Question

How does hydrogen chloride differ from hydrochloric acid?

Answer 1

They're almost the same thing.

At STP, hydrogen chloride is a colorless gas, and it condenses below `-85^@ "C"`. It can be dissolved into a solvent like water to give you hydrochloric acid. That is, hydrochloric acid is an aqueous solution of hydrogen chloride.

But they both have the chemical formula `"HCl"`. So if you want to differentiate between them, add the phase. `"HCl"(aq)` is hydrochloric acid, while `"HCl"(g)` is hydrogen chloride.

In addition we can say that hydrogen chloride, i.e.`"HCl"(g)`, exists in a molecular state where as hydrochloric acid ionizes to form `"H"_3"O"^(+)` (hydronium ion) and `"Cl"^(-)` (chloride ion) while interacting with water molecules:

`"H"_2"O"(l) + "HCl"(g) rightleftharpoons "H"_3"O"^(+)(aq) + "Cl"^(-)(aq)`