What happens when CaO is added to water

When CaO (calcium oxide or quicklime) is added to water, a highly exothermic and vigorous chemical reaction occurs, producing calcium hydroxide (slaked lime) and releasing substantial heat. The reaction can be so intense that the water boils, steam forms, hissing or crackling sounds are heard, and the solid quicklime swells, fractures, and crumbles into a fine powder or paste. The temperature can rise to 150°C or higher, and if the CaO is added too quickly to water, the violent heat release can cause splattering of hot, caustic material—creating a significant safety hazard.

The resulting calcium hydroxide has a pH around 12.4, making it a strong base that can cause chemical burns, which is why proper personal protective equipment is essential when handling this reaction. In construction and industrial applications, this slaking process must be carefully controlled by adding water gradually to large quantities of quicklime rather than dumping quicklime into water, allowing the heat to dissipate safely. The product, slaked lime, is widely used in mortar, plaster, whitewash, soil stabilization, and water treatment. Historically, this reaction's heat output was even exploited for cooking and heating applications. The dramatic nature of this reaction—combining rapid heat generation, physical disintegration of the solid, and formation of a caustic base—makes it one of the most striking examples of an exothermic chemical reaction and explains why quicklime must be stored away from moisture and handled with respect in any application.