list the chemical changes done by a molecuel of starch from it's place&mouth

Your question is not very clear. I am assuming you want to know about the digestion of starch beginning from mouth.

The process of starch digestion begins in the mouth when an enzyme in saliva (amylase) begins to break down starchy carbohydrates into disaccharide maltose. The salivary secretions accompany the food to the stomach and salivary digestion of starches continues in the stomach for a long time, if the food was eaten under correct conditions.

The partially digested starch proceeds to the duodenum, where further starch-splitting enzymes are secreted. Starch that escapes digestion in the stomach, may later be acted upon by pancreatic and intestinal enzymes. The disaccharides are finally converted into glucose in the small intestine.