When calcium oxide, CaO(s) dissolves in water, the water becomes hot. Write a chemical equation for this process and indicate whether it is exothermic or endothermic.

The interaction reaction of calcium oxide and water leads to the formation of calcium hydroxide, this reaction is exothermic, which indicates that energy is released.

CaO(s) + H2O ⇒ Ca(OH)2

Explanation:

Calcium oxide reacts violently with water, causing it to reach 90 C. It then forms calcium hydroxide, also called slaked lime. It is heated because it releases energy in the form of heat, because the new substance formed (slaked lime) has less internal energy than the original and this is released in the form of heat. As a general rule, acid and base solutions are exothermic.