Question

What is a chemical reaction that shows what happens when HCN dissolves in water?

Answer 1

The intensity or strength of an acid or a base is observed by the values of $p{{K}_{a}}$ for acids and $p{{K}_{b}}$ for bases. The more hydrogen, ${{H}^{+}}$ ions, and an acid liberates, stronger is the acid.

Complete answer: Any substance or compound that dissolves in water undergoes dissociation to liberate respective ions. Also this tells us the nature of that compound, either acidic or basic.
HCN, called as hydrogen cyanide, undergoes dissociation when dissolved in water to liberate hydrogen ${{H}^{+}}$ ions, along with cyanide anion. The reaction is in equilibrium, as the hydrogen ion is taken up by the water molecule to form hydronium cation. The chemical reaction is as follows:
$H-C\equiv N+{{H}_{2}}O\rightleftharpoons {{H}_{3}}{{O}^{+}}+{}^{-}C\equiv N$
This reaction shows that HCN acts as a weak Bronsted acid, it is because it has the ability to liberate the hydrogen ion, but this ion is taken up by the water molecule to form hydronium ions that show weak acidic character. Also the $p{{K}_{a}}$ value observed of hydrogen cyanide is 9.21.
Hence, HCN when dissolved in water acts as a weak acid.

Additional information: when a bronsted acid donates a hydrogen ion, then the remaining part or the anion is called its conjugate base. In this case $C{{N}^{-}}$ is the conjugate base.

Note: Like we take the pH readings, $p{{K}_{a}}$ values also tell the nature of acid or bases. As lower pH value tells us that the solution is acidic, the lower the value of $p{{K}_{a}}$, higher is the acidic value. Here the value of $p{{K}_{a}}$ is 9.21, hence the solution of HCN in water is weakly acidic.