An Arrhenius base is a substance that when dissolved in an aqueous solution, increases the concentration of hydroxide (-OH) in the solution. When Ca(OH)2 dissolves in water: Ca(OH)2 ⇒ Ca2- + 2OH-, thus increases the concentration of OH, so Ca(OH)2 is an Arrhenius base. KCl⇒K+ + Cl-, HCl⇒H+ +Cl-, they both don't increase -OH concentration, so they are not Arrhenius base. In fact HCl is an Arrhenius acid.
