# pH of liquid hydrogen chloride vs hydrochloric acid

Is hydrogen chloride in liquid state acidic like hydrochloric acid? I know that hydrochloric acid is hydrogen chloride dissolved in water but I was wondering if liquid hydrogen chloride was also an acidic substance. Is this the same for other acids?

• – Mithoron Jun 12 '16 at 1:37

That said, this is a clear-cut case. In truth, pure hydrogen chloride (gaseous or liquid) is a much stronger acid than aqueous hydrochloric acid. When hydrogen chloride is dissolved in water, it reacts quantitatively to produce hydronium ions $\ce{[H(H_2O)_{n}]^+}$. This is the actual acid present in hydrochloric acid solutions. The fact that this reaction between hydrogen chloride and water is quantitative, fast and very exothermic is a suggestion that the water is taming the acidity of the hydrogen chloride. This is a general phenomenon known as solvent levelling.
From the authors' data , gaseous hydrogen chloride at a pressure of $\mathrm{1\ bar}$ is an acid with an absolute pH ($\mathrm{pH_{abs}}$) of about 160, which is around 30 orders of magnitude stronger than even the most concentrated aqueous hydrochloric acid solution (pH in water approximately equal to -1), which would correspond to a $\mathrm{pH_{abs}}$ of about 190.