Question

Why do we use a double arrow in the dissociation equation for a weak acid answer?

Answer 1

Answer is: because weak acids do not dissociate completely.

The strength of an Arrhenius acid determines percentage of ionization of acid and the number of H⁺ ions formed. 
Strong acids completely ionize in water and give large amount ofhydrogen ions (H⁺), so we use only one arrow, because reaction goes in one direction and there no molecules of acid in solution.

For example hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq).

Weak acid partially ionize in water and give only a few hydrogen ions (H⁺), in the solution there molecules of acid and ions.

For example cyanide acid: HCN(aq) ⇄ H⁺(aq) + CN⁻(aq).