Saliva is generally acidic in nature. There are few acids (slightly acidic) present in saliva digests or disintegrates food particles that are stuck in mouth. Just like acids produced in stomach bag to digest food we eat.
If there are no digestive acids present in mouth, then the small food particles that get stuck when we eat will attract microbes that feed dead, which also affects other body parts too.
These acids damage our tooth which have Calcium, to compensate these acids that are produced in mouth, we brush our tooth with pastes that are basic in nature. And drinking water also controls pH levels in our mouth.
Human saliva is 99.5℅ water while the other 0.5 ℅ consists of mucus ,enzymes, and antibacterial compounds