Why is `HCl` predominantly covalent in the gaseous state but ionic in the aqueous solution?

Explanation

`HCl` is predominantly covalent in gaseous state, since `EN` difference between `CI` and `H` atom is `(3.0-2.1)=0.9.` However, being a polar molecule `(H^(+delta)-Cl^(-delta))`, when dissolved in `H_(2)O`, the polar `H_(2)O` molecule interacts with `HCl` molecule as follows:

As a result, the bond between `H` and `Cl` is broken and hydrated `H_(aq)^(o+)` and `Cl_(aq)^(ө)` is obtained in the solution.