Question

Why is $HCl(g)$ capable of being dissolved in water?

Answer 1

When a substance dissolves in a solvent, it may separate into ions or it may remain in its molecular form. If the substance separates into ions completely, it is called a strong electrolyte while if some ions are formed and some remain in their molecular form, then it is known as weak electrolyte. $HCl$ is a strong electrolyte as it completely separates into hydrogen and chloride ions.

Complete answer:
Hydrochloric acid most commonly appears in its gaseous phase and at molecular level, it consists of a hydrogen atom covalently bonded to a chlorine atom which means electrons in the outer shell of the atoms are equally shared. However, due to higher electronegativity of chlorine, it has higher electron affinity and thus, the chlorine atom will have comparatively higher electron density and will attain a partial negative charge while the hydrogen atom will have a partial positive charge due to less electron density.
In water, the oxygen atom is covalently bonded to two hydrogen atoms and because the oxygen atom is much more electronegative than hydrogen atom, the water molecule is polar. Now, when $HCl$ is added to water, the positive end of each molecule is attracted to the negative end of other molecules forming polar intermolecular bonds. Thus, $HCl$ is soluble in water.
Alternatively, the solubility of $HCl$ can be explained on the basis of its low ionization energy and high hydration energy. Hydrochloric acid is a strong acid, so it completely dissociates in the solution and requires a very less amount of energy to ionize and an appreciable amount of energy is released on dissolving it in water which shows it is a highly exothermic and spontaneous process. Thus, $HCl$ is capable of being dissolved in water.

Note:
It is important to note that we can practically demonstrate the solubility of $HCl$ in water with the help of a fountain experiment which can also be used to explain and exemplify the properties of gases like alkalinity and acidity.