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When a substance dissolves in water, it means that the particles of the substance are interacting with water molecules. If the solution becomes colder, it indicates that the dissolution process is absorbing heat energy from the surroundings. This is an endothermic process. In an exothermic process, the system releases heat to the surroundings, which would result in the solution becoming warmer instead. So, the process of ammonium chloride dissolving in water is an endothermic process.

A solution forms when the solute dissolves in the solvent. In this case, ammonium chloride is the solute, and water is the solvent. The formation of a solution depends on the balance between the attractive forces within the solute, the attractive forces within the solvent, and the attractive forces between the solute and the solvent. In order for a solution to form, the attractive forces between the solute and the solvent must overcome the attractive forces within the solute and the solvent. The water molecules surround the ammonium chloride ions, creating a hydration shell around each ion, and effectively separate them. This process is called solvation. As a result, the ammonium chloride dissociates into its individual ions (ammonium ions and chloride ions), forming an aqueous solution. The process of ammonium chloride solution formation in water occurs because the attractive forces between the solute and solvent are strong enough to overcome the attractive forces within the solute and the solvent.