Is #HCl# a strong acid or strong base? How is the determined?
Acidity (and we usually specify an aqueous solvent) is assessed on the basis of the completeness of the following equilibrium:
Hydrogen chloride is acidic because it increases the concentration of the characteristic cation of the solvent, i.e.
On the other hand, hydrogen fluoride is a weak acid, because the equivalent equilibrium lies farther to the left:
Fluoride is a stronger base than chloride, and hence hydrogen fluoride is a weak acid.