Why does alkali elements react vigorously with water?

Explanation:

Because the alkali metal is really willing to give up its electron much more so than Hydrogen.

The redox reaction looks like this

K ===> K+   +    1 e-    The reduction vo is very high which means that the K really does want to give up that electron.

2H+  +2e- > H2  The oxidation vo is almost 0 but the Hydrogen is the perfect recipient for the 2 electrons.

increased stability in the completion of fluorine's outer shell.

chemical digestion occurs in the stomach were hcl and pepsin forms chyme, the mouth in which salivary amylase breaks down starch to form bolus, and the small intestine specifically the duodenum.