Ammonium chloride (NH4Cl) is a salt that dissociates into ammonium ions (NH4+) and chloride ions (Cl-) when dissolved in water. The process of dissolving a solute in a solvent involves the breaking of intermolecular forces between the solute particles and the formation of new solute-solvent interactions. This process can either release or absorb energy, leading to changes in temperature.

When ammonium chloride is dissolved in water, the dissolution process is endothermic. This means that energy is absorbed from the surroundings to break the intermolecular forces between the ammonium and chloride ions in the solid state, and to separate them to form solvated ions in the solution. The energy required for this process is obtained from the surrounding water molecules. As a result, the temperature of the solution decreases, and it feels cold.

The endothermic nature of the dissolution of ammonium chloride in water can be explained by considering the energy changes involved. The breaking of ionic bonds within the solid requires energy, as it is an endothermic process. Additionally, the energy required to separate the water molecules and allow solvation of the ammonium and chloride ions also contributes to the overall endothermic nature of the process.

Furthermore, the dissolution of ammonium chloride in water is also accompanied by an increase in disorder or entropy. In the solid state, the ammonium and chloride ions are arranged in a regular lattice structure. However, when the salt is dissolved in water, the ions become dispersed and surrounded by water molecules, resulting in a more disordered state. The increase in entropy also contributes to the endothermic nature of the dissolution process.

In summary, when ammonium chloride is dissolved in water, the solution becomes cold due to the endothermic nature of the dissolution process. Energy is absorbed from the surroundings to break the ionic bonds and to solvate the ions, resulting in a decrease in temperature.