Tutoring chemistry, naming acids might arise. The tutor brings up a time to use hypo in naming an acid.

From a simple point of view, the hypo acid has one less oxygen than the -ous acid. For example:

chlorous acid: HClO₂

hypochlorous acid: HOCl

Source:

Mortimer, Charles E. Chemistry, sixth ed. Belmont: Wadsworth, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.