The process of digestion involves a series of chemical and physical changes to food substances. When we put food into our mouths and chew it, it is broken down into smaller pieces for easier digestion by the stomach. This is a physical change. But what happens in the mouth isn't just a physical change--the introduction of saliva triggers an enzymatic breakdown of foods. Sometimes, we can taste this chemical reaction as it occurs. When starchy foods are broken down by saliva and the action of chewing, they can take on a sweeter flavor. Try chewing on a plain cracker for as long as you can, and you may notice this sweet chemical change.
After food passes into the stomach, acids begin to break down the chewed-up mush even more. The actions of the upper digestive tract (including the mouth, esophagus, and stomach) work to break down foods as much as possible so that the nutrients within can be more readily absorbed by the lower digestive system.
In the small intestine, all of the nutrients in the now-soupy mixture of foods are beginning to be absorbed by the body. Secretions like bile can help further break down fatty substances, which is another chemical change. As this mush passes through the small intestine, much of the nutrients and liquid are absorbed through intestinal villi and put to work in the blood stream. From the large intestine onward, the changes are mostly physical, as more liquid is removed from the stool before it exits the body.