When quicklime (calcium oxide, CaO) dissolves in water, heat is evolved.
This process is known as slaking of lime. When CaO is added to water, it undergoes a vigorous reaction in which it combines with water (H2O) to form calcium hydroxide [Ca(OH)2]. The balanced chemical equation for this reaction is:
CaO + H2O -> Ca(OH)2 + heat
The heat is released because the reaction is exothermic. This means that the products (Ca(OH)2) have a lower energy than the reactants (CaO and H2O). The excess energy is released in the form of heat.
The heat evolved during the slaking of lime makes it useful in a variety of applications, such as in construction and in the treatment of water and wastewater. It can also be used as a source of heat in some industrial processes.
Therefore, when quicklime dissolves in water, heat is evolved.