What type of reaction is CaO(H2O) → Ca(OH)2

The reaction CaO + H2O → Ca(OH)2 is classified as a combination reaction (also called a synthesis reaction), where two or more reactants unite to form a single, more complex product. Specifically, this is also a hydration reaction, as water is being added to a compound to form a new substance. The reaction demonstrates classic combination reaction characteristics: multiple reactants (calcium oxide and water) merging to create one product (calcium hydroxide) without any byproducts.

From a broader chemical perspective, this reaction is also highly exothermic, meaning it releases considerable energy in the form of heat—a property that distinguishes it from many other combination reactions. While all combination reactions involve bonding new substances together, the CaO-water reaction is particularly notable for its violence and heat output. This dual classification as both a combination reaction and an exothermic hydration process helps chemists predict its behavior and understand why quicklime must be handled with such care in industrial and construction settings, where controlling the reaction rate through gradual water addition becomes critical for safety.