What chemical digestion occurs in the mouth

The first stage of digestion begins in your mouth. Chewing produces saliva, saliva contains enzymes which along with the mastication (chewing) starts to break down food so it's easier to digest once the food reaches your stomach.

The mouth is where carbohydrates begin their digestion, and also the amylase in the saliva digests starches.

"In humans, digestion begins in the oral cavity where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue. There are two types of saliva. One is a thin, watery secretion, and its purpose is to wet the food. The other is a thick, mucous secretion, and it acts as a lubricant and causes food particles to stick together and form a bolus. The saliva serves to clean the oral cavity and moisten the food, and contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucin, a glycoprotein which helps soften the food into a bolus."
Source and further information:
http://en.wikipedia.org/wiki/Digestion#Oral_cavity

chemical digestion changescomplex molecules int o maller molecules that can be easily absorbed and utilized by the cells.
in the mouth enzyme amylase degrades starch to simpler disacharrides and lipid digestion begins with the action of lingual lipase.

Ingestion begins in the mouth when the help of the teeth and tongue breaks down the food mechanically