acids are compounds that have been dissolved in water. In terms of this compund... eg Hydrogen Chloride for Hydrochloric Acid, when dissolved in water it undergoes the following change; HCl(g) --> HCl(aq) Hydrogen Chloride is a gas to start with - it could be considered volatile because if you had some in its liquid form, because the boiling point is below room temperature it would vaporise into its gaseous form. this volatility can be removed by dissolution in water, because of the state change from gas to aqueous. When dissolved in water, HCl doesn't exist as Hydrogen Chloride but rather as Cl- and H+ ions - the H+ ions (protons) being what gives the solution its acidity. Hydrogen Chloride is not an acid in its gaseous form because the Hydrogen ions are still part of the compound and cannot be donated. An acid is a substance which donates hydrogen ions (protons).