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Exothermic and endothermic reactions are types of chemical reactions that differ in their heat exchange with the surroundings. An exothermic reaction releases heat to the surroundings, causing the temperature to increase. On the other hand, an endothermic reaction absorbs heat from the surroundings, leading to a decrease in temperature.

When ammonium chloride (NH4Cl) dissolves in water, the solution becomes colder. This indicates that heat is being absorbed from the surroundings, as the temperature drops when the dissolution occurs.

Based on the observation that the temperature decreases when ammonium chloride dissolves in water, we can infer that the dissolution process is endothermic. This is because an endothermic reaction absorbs heat from its surroundings, causing the temperature of the solution to drop.

A solution is formed when the intermolecular forces between the solute and solvent are strong enough to overcome the forces holding their individual particles together. In the case of ammonium chloride dissolving in water, the positive and negative ions (NH4+ and Cl-) in the solute are attracted to the polar water molecules, forming strong ion-dipole interactions. These forces, combined with the entropy increase that occurs when a solute dissolves, lead to the formation of the solution. To summarize: a) The dissolution of ammonium chloride in water is an endothermic process since the solution becomes colder as heat is absorbed from the surroundings. b) The solution forms because the intermolecular forces between ammonium chloride and water molecules are strong enough to overcome the forces holding their individual particles together, and the entropy of the system increases upon dissolution.