Write the balanced chemical equation for the reaction of the weak acid HCN with water. Include the phase of each species.?

##"HCN"_ ((aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 3"O"_ ((aq))^(+) + "CN"_((aq))^(-)##

Don't get confused by the fact that you're dealing with a weak acid, hydrocyanic acid's reaction with water will still produce hydronium cations, ##"H"_3"O"^(+)##, and cyanide anions, ##"CN"^(-)##.

The only difference between a strong acid and a weak acid is the degree of ionization, i.e. how many molecules of the acid actually ionize in aqueous solution.

In the case of a weak acid, you only get partial ionization, meaning that most of molecules of acid will not donate their acidic proton. To symbolize this, use an equilibrium sign, ##rightleftharpoons##, instead of a normal arrow, ##->##.

The balanced chemical equation that describes hydrocyanic acid's ionization looks like this

##color(green)(|bar(ul(color(white)(a/a)color(black)(color(red)("H")"CN"_ ((aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 3"O"_ ((aq))^(color(red)(+)) + "CN"_((aq))^(-))color(white)(a/a)|)))##

Water is in the liquid state because it represents your . The hydrocyanic acid and the two ions that result from its ionization are in the aqueous state, which essentially means that they are dissolved in water.