* Formation: Calcium oxide is formed by the reaction of calcium (a metal) with oxygen. Metal oxides generally form bases.

* Reaction with Water: When calcium oxide reacts with water, it forms calcium hydroxide (Ca(OH)₂), which is a strong base. This reaction is exothermic and releases heat.

* pH: Calcium oxide solutions have a pH greater than 7, indicating a basic nature.

In summary: The chemical properties of calcium oxide, including its reaction with water and its high pH, classify it as a base.