Final answer:
An antacid tablet dissolving in water is an endothermic reaction because it absorbs heat from its surroundings.
Step-by-step explanation:
When an antacid tablet dissolves in water, it is considered an endothermic reaction because it absorbs heat from the surroundings. This process requires an input of energy in order to break the bonds of the tablet's compounds as they dissolve. According to the reference provided, although some dissolution processes can be exothermic, antacids typically consist of substances like Al(OH)3, Mg(OH)2, and NaHCO3 which, when dissolving, result in the water feeling cooler as heat is absorbed from the environment to facilitate the dissolution.