Answer
$HCN(aq) + H_2O(l) \lt -- \gt CN^-(aq) + H_3O^+(aq)$
Work Step by Step
$HCN$ is an acid, therefore, it will donate one proton to the other reactant, in this case, the water molecule.
When $HCN$ loses a proton, it produces $CN^-$, and when $H_2O$ receives this proton, it will produce $H_3O^+$.
