When naming an acid, what does the prefix hydro tell you?
Concerning or using water (Longman Dictionary of English Language and Culture, 1992).
Hydrochloric acid (HCl) or Hydrofluosilicic acid (H2SiF6) are acids that contain hydrogen.
Longman Dictionary (1992) Longman Dictionary of English Language and Culture. Essex, England.
It denotes that it is an acid formed via the dissolution of a hydrogen halide in water.
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
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".