Analysis of an unknown substance showed that it has a high boiling point and is brittle. It is an insulator as a solid but conducts electricity when melted. Which of the following substances would have those characteristics?A) HCl
B)Al
C)KBr
D)SiF4
E)I2

As you add more acid to the solution the amount of dissociation increases to compensate for it. But since the acid is weak, it will never have more ions dissociate than are in the concentrated solution being added. % ionization is the concentration of dissociated H+ ions divided by the initial concentration of the weak acid. 

% ionization = [H+] / [weak acid] x 100% 

As you increase the concentration of the weak acid, the numerator increases more than the denominator so the percentage goes down.