Question

When naming an acid, what does the prefix hydro tell you?

Answer 1

Concerning or using water (Longman Dictionary of English Language and Culture, 1992).

Explanation:

Hydrochloric acid (HCl) or Hydrofluosilicic acid (H2SiF6) are acids that contain hydrogen.

Reference:

Longman Dictionary (1992) Longman Dictionary of English Language and Culture. Essex, England.

Answer 2

It denotes that it is an acid formed via the dissolution of a hydrogen halide in water.

Explanation:

The hydrogen halides are diatomic compounds of hydrogen and either chlorine, fluorine, bromine or iodine (or astatine to be completely accurate).

Hydrogen halides dissolve in water and easily dissociate yielding `H_3O^+` ions (and halide ions in solution). Therefore the solutions are known as "hydrohalic acids" (hydrochloric, hydrofluoric, hydrobromic, etc).

NB: "Hydro" does not simply denote the presence of a hydrogen atom in the molecule. Acetic (ethanoic) acid has hydrogen atoms in the molecule, but is not named using the prefix "hydro".