The part of a longitudinal wave where the particles are spread apart is called a rarefaction.

Here's why:

* Longitudinal Waves: These waves travel by compressing and expanding the medium they travel through. Think of a slinky or a sound wave.

* Compression: The area where the particles are close together is called a compression.

* Rarefaction: The area where the particles are spread apart is called a rarefaction.

So, compressions and rarefactions alternate in a longitudinal wave, carrying energy through the medium.