Bronsted-Lowry theory states that an acid is a molecule that will drop off #H^+# ions, and a base is a molecule that will pick them up again.
For example,
#H_2O + HCl -> H_3O^+ + Cl^-#
In this situation, water is gaining a proton (#H^+# ion), so it is a base, while #HCl# is giving one away, so it is an acid, according to Bronsted-Lowry theory. This is because #"HCl"# is a stronger acid than #"H"_3"O"^(+)#.
However, in rare cases, it shouldn't be ruled out that #HCl# can be amphoteric, meaning it can act as an acid or a base. For example, in the reaction
#HBr + HCl -> Br^(-) + H_2Cl^+#
then hydrochloric acid is accepting a proton, meaning it is acting like a base. This is only plausible because #"HBr"# is a stronger acid.
But it depends on the #"pKa"# of #"H"_2"Cl"^(+)# relative to #"HBr"#. If the #"pKa"# of #"HBr"# is higher, then the reaction wouldn't go to completion, as #"Br"^(-)# would then want to grab a proton from #"H"_2"Cl"^(+)#.
