Is the stomach chemical or mechanical digestion?

It is both:

Mechanical: The stomach is a muscular sack. It contracts and relaxes, sloshing and squeezing the food inside to break it down and to ease the job of the 'chemicals'. Another example is how many birds swallow stones to enhance the crushing and churning in their stomachs.

Chemical: The conditions and enzymes within the stomach contribute to chemical digestion. Hydrochloric acid, for example, creates an acidic environment that denatures proteins; whilst enzymes such as pepsin react at a molecular level with peptides to cleave them apart.