The stomach is very acidic (pH = 2-3). So all food gets an acid bath. Most of the breakdown on the molecular level is done by enzymes. Each type of enzyme has a range of pH where it works best. For example, saliva contains an enzyme that breaks starch down into sugar (salivary amylase). When swallowed, the enzyme is denatured (destroyed) in the stomach. More amylase is added in the small intestine to continue the breakdown of starch.

When food is headed for the stomach, the stomach increases the acid output. Tasting, smelling, or even thinking about food gets the acid production up.

Pretty much everything goes through the same tract. What actually digests the food are enzymes - proteins that metabolize the actual food products. These enzymes have specific ranges of pH in which they function, which is why those pH values are needed in the parts of the digestive tract that work on particular food molecules.