• question_answer Why is$HCl$predominantly covalent in the gaseous state but is ionic in aqueous solution?

In$HCl$, the electro negativity difference in $Cl$ and H atoms (3.0 - 2.1) is 0.9. Therefore, it is predominantly covalent in the gaseous state. When dissolved in water which is of polar nature, there is an attraction between the atoms of the acid and polar${{H}_{2}}O$ molecules. Energy is released during attraction. This helps in the ionisation of the acid. Therefore, $HCl$ is ionic in aqueous solution.