To Pour Water on Calcium Oxide

Pouring water on calcium oxide (quicklime or CaO) produces a vigorous exothermic reaction that generates calcium hydroxide (slaked lime) and releases substantial heat, sometimes enough to cause the water to boil and produce steam. The chemical equation is: CaO + H₂O → Ca(OH)₂ + Heat. This reaction is highly exothermic, releasing approximately 65 kilojoules of energy per mole of calcium oxide, which means even small amounts can produce noticeable heat. The process is called slaking, and the resulting calcium hydroxide is widely used in construction (for making mortar and plaster), agriculture (to reduce soil acidity), and water treatment.

From a practical safety perspective, this reaction requires careful handling because the intense heat generation can cause water to splash or boil violently, potentially causing burns. In industrial and construction settings, calcium oxide is deliberately hydrated under controlled conditions to produce slaked lime for various applications. The reaction is also irreversible under normal conditions—once calcium hydroxide forms, it cannot easily be converted back to calcium oxide without heating it to very high temperatures (above 512°C) to drive off the water. This transformation demonstrates important principles of chemical reactivity and energy changes in reactions, making it a common demonstration in chemistry education to illustrate exothermic processes and the formation of bases from metal oxides.