Calcium oxide, also known as quicklime or burnt lime, is a white or grayish-white solid produced by heating limestone (calcium carbonate) at a high temperature. It is an alkaline oxide, meaning that it reacts with water to form a basic solution.

When calcium oxide is added to water, it undergoes a hydration reaction, forming calcium hydroxide:

$$CaO + H_2O → Ca(OH)_2$$

Calcium hydroxide is a strong base, which means that it dissociates completely in water to produce hydroxide ions:

$$Ca(OH)_2 → Ca^{2+} + 2OH^-$$

The presence of hydroxide ions in the solution makes calcium oxide an alkaline oxide.