Question

The dissolution of ammonia chloride in water is an endothermic process but still dissolves in water readily. Why?

Answer 1

Ammonia chloride is an ionic compound, thus there is dipole-dipole interaction between ammonium and chloride ions. In the compound ammonia chloride, chlorine ions are negatively charged and ammonia molecules are positively charged.

Complete answer:
When the compound ammonia chloride is dissolved in water, the electron density on chlorine ion ( $C{{l}^{-}}$ ) of ammonia chloride causes it to separate hydrogen ion ( ${{H}^{+}}$ ) from water molecule.
Now, as chlorine ion ( $C{{l}^{-}}$ ) starts separating from the ammonia chloride molecule, ammonia ion ( $N{{H}_{4}}^{+}$ ) also starts separating. But, the energy of electron density on chlorine ions is not enough to break the bond of hydrogen and oxygen in water molecules.
Therefore, it takes energy from outside and breaks the hydrogen bond between oxygen and hydrogen. As the solution takes some heat energy from the surrounding to completely break the hydrogen ion free, it is an endothermic reaction.
Thus, four ions then exist in the solution, $N{{H}_{4}}^{+}$ (ammonia ion), $C{{l}^{-}}$ (chlorine ion), ${{H}^{+}}$ (hydrogen ion) and $O{{H}^{-}}$ (hydroxide ion). So, ammonia ions and hydroxide ions react together to form ammonia hydroxide.
However, the chlorine ions don’t react much with the hydrogen ions and thus more and more hydrogen ions get accumulated in the solution. Therefore, the solution becomes acidic.
Also, theoretically when ammonia and chlorine ions get separated from solid (crystalline) to liquid (free) form, it starts to move around randomly in the solution. Thus, the entropy of the system increases a lot and thus, $\Delta S$ becomes positive.
Increased entropy makes the term $T\Delta S$ more positive than $\Delta H$ , so Gibbs free energy becomes negative which shows that the reaction can occur on its own.

Note:
The enthalpy of the system decreases as the heat is getting transferred from the surrounding to the system. And entropy increases as the ions get free, thus ultimately the value of Gibbs free energy becomes negative. The formula of Gibbs free energy is given by $\Delta G=\Delta H-T\Delta S$ .