The reaction of calcium oxide (CaO) with water (H₂O) is highly exothermic, meaning it releases energy in the form of heat. This is due to the strong affinity of calcium oxide for water molecules. As CaO reacts with water, it forms calcium hydroxide {Ca(OH)₂}, and the process releases a significant amount of heat. The energy change is negative, indicating an exothermic reaction. This heat release makes the reaction useful in various applications, including the preparation of slaked lime in construction and as a source of heat in self-heating food and beverage containers.