What happens when calcium oxide reacts with water

When calcium oxide (quicklime) reacts with water, it undergoes a vigorous exothermic reaction that produces calcium hydroxide (slaked lime) and releases substantial heat—enough to boil the water in some cases. The reaction is immediate and dramatic, often accompanied by hissing sounds, steam formation, and the material swelling or crumbling as the solid CaO transforms into a white powder or paste. This process, called slaking, can raise temperatures to 150°C or higher, making it essential to add water gradually and use appropriate safety equipment when working with quicklime.

The resulting calcium hydroxide has numerous practical applications that make this reaction commercially important. In construction, slaked lime serves as a key ingredient in mortar and plaster, while in agriculture it's used to neutralize acidic soils and improve crop yields. The reaction's heat generation has historically been exploited for cooking applications and chemical heating packs. Because the reaction is so energetic, quicklime must be stored in moisture-free conditions—even humidity in the air can trigger the reaction over time, causing the material to degrade and potentially creating hazardous conditions if large quantities react simultaneously.