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According to the Arrhenius definition of acids: An Arrhenius acid is any species that increases the concentration of hydrogen ions in its aqueous solution.

Hydrogen Chloride gas ionises in water to give hydrogen and chlorine ions. Thus, it increases the concentration of hydrogen ions in the solution. So is hydrogen chloride gas considered to be an Arrhenius acid or is the aqueous solution of hydrogen chloride, i.e., HCl(aq.) gas an Arrhenius acid?

If the aqueous solution is an acid then why is hydrogen chloride gas referred to as an ‘acidic’ gas?

Similarly, is NaOH(s) an Arrhenius base or is NaOH(aq.) an Arrhenius base?

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    $\begingroup$ The thing you're missing is that substances rather act as acid or base, depending on situation, then are an acid or base. Both HCl and NaOH do act as Arrhenius acid and base in aq. sol. Not so much in solid or gaseous phase. $\endgroup$ – Mithoron Sep 3 '20 at 19:12
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    $\begingroup$ As a complement to Mithoron, I would add that pure gaseous $\ce{HCl}$ is $NOT$ an acid. For example, it does not react with solid and anhydrous calcium carbonate according to : $$\ce{CaCO3(s) + HCl(g) -> no reaction }$$ As soon as some droplet of water exists, HCl reacts according to : $$\ce{CaCO3(s) + 2 HCl(aq) -> CaCl2 + H2O + CO2}$$ Of course $\ce{HCl(aq)}$ should be rewritten as made of ions $\ce{H+ + Cl-}$ or even $\ce{H3O+ + Cl-}$ $\endgroup$ – Maurice Sep 3 '20 at 19:50

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