Calcium oxide (Cao), commonly known as Quick lime or Burnt lime is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature.

Preparation

Calcium oxide is usually made by the thermal decomposition of materials, such as limestone or seashells, that contain calcium carbonate (CaCO3; mineral calcite) in a lime kiln. This is accomplished by heating the material to 1000 °C (1835 °F), a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO2), leaving quicklime.

CaCO₃(s) CaO(s) + CO₂(g)

The quicklime is not stable and, when cooled, will spontaneously react with CO2 from the air until, after enough time, it will be completely converted back to calcium carbonate unless slaked with water to set as lime plaster or lime mortar.

Approximately 1.8 ton of limestone is required to get 1.0 ton of quicklime. Quicklime has a high affinity for water and is a more efficient desiccant than silica gel. The reaction of quicklime with water is associated with an increase in volume by a factor of at least 2.5.