Final answer:
The prefix 'hydro-' is used when naming acids if the anion in the compound ends with the suffix -ide, resulting in names such as hydrochloric acid for HCl.
Step-by-step explanation:
When naming acids, the prefix hydro- is used when the name of the acid anion ends in -ide. This applies to acids where the anion accompanying the hydrogen is a monatomic ion.
For example, the acid composed of hydrogen and chlorine (Cl-) is called hydrochloric acid, and this naming convention is followed because the chlorine anion is a chloride ion. Similarly, HCN is named hydrocyanic acid because the anion part is cyanide, which also ends in -ide.
In the case of polyatomic ions ending with -ate and -ite, the naming conventions are different. For anions ending in -ate, the corresponding acid name ends in -ic acid but without the hydro- prefix.
For example, nitrate (NO3-) forms nitric acid (HNO3). For anions ending with -ite, the acid name ends with -ous acid, as in the transition from nitrite (NO2-) to nitrous acid (HNO2).