Gaseous HCl is not an acid at all—it shows no tendency to dissociate into H⁺ and Cl− as the H–Cl bond is strong. But hydrochloric acid—that is, a solution of HCl in water—is a strong acid. The difference is that an isolated proton H+ is too unstable to be encountered under normal conditions, but in water the hydrogen of HCl is transferred to a water molecule and not released as a free species.

The chloride anion is the same in both cases: the only difference is that a very unstable naked proton would have to be the other product in the gas phase but a much more stable H3O+ cation would be formed in water. In fact, it’s even better than that, as other molecules of water cluster round (‘solvate’) the H₃O⁺ cation, stabilizing it with a network of hydrogen bonds. That is why HCl is an acid in water. But how strong an acid is it? This is where chloride plays a role: hydrochloric acid is a strong acid because chloride ion is a stable anion. The sea is full of it! Water is needed to reveal the acidic quality of HCl, and acidity is determined in water as the standard solvent. If we measure acidity in water, what we are really measuring is how much our acid transfers a proton to a water molecule. HCl transfers its proton almost completely to water, and is a strong acid. But the transfer of protons to water from carboxylic acids is only partial. That is why carboxylic acids are weak acids. Unlike the reaction of HCl with water, the reaction below is an equilibrium.