

* Compression: When the molecules are close together, it's called a compression.

* Rarefaction: When the molecules are spread apart, it's called a rarefaction.

Think of it like a slinky: You can push the coils together (compression) or pull them apart (rarefaction). These alternating compressions and rarefactions create the sound wave.