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

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).